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Building an application without the C library

Creating an application that has a main() function causes the C library initialization functions to be included as part of __rt_lib_init.

If your application does not have a main() function, the C library is not initialized and the following functions are not available in your application:

  • Low-level stdio functions that have the prefix _sys_.

  • Signal-handling functions, signal() and raise() in signal.h .

  • Other functions, such as atexit().

Table 5 shows header files, and the functions they contain, that are available with an uninitialized library. Some otherwise unavailable functions can be used if the library functions they depend on are re-implemented.

Table 5. Standalone C library functions
FunctionDescription
alloca.h

Functions in this file work without any library initialization or function re-implementation. You must know how to build an application with the C library to use this header file.

assert.h

Functions listed in this file require high-level stdio, __rt_raise(), and _sys_exit(). You must be familiar with tailoring error signaling, error handling, and program exit to use this header file.

ctype.h

Functions listed in this file require the locale functions.

errno.h

Functions in this file work without the requirement for any library initialization or function re-implementation.

fenv.h

Functions in this file work without the requirement for any library initialization and only require the re-implementation of __rt_raise().

float.h

This file does not contain any code. The definitions in the file do not require library initialization or function re-implementation.

inttypes.h

Functions listed in this file require the locale functions.

limits.h

Functions in this file work without the requirement for any library initialization or function re-implementation.

locale.h

Call setlocale() before calling any function that uses locale functions. For example:

setlocale(LC_ALL, "C")

See the contents of locale.h for more information on the following functions and data structures:

  • setlocale() selects the appropriate locale as specified by the category and locale arguments.

  • lconv is the structure used by locale functions for formatting numeric quantities according to the rules of the current locale.

  • localeconv() creates an lconv structure and returns a pointer to it.

  • _get_lconv() fills the lconv structure pointed to by the parameter. This ISO extension removes the requirement for static data within the library.

locale.h also contains constant declarations used with locale functions.

math.h

For functions in this file to work, you must first call _fp_init() and re-implement __rt_raise().

setjmp.h

Functions in this file work without any library initialization or function re-implementation.

signal.h

Functions listed in this file are not available without library initialization. You must know how to build an application with the C library to use this header file.

__rt_raise() can be re-implemented for error and exit handling. You must be familiar with tailoring error signaling, error handling, and program exit.

stdarg.h

Functions listed in this file work without any library initialization or function re-implementation.

stddef.h

This file does not contain any code. The definitions in the file do not require library initialization or function re-implementation.

stdint.h

This file does not contain any code. The definitions in the file do not require library initialization or function re-implementation.

stdio.h

The following dependencies or limitations apply to these functions:

  • The high-level functions such as printf(), scanf(), puts(), fgets(), fread(), fwrite(), and perror() depend on lower-level stdio functions fgetc(), fputc(), and __backspace(). You must re-implement these lower-level functions when using the standalone C library.

    However, you cannot re-implement the _sys_ prefixed functions (for example, _sys_read()) when using the standalone C library because they require library initialization.

    You must be familiar with tailoring the input/output functions in the C and C++ libraries.

  • The printf() and scanf() family of functions require locale.

  • The remove() and rename() functions are system-specific and probably not usable in your application.

stdlib.h

Most functions in this file work without any library initialization or function re-implementation. The following functions depend on other functions being instantiated correctly:

  • ato*() requires locale.

  • strto*() requires locale.

  • malloc(), calloc(), realloc(), and free() require heap functions.

  • atexit() is not available when building an application without the C library.

string.h

Functions in this file work without any library initialization, with the exception of strcoll() and strxfrm(), that require locale.

time.h

mktime() and localtime() can be used immediately

time() and clock() are system-specific and are probably not usable unless re-implemented

asctime(), ctime(), and strftime() require locale.

wchar.h

Wide character library functions added to ISO C by Normative Addendum 1 in 1994.

  • Support for wide-character output and format strings, swprintf(), vswprintf(), swscanf(), and vswscanf().

  • All the conversion functions (for example, btowc, wctob, mbrtowc, and wcrtomb) require locale.

  • wcscoll() and wcsxfrm() require locale.

wctype.h

Wide character library functions added to ISO C by Normative Addendum 1 in 1994. This requires locale.


See also

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