# The UDUNITS-2 C API Guide

## Table of Contents

- 1 Synopsis
- 2 What’s a Unit Package Good For?
- 3 Unit-Systems
  - 3.1 Obtaining a Unit-System
  - 3.2 Extracting Units from a Unit-System
  - 3.3 Adding Units to a Unit-System
  - 3.4 Adding Unit-Prefixes to a Unit-System
  - 3.5 Miscellaneous Operations on Unit-Systems
- 4 Converting Values Between Units
- 5 Parsing a String into a Unit
- 6 Unit Syntax
  - 6.1 Unit Specification Examples
  - 6.2 Unit Grammar
- 7 Formatting a Unit into a String
- 8 Unit Operations
  - 8.1 Unary Unit Operations
  - 8.2 Binary Unit Operations
- 9 Mapping Between Identifiers and Units
  - 9.1 Names
  - 9.2 Symbols
- 10 The Handling of Time
- 11 Error Handling
  - 11.1 Status of Last Operation
  - 11.2 Error-Messages
- 12 The Units Database
- 13 Data Types
- Index

Next: Synopsis, Previous: (dir), Up: (dir) [Contents][Index]

# UDUNITS-2

This manual describes how to use the C API of the UDUNITS-2 library. Among other things, the library allows C code to obtain a binary representation of a unit of a physical quantity, to operate on such units, and to convert numeric values between compatible units.

The library comes with an extensive database of units all referenced to the SI system of units.

Copyright 2014 University Corporation for Atmospheric Research and contributors. All rights reserved.

This software was developed by the Unidata Program Center of the University Corporation for Atmospheric Research (UCAR) <http://www.unidata.ucar.edu>.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1) Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2) Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3) Neither the names of the development group, the copyright holders, nor the names of contributors may be used to endorse or promote products derived from this software without specific prior written permission. 4) This license shall terminate automatically and you may no longer exercise any of the rights granted to you by this license as of the date you commence an action, including a cross-claim or counterclaim, against the copyright holders or any contributor alleging that this software infringes a patent. This termination provision shall not apply for an action alleging patent infringement by combinations of this software with other software or hardware.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE SOFTWARE.

Next: Why, Previous: Top, Up: Top [Contents][Index]

## 1 Synopsis

Coding:

    #include <udunits2.h>

>     const char*
>     ut_get_path_xml(const char* path, ut_status* status);
>     ut_system*
>     ut_read_xml(const char* path);
>     ut_system*
>     ut_new_system(void);
>     void
>     ut_free_system(ut_system* system);
>     ut_system*
>     ut_get_system(const ut_unit* unit);
>     ut_unit*
>     ut_get_dimensionless_unit_one(const ut_system* system);
>     ut_unit*
>     ut_get_unit_by_name(const ut_system* system, const char* name);
>     ut_unit*
>     ut_get_unit_by_symbol(const ut_system* system, const char* symbol);
>     ut_status
>     ut_set_second(const ut_unit* second);
>     ut_status
>     ut_add_name_prefix(ut_system* system, const char* name, double value);
>     ut_status
>     ut_add_symbol_prefix(ut_system* system, const char* symbol, double value);
>     ut_unit*
>     ut_new_base_unit(ut_system* system);
>     ut_unit*
>     ut_new_dimensionless_unit(ut_system* system);
>     ut_unit*
>     ut_clone(const ut_unit* unit);
>     void
>     ut_free(ut_unit* unit);
>     const char*
>     ut_get_name(const ut_unit* unit, ut_encoding encoding);
>     ut_status
>     ut_map_name_to_unit(const char* name, const ut_encoding encoding, const ut_unit* unit);
>     ut_status
>     ut_unmap_name_to_unit(ut_system* system, const char* name, const ut_encoding encoding);
>     ut_status
>     ut_map_unit_to_name(const ut_unit* unit, const char* name, ut_encoding encoding);
>     ut_status
>     ut_unmap_unit_to_name(const ut_unit* unit, ut_encoding encoding);
>     const char*
>     ut_get_symbol(const ut_unit* unit, ut_encoding encoding);
>     ut_status
>     ut_map_symbol_to_unit(const char* symbol, const ut_encoding encoding, const ut_unit* unit);
>     ut_status
>     ut_unmap_symbol_to_unit(ut_system* system, const char* symbol, const ut_encoding encoding);
>     ut_status
>     ut_map_unit_to_symbol(const ut_unit* unit, const char* symbol, ut_encoding encoding);
>     ut_status
>     ut_unmap_unit_to_symbol(const ut_unit* unit, ut_encoding encoding);
>     int
>     ut_is_dimensionless(const ut_unit* unit);
>     int
>     ut_same_system(const ut_unit* unit1, const ut_unit* unit2);
>     int
>     ut_compare(const ut_unit* unit1, const ut_unit* unit2);
>     int
>     ut_are_convertible(const ut_unit* unit1, const ut_unit* unit2);
>     cv_converter*
>     ut_get_converter(ut_unit* from, ut_unit* to);
>     ut_unit*
>     ut_scale(double factor, const ut_unit* unit);
>     ut_unit*
>     ut_offset(const ut_unit* unit, double offset);
>     ut_unit*
>     ut_offset_by_time(const ut_unit* unit, double origin);
>     ut_unit*
>     ut_multiply(const ut_unit* unit1, const ut_unit* unit2);
>     ut_unit*
>     ut_invert(const ut_unit* unit);
>     ut_unit*
>     ut_divide(const ut_unit* numer, const ut_unit* denom);
>     ut_unit*
>     ut_raise(const ut_unit* unit, int power);
>     ut_unit*
>     ut_root(const ut_unit* unit, int root);
>     ut_unit*
>     ut_log(double base, const ut_unit* reference);
>     ut_unit*
>     ut_parse(const ut_system* system, const char* string, ut_encoding encoding);
>     char*
>     ut_trim(char* string, ut_encoding encoding);
>     int
>     ut_format(const ut_unit* unit, char* buf, size_t size, unsigned opts);
>     ut_status
>     ut_accept_visitor(const ut_unit* unit, const ut_visitor* visitor, void* arg);
>     double
>     ut_encode_date(int year, int month, int day);
>     double
>     ut_encode_clock(int hours, int minutes, double seconds);
>     double
>     ut_encode_time(int year, int month, int day, int hour, int minute, double second);
>     void
>     ut_decode_time(double value, int* year, int* month, int* day, int* hour, int* minute, double* second, double* resolution);
>     ut_status
>     ut_get_status(void);
>     void
>     ut_set_status(ut_status status);
>     int
>     ut_handle_error_message(const char* fmt, ...);
>     ut_error_message_handler
>     ut_set_error_message_handler(ut_error_message_handler handler);
>     int
>     ut_write_to_stderr(const char* fmt, va_list args);
>     int
>     ut_ignore(const char* fmt, va_list args);
>     float
>     cv_convert_float(const cv_converter* converter, float value);
>     double
>     cv_convert_double(const cv_converter* converter, double value);
>     float*
>     cv_convert_floats(const cv_converter* converter, const float* in, size_t count, float* out);
>     double*
>     cv_convert_doubles(const cv_converter* converter, const double* const in, size_t count, double* out);
>     void
>     cv_free(cv_converter* conv);

Compiling:

    c89 -I includedir ...

