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If you create an application that includes a main() function,
the linker automatically includes the initialization code necessary
for the execution environment. See Building an application with the C
library for instructions. There are situations though
where this is not desirable or possible.
You can create an application that consists of customized startup code and still use many of the library functions. You must either:
avoid functions that require initialization
provide the initialization and low-level support functions.
The functions you must reimplement depend on how much of the library functionality you require:
If
you want only the compiler support functions for division, structure
copy, and FP arithmetic, you must provide __rt_raise().
This also enables very simple library functions such as those in errno.h, setjmp.h,
and most of string.h to work.
If you call setlocale() explicitly,
locale-dependent functions are activated. This enables you to use
the atoi family, sprintf(), sscanf(),
and the functions in ctype.h.
Programs that use floating-point must call _fp_init().
If you select software floating-point, the program must also provide __rt_fp_status_addr().
If this function is not reimplemented, the default action is to
create a __user_libspace area. See The __user_libspace static data area for a description
of the __user_libspace area.
Implementing high-level input/output support is
necessary for functions that use fprintf() or fputs().
The high-level output functions depend on fputc() and ferror().
The high-level input functions depend on fgetc() and __backspace().
Implementing these functions and the heap enables you to use almost the entire library.
If you are using the libraries in an application that does
not have a main() function, you must reimplement
some functions in the library. See The standalone C library functions for more information.
__rt_raise() is essential. It is required
by all FP functions, by integer division (so that divide-by-zero
can be reported), and by some other library routines. You probably cannot
write a non trivial program without doing something that requires __rt_raise().
If rand() is called, srand() must be
called first. This is done automatically during library initialization
but not when you avoid the library initialization.
High-level I/O functions, fprintf() for
example, can be used if the low-level functions, fputc() for
example, are reimplemented. Most of the formatted output functions
also require a call to setlocale(). See Tailoring the input/output functions for instructions.
Anything that uses locale must not be
called before first calling setlocale() to
initialize it, for example, call setlocale(LC_ALL, "C").
Locale-using functions are described in The standalone C library functions. These include the functions in ctype.h and locale.h,
the printf() family, the scanf() family, ato*, strto*, strcoll/strxfrm,
and much of time.h.
| Copyright © 2007-2010 ARM. All rights reserved. | ARM DUI 0349C |
| Non-Confidential | ID101213 |
| PDF version | |
| Home > The C and C++ Libraries > Building an application without the C library > Exploiting the C library |