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.
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• Synopsis: | Terse usage display | |
• Why: | What’s a unit package good for? | |
• Unit-Systems: | Explanation of unit-systems and how to get one | |
• Parsing: | Converting strings into units | |
• Syntax: | Syntax for string representation of units | |
• Formatting: | Converting units into strings | |
• Value Conversion: | Converting values between units | |
• Prefixes: | Defining unit prefixes | |
• Mapping: | Mapping between units and identifiers | |
• Operations: | Operations on units | |
• Time: | Handling time | |
• Errors: | Error-handling | |
• Database: | The units database | |
• Types: | Data types | |
• Complete Index: | Complete index |
Coding:
#include <udunits2.h>
const char* ut_get_path_xml(const char* path); 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]
The existance 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
ut_parse()
.
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]
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:
• Obtaining: | How to obtain a unit-system. | |
• Extracting: | Getting a unit from a unit-system. | |
• Adding: | Adding new units to a unit-system. | |
• Prefixes: | Add new unit-prefixes to a unit-system. | |
• Misc: | Miscelaneous unit-system operations. |
Next: Extracting, Up: Unit-Systems [Contents][Index]
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):
ut_read_xml(NULL)
.
ut_read_xml()
with the pathname of the customized
database to obtain a customized unit-system.
ut_new_base_unit()
and
ut_new_dimensionless_unit()
.
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.
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.
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.
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]
NOTE\: This section covers low-level access to the indidual units of a unit-system. General parsing of arbitrary unit specifications is coverted 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()
.
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.
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.
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]
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.
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.
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]
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.
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
.
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]
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.
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]
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.
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.
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
.
float
cv_convert_float (const cv_converter* converter, const float value)
Converts the single floating-point value value and returns the new value.
double
cv_convert_double (const cv_converter* converter, const double value)
Converts the single double-precision value value and returns the new value.
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.
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.
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]
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. */ }
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.
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]
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”).
• Examples: | Examples of unit specifications | |
• Grammar: | Formal unit grammar |
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.
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>)? TIMSTAMP: <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]
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 */ }
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]
You can use unit operations to construct new units, get information about units, or compare units.
• Unary: | Operations on a single unit | |
• Binary: | Operations on pairs of units |
Next: Binary, Up: Operations [Contents][Index]
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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
.
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 dimensionfull;
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.
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.
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.
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]
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.
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.
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.
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
.
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]
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.
• Names: | Mapping between units and names. | |
• Symbols: | Mapping between units and symbols. |
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.
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.
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
.
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.
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
.
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.
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.
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
.
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.
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
.
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.
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)
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.
double
ut_encode_clock (int hour, int minute, double second)
Encodes a clock-time as a double-precision value. You probably won’t use this 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.
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:
• Status: | The status of the last operation. | |
• Messages: | The handling of error-messages. |
UDUNITS-2 functions set their status by calling ut_set_status()
.
You can use the function ut_get_status()
to retrieve that
status.
ut_status
ut_get_status (void)
Returns the value specified in the last call to
ut_set_status()
void
ut_set_status (ut_status status)
Set the status of the units module to 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
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.
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()
.
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
.
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.
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);
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]
The data types ut_visitor
, ut_status
, and
ut_error_message_handler
are documented elsewhere.
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.
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