Where includedir is the installation-directory for C header files (e.g., `/usr/local/include`).

Linking:

    c89 ... -Llibdir -ludunits2 -lexpat ... -lm

Where libdir is the installation-directory for object code libraries (e.g., `/usr/local/lib`).

Next: Unit-Systems, Previous: Synopsis, Up: Top [Contents][Index]

## 2 What’s a Unit Package Good For?

The existence of a software package is justified by what you can do with it. The three main things you can do with the UDUNIT-2 package are

1. Convert numeric values between compatible units.
2. Convert a string representation of a unit into a binary one — enabling the programatic manipulation of units. There are three ways to do this:
   - Get the unit from a unit-system. This requires that you know the unit’s name or symbol and that the unit is in a unit-system.
   - Parse a string representation of the unit into its binary representation. This requires that the string be parsable by `ut_parse()`.
   - Explicity construct the unit from subcomponent units using unit operations.
3. Convert a binary representation of a unit into a string — enabling the printing and storing of units in a human-readable form.

While the above might seem to be trivial activities, their general availability at the time might have helped prevent the Mars Climate Orbiter fiasco.

Next: Value Conversion, Previous: Why, Up: Top [Contents][Index]

## 3 Unit-Systems

A unit-system is a set of units that are all defined in terms of the same set of base units. In the SI system of units, for example, the base units are the meter, kilogram, second, ampere, kelvin, mole, and candela. (For definitions of these base units, see http://physics.nist.gov/cuu/Units/current.html.)

In the UDUNITS-2 package, every accessible unit belongs to one and only one unit-system. It is not possible to convert numeric values between units of different unit-systems. Similarly, units belonging to different unit-systems always compare unequal.

There are several categories of operations on unit-systems:

Next: Extracting, Up: Unit-Systems [Contents][Index]

### 3.1 Obtaining a Unit-System

Typically, you would obtain a unit-system of predefined units by reading the default unit database using `ut_read_xml()` with a `NULL` pathname argument. If this doesn’t quite match your needs, then there are alternatives. Together with the typical solution, the means for obtaining a useful unit-system are (in order of increasing complexity):

- Obtain the default unit-system using `ut_read_xml(NULL)`.
- Copy and customize the unit database and then call `ut_read_xml()` with the pathname of the customized database to obtain a customized unit-system.
- Same as either of the above but then adding new units to the unit-system using `ut_new_base_unit()` and `ut_new_dimensionless_unit()`.
- Same as the above but also deriving new units using unit operations and then adding them to the unit-system using unit mapping.
- Same as the above but starting with an empty unit-system obtained from `ut_new_system()`, in which case you will definitely have to start with `ut_new_base_unit()` and `ut_new_dimensionless_unit()`.

You should pass every unit-system pointer to `ut_free_system()` when you no longer need the corresponding unit-system.

Function: `const char*` ut_get_path_xml `(const char* path, ut_status* status)`

  Returns the pathname of the XML-formatted unit-database corresponding to path. If path is non-`NULL`, then it is returned; otherwise, if the environment variable `UDUNITS2_XML_PATH` is set, then its value is returned; otherwise, the pathname of the default unit-database is returned. The value of `*status` indicates which of these possibilities occurred:

  `UT_OPEN_ARG`

    path is non-`NULL` and was returned.

  `UT_OPEN_ENV`

    path is `NULL`, the environment variable `UDUNITS2_XML_PATH` is set, and its value was returned.

  `UT_OPEN_DEFAULT`

    path is `NULL`, the environment variable `UDUNITS2_XML_PATH` is unset, and the pathname of the default unit-database was returned.

Function: `ut_system*` ut_read_xml `(const char* path)`

  Reads the XML-formatted unit-database specified by path and returns the corresponding unit-sytem. If path is `NULL`, then the pathname specified by the environment variable `UDUNITS2_XML_PATH` is used if set; otherwise, the compile-time pathname of the installed, default, unit database is used. You should pass the returned pointer to `ut_free_system()` when you no longer need the unit-system. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_OPEN_ARG`

    path is non-`NULL` but the file couldn’t be opened. See `errno` for the reason.

  `UT_OPEN_ENV`

    path is `NULL` and environment variable `UDUNITS2_XML_PATH` is set but the file couldn’t be opened. See `errno` for the reason.

  `UT_OPEN_DEFAULT`

    path is `NULL`, environment variable `UDUNITS2_XML_PATH` is unset, and the installed, default, unit database couldn’t be opened. See `errno` for the reason.

  `UT_OS`

    Operating-system error. See `errno`.

  `UT_PARSE`

    The database file couldn’t be parsed.

Function: `ut_system*` ut_new_system `(void)`

  Creates and returns a new unit-system. On success, the unit-system will be empty except for the dimensionless unit one. You should pass the returned pointer to `ut_free_system()` when you no longer need the unit-system. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return the following:

  `UT_OS`

    Operating-system error. See `errno`.

Next: Adding, Previous: Obtaining, Up: Unit-Systems [Contents][Index]

### 3.2 Extracting Units from a Unit-System

NOTE\: This section covers low-level access to the indidual units of a unit-system. General parsing of arbitrary unit specifications is covered in the section Parsing.

A unit-system contains mappings from identifiers to units (and vice versa). Consequently, once you have a unit-system, you can easily obtain a unit for which you know the name or symbol using the function `ut_get_unit_by_name()` or `ut_get_unit_by_symbol()`.

Function: `ut_unit*` ut_get_unit_by_name `(const ut_system* system, const char* name)`

  Returns the unit to which name maps from the unit-system referenced by system or `NULL` if no such unit exists. Name comparisons are case-insensitive. If this function returns `NULL`, then `ut_get_status()` will return one of the following:

  `UT_SUCCESS`

    name doesn’t map to a unit of system.

  `UT_BAD_ARG`

    system or name is `NULL`.

  You should pass the returned unit to `ut_free()` when it is no longer needed.

Function: `ut_unit*` ut_get_unit_by_symbol `(const ut_system* system, const char* symbol)`

  Returns the unit to which symbol maps from the unit-system referenced by system or `NULL` if no such unit exists. Symbol comparisons are case-sensitive. If this function returns `NULL`, then `ut_get_status()` will return one of the following:

  `UT_SUCCESS`

    symbol doesn’t map to a unit of system.

  `UT_BAD_ARG`

    system or symbol is `NULL`.

  You should pass the returned unit to `ut_free()` when it is no longer needed.

Function: `ut_unit*` ut_get_dimensionless_unit_one `(const ut_system* system)`

  Returns the dimensionless unit one of the unit-system referenced by system. While not necessary, the returned pointer may be passed to `ut_free()` when you no longer need the unit. If system is `NULL`, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return `UT_BAD_ARG`.

Next: Prefixes, Previous: Extracting, Up: Unit-Systems [Contents][Index]

### 3.3 Adding Units to a Unit-System

If you use `ut_read_xml()`, then you should not normally need to add any new units to a unit-system.

Because you get units via their names or symbols, adding a unit to a unit-system actually means mapping one or more identifiers (i.e., names or symbols) to the unit. Thereafter, you can use `ut_get_unit_by_name()` and `ut_get_unit_by_symbol()` to retrieve the unit. The mapping of identifiers to units is covered here.

Having said that, it is possible to create a new base or dimensionless unit within a unit-system using `ut_new_base_unit()` or `ut_new_dimensionless_unit()`—you’ll just also have to map identifiers to the newly-created unit in order to be able to retrieve it later by identifier.

Function: `ut_unit*` ut_new_base_unit `(ut_system* system)`

  Creates and adds a new base-unit to the unit-system referenced by system. This function returns the new base-unit. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    system is `NULL`.

  `UT_OS`

    Operating-system failure. See `errno`.

  If you use `ut_read_xml()`, then you should not normally need to call this function.

Function: `ut_unit*` ut_new_dimensionless_unit `(ut_system* system)`

  Creates and adds a new dimensionless-unit to the unit-system referenced by system. This function returns the new dimensionless-unit. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    system is `NULL`.

  `UT_OS`

    Operating-system failure. See `errno`.

  If you use `ut_read_xml()`, then you should not normally need to call this function.

Next: Misc, Previous: Adding, Up: Unit-Systems [Contents][Index]

### 3.4 Adding Unit-Prefixes to a Unit-System

A prefix is a word or symbol that is appended to the beginning of a word or symbol that represents a unit in order to modify the value of that unit. For example, the prefix “kilo” in the word “kiloamperes” changes the value from one ampere to one-thousand amperes.

If you use `ut_read_xml()`, then you should not normally need to add any new prefixes to a unit-system.

Function: `ut_status` ut_add_name_prefix `(ut_system* system, const char* name, double value)`

  Adds the name-prefix name with the value value to the unit-system system. A name-prefix is something like “mega” or “milli”. Comparisons between name-prefixes are case-insensitive. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    system or name is `NULL`, or value is `0`.

  `UT_EXISTS`

    name already maps to a different value.

  `UT_OS`

    Operating-system failure. See `errno`.

Function: `ut_status` ut_add_symbol_prefix `(ut_system* system, const char* symbol, double value)`

  Adds the symbol-prefix symbol with the value value to the unit-system system. A symbol-prefix is something like “M” or “m”. Comparisons between symbol-prefixes are case-sensitive. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    system or symbol is `NULL`, or value is `0`.

  `UT_EXISTS`

    symbol already maps to a different value.

  `UT_OS`

    Operating-system failure. See `errno`.

Previous: Prefixes, Up: Unit-Systems [Contents][Index]

### 3.5 Miscellaneous Operations on Unit-Systems

Function: `void` ut_free_system `(ut_system* system)`

  Frees the unit-system referenced by system. All unit-to-identifier and identifier-to-unit mappings are removed. Use of `system` after this function returns results in undefined behavior.

Function: `ut_status` ut_set_second `(const ut_unit* second)`

  Sets the “second” unit of a unit-system. This function must be called before the first call to `ut_offset_by_time()` for a unit in the same unit-system. `ut_read_xml()` calls this function if the unit-system it’s reading contains a unit named “second”. This function returns one of the following:

  `UT_SUCCESS`

    The “second” unit of system was successfully set.

  `UT_EXISTS`

    The “second” unit of system is set to a different unit.

  `UT_BAD_ARG`

    second is `NULL`.

Next: Parsing, Previous: Unit-Systems, Up: Top [Contents][Index]

## 4 Converting Values Between Units

You can convert numeric values in one unit to equivalent values in another, compatible unit by means of a converter. For example

    #include <udunits2.h>
    ...
        ut_unit*      from = ...;
        ut_unit*      to = ...;
        cv_converter* converter = ut_get_converter(from, to);
        double       fromValue = ...;
        double       toValue = cv_convert_double(converter, fromValue);

        cv_free(converter);

The converter API is declared in the header-file `<converter.h>`, which is automatically included by the UDUNITS-2 header-file (`<udunits2.h>`) so you don’t need to explicitly include it.

Function: `int` ut_are_convertible `(const ut_unit* unit1, uconst t_unit* unit2)`

  Indicates if numeric values in unit unit1 are convertible to numeric values in unit unit2 via ut_get_converter(). In making this determination, dimensionless units are ignored. This function returns a non-zero value if conversion is possible; otherwise, `0` is returned and ut_get_status() will return one of the following:

  `UT_BAD_ARG`

    unit1 or unit2 is `NULL`.

  `UT_NOT_SAME_SYSTEM`

    unit1 and unit2 belong to different unit-systems.

  `UT_SUCCESS`

    Conversion between the units is not possible (e.g., unit1 refers to a meter and unit2 refers to a kilogram.

Function: `cv_converter*` ut_get_converter `(ut_unit* const from, ut_unit* const to)`

  Creates and returns a converter of numeric values in the from unit to equivalent values in the to unit. You should pass the returned pointer to `cv_free()` when you no longer need the converter. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    from or to is `NULL`.

  `UT_NOT_SAME_SYSTEM`

    The units from and to don’t belong to the same unit-system.

  `UT_MEANINGLESS`

    The units belong to the same unit-system but conversion between them is meaningless (e.g., conversion between seconds and kilograms is meaningless).

  `UT_OS`

    Operating-system failure. See `errno`.

Function: `float` cv_convert_float `(const cv_converter* converter, const float value)`

  Converts the single floating-point value value and returns the new value.

Function: `double` cv_convert_double `(const cv_converter* converter, const double value)`

  Converts the single double-precision value value and returns the new value.

Function: `float*` cv_convert_floats `(const cv_converter* converter, const float* in, size_t count, float* out)`

  Converts the count floating-point values starting at in, writing the new values starting at out and, as a convenience, returns out. The input and output arrays may overlap or be identical.

Function: `double*` cv_convert_doubles `(const cv_converter* converter, const double* in, size_t count, double* out)`

  Converts the count double-precision values starting at in, writing the new values starting at out and, as a convenience, returns out. The input and output arrays may overlap or be identical.

Function: `void` cv_free `(cv_converter* conv)`;

  Frees resources associated with the converter referenced by conv. You should call this function when you no longer need the converter. Use of conv upon return results in undefined behavior.

Next: Syntax, Previous: Value Conversion, Up: Top [Contents][Index]

## 5 Parsing a String into a Unit

Here’s an example of parsing a string representation of a unit into its binary representation:

    #include <stdlib.h>
    #include <udunits2.h>
    ...
        ut_system*   unitSystem = ut_read_xml(NULL);
        const char* string = "kg.m2/s3";
        ut_unit*     watt = ut_parse(unitSystem, string, UT_ASCII);

        if (watt == NULL) {
            /* Unable to parse string. */
        }
        else {
            /* Life is good. */
        }

Function: `ut_unit*` ut_parse `(const ut_system* system, const char* string, ut_encoding encoding)`

  Returns the binary unit representation corresponding to the string unit representation string in the character-set encoding using the unit-system system. string must have no leading or trailing whitespace (see `ut_trim()`). If an error occurs, then this function returns `NULL` and `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    system or string is `NULL`.

  `UT_SYNTAX`

    string contained a syntax error.

  `UT_UNKNOWN`

    string contained an unknown identifier.

  `UT_OS`

    Operating-system failure. See `errno` for the reason.

  You should pass the returned unit to `ut_free()` when it is no longer needed.

Function: `size_t` ut_trim `(char* string, ut_encoding encoding)`

  Removes all leading and trailing whitespace from the NUL-terminated string string. Returns string, which is modified if it contained leading or trailing whitespace.

Next: Formatting, Previous: Parsing, Up: Top [Contents][Index]

## 6 Unit Syntax

For the most part, the UDUNITS-2 package follows the syntax for unit-strings promulgated by the US National Institute for Standards and Technology (NIST). Details, of which, can be found at http://physics.nist.gov/cuu/Units/index.html. The one general exception to this is the invention of a syntax for “offset”-units (e.g., the definition of the degree Celsius is “K @ 273.15”).

Next: Grammar, Up: Syntax [Contents][Index]

### 6.1 Unit Specification Examples

> String Type | Using Names | Using Symbols | Comment
> --- | --- | --- | ---
> Simple | meter | m
> Raised | meter^2 | m2 | higher precedence than multiplying or dividing
> Product | newton meter | N.m
> Quotient | meter per second | m/s
> Scaled | 60 second | 60 s
> Prefixed | kilometer | km
> Offset | kelvin from 273.15 | K @ 273.15 | lower precedence than multiplying or dividing
> Logarithmic | lg(re milliwatt) | lg(re mW) | "lg" is base 10, "ln" is base e, and "lb" is base 2
> Grouped | (5 meter)/(30 second) | (5 m)/(30 s)

The above may be combined, e.g., "0.1 lg(re m/(5 s)^2) @ 50".

You may also look at the `<def>` elements in the units database to see examples of string unit specifications.

You may use the `(udunits2prog)udunits2` utility to experiment with string unit specifications.

Previous: Examples, Up: Syntax [Contents][Index]

### 6.2 Unit Grammar

Here is the unit-syntax understood by the UDUNITS-2 package. Words printed Thusly indicate non-terminals; words printed THUSLY indicate terminals; and words printed <thusly> indicate lexical elements.

    Unit-Spec: one of
            nothing
            Shift-Spec

    Shift-Spec: one of
            Product-Spec
            Product-Spec SHIFT REAL
            Product-Spec SHIFT INT
            Product-Spec SHIFT Timestamp

    Product-Spec: one of
            Power-Spec
            Product-Spec Power-Spec
            Product-Spec MULTIPLY Power-Spec
            Product-Spec DIVIDE Power-Spec

    Power-Spec: one of
            Basic-Spec
            Basic-Spec INT
            Basic-Spec EXPONENT
            Basic-Spec RAISE INT

    Basic-Spec: one of
            ID
            "(" Shift-Spec ")"
            LOGREF Product_Spec ")"
            Number

    Number: one of
            INT
            REAL

    Timestamp: one of
            DATE
            DATE CLOCK
            DATE CLOCK CLOCK
            DATE CLOCK INT
            DATE CLOCK ID
            TIMESTAMP
            TIMESTAMP INT
            TIMESTAMP ID

    SHIFT:
            <space>* <shift_op> <space>*

    <shift_op>: one of
            "@"
            "after"
            "from"
            "since"
            "ref"

    REAL:
            the usual floating-point format

    INT:
            the usual integer format

    MULTIPLY: one of
            "-"
            "."
            "*"
            <space>+
            <centered middot>

    DIVIDE:
            <space>* <divide_op> <space>*

    <divide_op>: one of
            per
            PER
            "/"

    EXPONENT:
            ISO-8859-9 or UTF-8 encoded exponent characters

    RAISE: one of
            "^"
            "**"

    ID: one of
            <id>
            "%"
            "'"
            "\""
            degree sign
            greek mu character

    <id>:
            <alpha> <alphanum>*

    <alpha>:
            [A-Za-z_]
            ISO-8859-1 alphabetic characters
            non-breaking space

    <alphanum>: one of
            <alpha>
            <digit>

    <digit>:
            [0-9]

    LOGREF:
            <log> <space>* <logref>

    <log>: one of
            "log"
            "lg"
            "ln"
            "lb"

    <logref>:
            "(" <space>* <re> ":"? <space>*

    DATE:
            <year> "-" <month> ("-" <day>)?

    <year>:
            [+-]?[0-9]{1,4}

    <month>:
            "0"?[1-9]|1[0-2]

    <day>:
            "0"?[1-9]|[1-2][0-9]|"30"|"31"

    CLOCK:
            <hour> ":" <minute> (":" <second>)?

    TIMESTAMP:
            <year> (<month> <day>?)? "T" <hour> (<minute> <second>?)?

    <hour>:
            [+-]?[0-1]?[0-9]|2[0-3]

    <minute>:
            [0-5]?[0-9]

    <second>:
            (<minute>|60) (\.[0-9]*)?

Next: Operations, Previous: Syntax, Up: Top [Contents][Index]

## 7 Formatting a Unit into a String

Use the `ut_format()` function to obtain the string representation of a binary unit. For example, the following gets the definition of the unit "watt" in ASCII characters using unit-symbols rather than unit-names:

    ut_unit*     watt = ...;
    char        buf[128];
    unsigned    opts = UT_ASCII | UT_DEFINITION;
    int         len = ut_format(watt, buf, sizeof(buf), opts);

    if (len == -1) {
        /* Couldn't get string */
    }
    else if (len == sizeof(buf)) {
        /* Entire buffer used: no terminating NUL */
    }
    else {
        /* Have string with terminating NUL */
    }

Function: `int` ut_format `(const ut_unit* unit, char* buf, size_t size, unsigned opts)`

  Formats the unit unit (i.e., returns its string representation) into the buffer pointed-to by buf of size size. The argument opts specifies how the formatting is to be done and is a bitwise OR of a ut_encoding value and zero or more of the following:

  `UT_NAMES`

    Use unit names instead of symbols.

  `UT_DEFINITION`

    The formatted string should be the definition of unit in terms of basic-units instead of stopping any expansion at the highest level possible.

  On success, this function returns either the number of bytes – excluding the terminating `NUL` – that were written into `buf` or the number of bytes that would have been written. The difference is due to the runtime `snprinf()` function that was used.

  On failure, this function returns `-1` and ut_get_status() will return one of the following:

  `UT_BAD_ARG`

    unit or buf is `NULL`, or opts contains the bit patterns of both `UT_LATIN1` and `UT_UTF8`.

  `UT_CANT_FORMAT`

    unit can’t be formatted in the desired manner (e.g., opts contains `UT_ASCII` but unit doesn’t have an identifier in that character-set or opts doesn’t contain UT_NAMES and a necessary symbol doesn’t exist).

Next: Mapping, Previous: Formatting, Up: Top [Contents][Index]

## 8 Unit Operations

You can use unit operations to construct new units, get information about units, or compare units.

Next: Binary, Up: Operations [Contents][Index]

### 8.1 Unary Unit Operations

Function: `void` ut_free `(ut_unit* unit)`

  Frees resources associated with unit. You should invoke this function on every unit that you no longer need. Use of unit upon return from this function results in undefined behavior.

Function: `ut_unit*` ut_scale `(double factor, const ut_unit* unit)`

  Returns a unit equivalent to another unit scaled by a numeric factor. For example:

      const ut_unit*   meter = ...
      const ut_unit*   kilometer = ut_scale(1000, meter);

  The returned unit is equivalent to unit multiplied by factor. You should pass the returned pointer to `ut_free()` when you no longer need the unit.

Function: `ut_unit*` ut_offset `(const ut_unit* unit, double offset)`

  Returns a unit equivalent to another unit relative to a particular origin. For example:

      const ut_unit*   kelvin = ...
      const ut_unit*   celsius = ut_offset(kelvin, 273.15);

  The returned unit is equivalent to unit with an origin of offset. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function returns `NULL` and `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    unit is `NULL`.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

Function: `ut_unit*` ut_offset_by_time `(const ut_unit* const unit, const double origin)`

  Returns a timestamp-unit equivalent to the time unit unit referenced to the time-origin origin (as returned by `ut_encode_time()`). For example:

      const ut_unit*   second = ...
      const ut_unit*   secondsSinceTheEpoch =
          ut_offset_by_time(second, ut_encode_time(1970, 1, 1, 0, 0, 0.0));

  Leap seconds are not taken into account. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function returns `NULL` and `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    unit is `NULL`.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

  `UT_MEANINGLESS`

    Creation of a timestamp unit based on unit is not meaningful. It might not be a time-unit, for example.

  `UT_NO_SECOND`

    The associated unit-system doesn’t contain a “second” unit. See `ut_set_second()`.

  CAUTION: The timestamp-unit was created to be analogous to, for example, the degree celsius—but for the time dimension. I’ve come to believe, however, that creating such a unit was a mistake, primarily because users try to use the unit in ways for which it was not designed (such as converting dates in a calendar whose year is exactly 365 days long). Such activities are much better handled by a dedicated calendar package. Please be careful about using timestamp-units. See also the section on The Handling of Time.

Function: `ut_unit*` ut_invert `(const ut_unit* unit)`

  Returns the inverse (i.e., reciprocal) of the unit unit. This convenience function is equal to `ut_raise(unit,-1)`. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    unit is `NULL`.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

Function: `ut_unit*` ut_raise `(const ut_unit* unit, int power)`

  Returns the unit equal to unit unit raised to the power power. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    unit is `NULL`.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

Function: `ut_unit*` ut_root `(const ut_unit* unit, int root)`

  Returns the unit equal to the root root of unit unit. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    unit is `NULL`.

  `UT_MEANINGLESS`

    It’s meaningless to take the given root of the given unit. This could be because the resulting unit would have fractional (i.e., non-integral) dimensionality, or because the unit is, for example, a logarithmic unit.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

Function: `ut_unit*` ut_log `(double base, const ut_unit* reference)`

  Returns the logarithmic unit corresponding to the logarithmic base base and a reference level specified as the unit reference. For example, the following creates a decibel unit with a one milliwatt reference level:

           const ut_unit* milliWatt = ...;
           const ut_unit* bel_1_mW = ut_log(10.0, milliWatt);

           if (bel_1_mW != NULL) {
               const ut_unit* decibel_1_mW = ut_scale(0.1, bel_1_mW);

               ut_free(bel_1_mW);   /* no longer needed */

               if (decibel_1_mW != NULL) {
                   /* Have decibel unit with 1 mW reference */
                   ...
                   ut_free(decibel_1_mW);
               }                 /* "decibel_1_mW" allocated */
           }

  You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    reference is `NULL`.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

  `UT_BAD_ARG`

    base is invalid (e.g., it must be greater than one).

Function: `const char*` ut_get_name `(const ut_unit* unit, ut_encoding encoding)`

  Returns the name to which the unit referenced by unit maps in the character-encoding specified by encoding. If this function returns `NULL`, then `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    name is `NULL`.

  `UT_SUCCESS`

    unit doesn’t map to a name in the given character-set.

Function: `const char*` ut_get_symbol `(const ut_unit* unit, ut_encoding encoding)`

  Returns the symbol to which the unit referenced by unit maps in the character-encoding specified by encoding. If this function returns `NULL`, then `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    symbol is `NULL`.

  `UT_SUCCESS`

    unit doesn’t map to a symbol in the given character-set.

Function: `ut_system*` ut_get_system `(const ut_unit* unit)`

  Returns the unit-system to which the unit referenced by unit belongs. If unit is `NULL`, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return `UT_BAD_ARG`.

Function: `int` ut_is_dimensionless `(const ut_unit* unit)`

  Indicates if unit unit is dimensionless (like “radian”). This function returns a non-zero value if the unit is dimensionless; otherwise, `0` is returned and ut_get_status() will return one of the following:

  `UT_BAD_ARG`

    unit1 is `NULL`.

  `UT_SUCCESS`

    The unit is dimensionless.

Function: `ut_unit*` ut_clone `(const ut_unit* unit)`

  Returns a copy of the unit referenced by unit. You should pass the returned pointer to `ut_free()` when you no longer need the unit. If an error occurs, then this function writes an error-message using `ut_handle_error_message()` and returns `NULL`. Also, `ut_get_status()` will return one of the following:

  `UT_BAD_ARG`

    unit is `NULL`.

  `UT_OS`

    Operating-system failure. See `errno`.

  If you use `ut_read_xml()`, then you should not normally need to call this function.

Function: `ut_status` ut_accept_visitor `(const ut_unit* unit, const ut_visitor* visitor, void* arg)`

  Accepts the visitor visitor to the unit unit. The argument arg is passed to the visitor’s functions. This function returns one of the following:

  `UT_BAD_ARG`

    visitor or unit is `NULL`.

  `UT_VISIT_ERROR`

    An error occurred in visitor while visiting unit.

  `UT_SUCCESS`

    Success.

Data type: ut_visitor int foo(int) int bar(int, int)

  You pass a pointer to a data object of this type if and when you call `ut_accept_visitor()`. It contains the following pointers to functions that implement your unit-visitor:

  `ut_status (*visit_basic)(const ut_unit* unit, void* arg);`

    Visits the basic-unit unit. A basic-unit is a base unit like “meter” or a non-dimensional but named unit like “radian”. This function returns `UT_SUCCESS` on and only on success.

  `ut_status (*visit_product)(const ut_unit* unit, int count, const ut_unit* const* basicUnits, const int* powers, void* arg);`

    Visits the product-unit unit. The product-unit is a product of the count basic-units referenced by basicUnits, each raised to their respective, non-zero power in powers. This function returns `UT_SUCCESS` on and only on success.

  `ut_status (*visit_galilean)(const ut_unit* unit, double scale, const ut_unit* underlyingUnit, double origin, void* arg);`

    Visits the Galilean-unit unit. The Galilean-unit has the underlying unit underlyingUnit and either the non-unity scale factor scale or the non-zero origin origin, or both. This function returns `UT_SUCCESS` on and only on success.

  `ut_status (*visit_timestamp)(const ut_unit* unit, const ut_unit* timeUnit, double origin, void* arg);`

    Visits the timestamp-unit unit. The timestamp-unit has the underlying unit of time timeUnit and the `ut_encode_time()`-encoded time-origin origin. This function returns `UT_SUCCESS` on and only on success.

  `ut_status (*visit_logarithmic)(const ut_unit* unit, double base, const ut_unit* reference, void* arg);`

    Visits the logarithmic-unit unit. The logarithmic-unit has the logarithmic base base and the reference-level is specified by the unit reference. This function returns `UT_SUCCESS` on and only on success.

Previous: Unary, Up: Operations [Contents][Index]

### 8.2 Binary Unit Operations

Binary unit operations act on two units.

NOTE\: The functions `ut_are_convertible()` and `ut_get_converter()` are also binary unit operations but are documented elsewhere.

Function: `ut_unit*` ut_multiply `(const ut_unit* unit1, const ut_unit* unit2)`

  Returns the result of multiplying unit unit1 by unit unit2. You should pass the pointer to ut_free() when you no longer need the unit On failure, this function returns `NULL` and ut_get_status() will return one of the following:

  `UT_BAD_ARG`

    unit1 or unit2 is `NULL`.

  `UT_NOT_SAME_SYSTEM`

    unit1 and unit2 belong to different unit-systems.

  `UT_OS`

    Operating-system error. See errno for the reason.

Function: `ut_unit*` ut_divide `(const ut_unit* numer, const ut_unit* denom)`

  Returns the result of dividing unit numer by unit denom. You should pass the pointer to ut_free() when you no longer need the unit On failure, this function returns `NULL` and ut_get_status() will return one of the following:

  `UT_BAD_ARG`

    numer or denom is `NULL`.

  `UT_NOT_SAME_SYSTEM`

    unit1 and unit2 belong to different unit-systems.

  `UT_OS`

    Operating-system error. See `errno` for the reason.

Function: `int` ut_compare `(const ut_unit* unit1, const ut_unit* unit2)`

  Compares two units. Returns a value less than, equal to, or greater than zero as unit1 is considered less than, equal to, or greater than unit2, respectively. Units from different unit-systems never compare equal. The value zero is also returned if both unit pointers are `NULL`.

Function: `int` ut_same_system `(const ut_unit* unit1, const ut_unit* unit2)`

  Indicates if two units belong to the same unit-system. This function returns a non-zero value if the two units belong to the same unit-system; otherwise, `0` is returned and ut_get_status() will return one of the following:

  `UT_BAD_ARG`

    unit1 or unit2 is `NULL`.

  `UT_SUCCESS`

    The units belong to different unit-systems.

Next: Time, Previous: Operations, Up: Top [Contents][Index]

## 9 Mapping Between Identifiers and Units

Within a unit-system, you can map an identifier to a unit and vice versa. If an identifier maps to a unit, then the unit can be retrieved from the unit-system via the identifier. Similarly, if a unit maps to an identifier, then the unit can be printed using the identifier.

There a two kinds of identifiers: names and symbols.

Next: Symbols, Up: Mapping [Contents][Index]

### 9.1 Names

You can map a name to a unit and vice versa. If you use `ut_read_xml()`, then you shouldn’t normally need to do this.

Function: `ut_status` ut_map_name_to_unit `(const char* name, const ut_encoding encoding, const ut_unit* unit)`

  Maps the name referenced by name, in character-set encoding, to the unit referenced by unit in the unit-system that contains unit. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    name or unit is `NULL`.

  `UT_OS`

    Operating-system failure. See `errno`.

  `UT_EXISTS`

    name already maps to a different unit.

Function: `ut_status` ut_unmap_name_to_unit `(ut_system* system, const char* name, const ut_encoding encoding)`

  Removes any mapping from name name, in character-set encoding, to a unit in unit-system system. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    system or name is `NULL`.

Function: `ut_status` ut_map_unit_to_name `(const ut_unit* unit, const char* name, ut_encoding encoding)`

  Maps the unit unit to the name name, which is in character-set encoding, in the unit-system that contains the unit. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    unit or name is `NULL`, or name is not in the character-set encoding.

  `UT_OS`

    Operating-system failure. See `errno`.

  `UT_EXISTS`

    unit already maps to a different name.

Function: `ut_status` ut_unmap_unit_to_name `(const ut_unit* unit, ut_encoding encoding)`

  Removes any mapping from unit unit to a name in character-set encoding from the unit-system that contains the unit. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    unit is `NULL`.

Previous: Names, Up: Mapping [Contents][Index]

### 9.2 Symbols

You can map a symbol to a unit and vice versa. If you use `ut_read_xml()`, then you shouldn’t normally need to do this.

Function: `ut_status` ut_map_symbol_to_unit `(const char* symbol, const ut_encoding encoding, const ut_unit* unit)`

  Maps the symbol referenced by symbol, in character-set encoding, to the unit referenced by unit in the unit-system that contains unit. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    symbol or unit is `NULL`.

  `UT_OS`

    Operating-system failure. See `errno`.

  `UT_EXISTS`

    symbol already maps to a different unit.

Function: `ut_status` ut_unmap_symbol_to_unit `(ut_system* system, const char* symbol, const ut_encoding encoding)`

  Removes any mapping from symbol symbol, in character-set encoding, to a unit in unit-system system. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    system or symbol is `NULL`.

Function: `ut_status` ut_map_unit_to_symbol `(const ut_unit* unit, const char* symbol, ut_encoding encoding)`

  Maps the unit unit to the symbol symbol, which is in character-set encoding, in the unit-system that contains the unit. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    unit or symbol is `NULL`.

  `UT_BAD_ARG`

    Symbol symbol is not in the character-set encoding.

  `UT_OS`

    Operating-system failure. See `errno`.

  `UT_EXISTS`

    unit already maps to a different symbol.

Function: `ut_status` ut_unmap_unit_to_symbol `(const ut_unit* unit, ut_encoding encoding)`

  Removes any mapping from unit unit to a symbol in character-set encoding from the unit-system that contains the unit. This function returns one of the following:

  `UT_SUCCESS`

    Success.

  `UT_BAD_ARG`

    unit is `NULL`.

Next: Errors, Previous: Mapping, Up: Top [Contents][Index]

## 10 The Handling of Time

You should use a true calendar package rather than the UDUNITS-2 package to handle time. Having said that, many people use the time-handling capabilities of the UDUNITS-2 package because it supports "units" like "`seconds since 1970-01-01`". You should be aware, however, that the hybrid Gregorian/Julian calendar used by the UDUNITS-2 package cannot be changed. Dates on or after 1582-10-15 are assumed to be Gregorian dates; dates before that are assumed to be Julian dates. In particular, the year 1 BCE is immediately followed by the year 1 CE.

In general, the UDUNITS-2 package handles time by encoding it as double-precision value, which can then be acted upon arithmetically.

Function: `double` ut_encode_time `(int year, int month, int day, int hour, int minute, double second)`

  Encodes a time as a double-precision value. This convenience function is equivalent to

      ut_encode_date(year,month,day) + ut_encode_clock(hour,minute,second)

Function: `double` ut_encode_date `(int year, int month, int day)`

  Encodes a date as a double-precision value. You probably won’t use this function. Dates on or after 1582-10-15 are assumed to be Gregorian dates; dates before that are assumed to be Julian dates. In particular, the year 1 BCE is immediately followed by the year 1 CE.

Function: `double` ut_encode_clock `(int hour, int minute, double second)`

  Encodes a clock-time as a double-precision value. `abs(hour)` must be less than 24; `abs(minute)` must be less than 60; and `fabs(second)` must be less than or equal to 62. You probably won’t use this function.

Function: `void` ut_decode_time `(double time, int* year, int* month, int* day, int* hour, int* minute, double* second, double* resolution)`

  Decodes a time from a double-precision value into its individual components. The variable referenced by resolution will be set to the resolution (i.e., uncertainty) of the time in seconds.

Next: Database, Previous: Time, Up: Top [Contents][Index]

## 11 Error Handling

Error-handling in the units module has two aspects: the status of the last operation performed by the module and the handling of error-messages:

Next: Messages, Up: Errors [Contents][Index]

### 11.1 Status of Last Operation

UDUNITS-2 functions set their status by calling `ut_set_status()`. You can use the function `ut_get_status()` to retrieve that status.

Function: `ut_status` ut_get_status `(void)`

  Returns the value specified in the last call to `ut_set_status()`

Function: `void` ut_set_status `(ut_status status)`

  Set the status of the units module to status.

Data type: ut_status

  This enumeration has the following values:

  `UT_SUCCESS`

    Success

  `UT_BAD_ARG`

    An argument violates the the function’s contract (e.g., it’s `NULL`).

  `UT_EXISTS`

    Unit, prefix, or identifier already exists

  `UT_NO_UNIT`

    No such unit exists

  `UT_OS`

    Operating-system error. See `errno` for the reason.

  `UT_NOT_SAME_SYSTEM`

    The units belong to different unit-systems

  `UT_MEANINGLESS`

    The operation on the unit or units is meaningless

  `UT_NO_SECOND`

    The unit-system doesn’t have a unit named “second”

  `UT_VISIT_ERROR`

    An error occurred while visiting a unit

  `UT_CANT_FORMAT`

    A unit can’t be formatted in the desired manner

  `UT_SYNTAX`

    String unit representation contains syntax error

  `UT_UNKNOWN`

    String unit representation contains unknown word

  `UT_OPEN_ARG`

    Can’t open argument-specified unit database

  `UT_OPEN_ENV`

    Can’t open environment-specified unit database

  `UT_OPEN_DEFAULT`

    Can’t open installed, default, unit database

  `UT_PARSE`

    Error parsing unit database

Previous: Status, Up: Errors [Contents][Index]

### 11.2 Error-Messages

Function: `int` ut_handle_error_message `(const char* fmt, ...)`

  Handles the error-message corresponding to the format-string fmt and any subsequent arguments referenced by it. The interpretation of the formatting-string is identical to that of the UNIX function `printf()`. On success, this function returns the number of bytes in the error-message; otherwise, this function returns `-1`.

  Use the function `ut_set_error_message_handler()` to change how error-messages are handled.

Function: `ut_error_message_handler` ut_set_error_message_handler `(ut_error_message_handler handler)`

  Sets the function that handles error-messages and returns the previous error-message handler. The initial error-message handler is `ut_write_to_stderr()`.

Function: `int` ut_write_to_stderr `(const char* fmt, va_list args)`

  Writes the variadic error-message corresponding to formatting-string fmt and arguments args to the standard-error stream and appends a newline. The interpretation of the formatting-string is identical to that of the UNIX function `printf()`. On success, this function returns the number of bytes in the error-message; otherwise, this function returns `-1`.

Function: `int` ut_ignore `(const char* fmt, va_list args)`

  Does nothing. In particular, it ignores the variadic error-message corresponding to formatting-string fmt and arguments args. Pass this function to `ut_set_error_message_handler()` when you don’t want the unit module to print any error-messages.

Data type: ut_error_message_handler

  This is the type of an error-message handler. It’s definition is

      typedef int (*ut_error_message_handler)(const char* fmt, va_list args);

Next: Types, Previous: Errors, Up: Top [Contents][Index]

## 12 The Units Database

The database of units that comes with the UDUNITS-2 package is an XML-formatted file that is based on the SI system of units. It contains the names and symbols of most of the units that you will ever encounter. The pathname of the installed file is `datadir/udunits2.xml`, where datadir is the installation-directory for read-only, architecture-independent data (e.g., `/usr/local/share`). This pathname is the default that `ut_read_xml()` uses.

Naturally, because the database is a regular file, it can be edited to add new units or remove existing ones. Be very careful about doing this, however, because you might lose the benefit of exchanging unit-based information with others who haven’t modified their database.

Next: Complete Index, Previous: Database, Up: Top [Contents][Index]

## 13 Data Types

The data types `ut_visitor`, `ut_status`, and `ut_error_message_handler` are documented elsewhere.

Data type: ut_encoding

  This enumeration has the following values:

  `UT_ASCII`

    US ASCII character-set.

  `UT_ISO_8859_1`

    The ISO-8859-1 character-set.

  `UT_LATIN1`

    Synonym for `UT_ISO_8859_1`.

  `UT_UTF8`

    The UTF-8 encoding of the Unicode character-set.

Previous: Types, Up: Top [Contents][Index]

## Index

Jump to: | A B C D E F G M N O P S T U

| Index Entry |  | Section
A |  |
| adding prefixes to a unit-system: |  | Prefixes
| adding units to a unit-system: |  | Adding
B |  |
| base unit: |  | Unit-Systems
| binary unit operations: |  | Binary
C |  |
| converting values between units: |  | Value Conversion
| `cv_convert_double`: |  | Value Conversion
| `cv_convert_doubles`: |  | Value Conversion
| `cv_convert_float`: |  | Value Conversion
| `cv_convert_floats`: |  | Value Conversion
| `cv_free`: |  | Value Conversion
D |  |
| data types: |  | Types
| database, unit, obtaining predefined: |  | Obtaining
| database, units: |  | Database
E |  |
| error handling: |  | Errors
| error-messages: |  | Messages
| examples, unit specification: |  | Examples
F |  |
| formatting a unit into a string: |  | Formatting
G |  |
| getting a unit by its name: |  | Extracting
| getting a unit by its symbol: |  | Extracting
| grammar, unit: |  | Grammar
M |  |
| mapping identifiers: |  | Mapping
| mapping units: |  | Mapping
| messages, error: |  | Messages
| module status: |  | Status
N |  |
| names: |  | Names
O |  |
| operations, unit: |  | Operations
P |  |
| parsing a string into a unit: |  | Parsing
| prefixes, adding to a unit-system: |  | Prefixes
S |  |
| status of last operation: |  | Status
| string, formatting a unit into a: |  | Formatting
| string, parsing into a unit: |  | Parsing
| symbols: |  | Symbols
| synopsis: |  | Synopsis
| syntax, unit: |  | Syntax
| system of units: |  | Unit-Systems
T |  |
| time, handling of: |  | Time
| types, data: |  | Types
U |  |
| unary unit operations: |  | Unary
| unit conversion: |  | Value Conversion
| unit database, obtaining predefined: |  | Obtaining
| unit grammar: |  | Grammar
| unit operations: |  | Operations
| unit specification examples: |  | Examples
| unit syntax: |  | Syntax
| unit, adding to a unit-system: |  | Adding
| unit, base: |  | Unit-Systems
| unit, formatting into a string: |  | Formatting
| unit, getting by name: |  | Extracting
| unit, getting by symbol: |  | Extracting
| unit-system: |  | Unit-Systems
| unit-system, adding a unit to: |  | Adding
| unit-system, adding prefixes to a: |  | Prefixes
| unit-system, obtaining predefined: |  | Obtaining
| units database: |  | Database
| units, mapping to identifiers: |  | Mapping
| units, obtaining predefined: |  | Obtaining
| `ut_accept_visitor`: |  | Unary
| `ut_add_name_prefix`: |  | Prefixes
| `ut_add_symbol_prefix`: |  | Prefixes
| `ut_are_convertible`: |  | Value Conversion
| `ut_clone`: |  | Unary
| `ut_compare`: |  | Binary
| `ut_decode_time`: |  | Time
| `ut_divide`: |  | Binary
| `ut_encode_clock`: |  | Time
| `ut_encode_date`: |  | Time
| `ut_encode_time`: |  | Time
| `ut_encoding`: |  | Types
| `ut_error_message_handler`: |  | Messages
| `ut_format`: |  | Formatting
| `ut_free`: |  | Unary
| `ut_free_system`: |  | Misc
| `ut_get_converter`: |  | Value Conversion
| `ut_get_dimensionless_unit_one`: |  | Extracting
| `ut_get_name`: |  | Unary
| `ut_get_path_xml`: |  | Obtaining
| `ut_get_status`: |  | Status
| `ut_get_symbol`: |  | Unary
| `ut_get_system`: |  | Unary
| `ut_get_unit_by_name`: |  | Extracting
| `ut_get_unit_by_symbol`: |  | Extracting
| `ut_handle_error_message`: |  | Messages
| `ut_ignore`: |  | Messages
| `ut_invert`: |  | Unary
| `ut_is_dimensionless`: |  | Unary
| `ut_log`: |  | Unary
| `ut_map_name_to_unit`: |  | Names
| `ut_map_symbol_to_unit`: |  | Symbols
| `ut_map_unit_to_name`: |  | Names
| `ut_map_unit_to_symbol`: |  | Symbols
| `ut_multiply`: |  | Binary
| `ut_new_base_unit`: |  | Adding
| `ut_new_dimensionless_unit`: |  | Adding
| `ut_new_system`: |  | Obtaining
| `ut_offset`: |  | Unary
| `ut_offset_by_time`: |  | Unary
| `ut_parse`: |  | Parsing
| `ut_raise`: |  | Unary
| `ut_read_xml`: |  | Obtaining
| `ut_read_xml()`, discussion of: |  | Obtaining
| `ut_root`: |  | Unary
| `ut_same_system`: |  | Binary
| `ut_scale`: |  | Unary
| `ut_set_error_message_handler`: |  | Messages
| `ut_set_second`: |  | Misc
| `ut_set_status`: |  | Status
| `ut_status`: |  | Status
| `ut_trim`: |  | Parsing
| `ut_unmap_name_to_unit`: |  | Names
| `ut_unmap_symbol_to_unit`: |  | Symbols
| `ut_unmap_unit_to_name`: |  | Names
| `ut_unmap_unit_to_symbol`: |  | Symbols
| `ut_visitor`: |  | Unary
| `ut_write_to_stderr`: |  | Messages

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