Reentrancy - Untitled
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## 10 Reentrancy

Reentrancy is a characteristic of library functions which allows multiple processes to use the same address space with assurance that the values stored in those spaces will remain constant between calls. The Red Hat newlib implementation of the library functions ensures that whenever possible, these library functions are reentrant. However, there are some functions that can not be trivially made reentrant. Hooks have been provided to allow you to use these functions in a fully reentrant fashion.

These hooks use the structure `_reent` defined in `reent.h`. A variable defined as ‘`struct _reent`’ is called a reentrancy structure. All functions which must manipulate global information are available in two versions. The first version has the usual name, and uses a single global instance of the reentrancy structure. The second has a different name, normally formed by prepending ‘`_`’ and appending ‘`_r`’, and takes a pointer to the particular reentrancy structure to use.

For example, the function `fopen` takes two arguments, file and mode, and uses the global reentrancy structure. The function `_fopen_r` takes the arguments, struct_reent, which is a pointer to an instance of the reentrancy structure, file and mode.

There are two versions of ‘`struct _reent`’, a normal one and one for small memory systems, controlled by the `_REENT_SMALL` definition from the (automatically included) `<sys/config.h>`.

Each function which uses the global reentrancy structure uses the global variable `_impure_ptr`, which points to a reentrancy structure.

This means that you have two ways to achieve reentrancy. Both require that each thread of execution control initialize a unique global variable of type ‘`struct _reent`’:

1. Use the reentrant versions of the library functions, after initializing a global reentrancy structure for each process. Use the pointer to this structure as the extra argument for all library functions.
2. Ensure that each thread of execution control has a pointer to its own unique reentrancy structure in the global variable `_impure_ptr`, and call the standard library subroutines.

The following functions are provided in both reentrant and non-reentrant versions.

    Equivalent for errno variable:

         _errno_r

    Locale functions:

         _localeconv_r  _setlocale_r

    Equivalents for stdio variables:

         _stdin_r        _stdout_r       _stderr_r

    Stdio functions:

         _fdopen_r       _perror_r       _tempnam_r
         _fopen_r        _putchar_r      _tmpnam_r
         _getchar_r      _puts_r         _tmpfile_r
         _gets_r         _remove_r       _vfprintf_r
         _iprintf_r      _rename_r       _vsnprintf_r
         _mkstemp_r      _snprintf_r     _vsprintf_r
         _mktemp_t       _sprintf_r

    Signal functions:

         _init_signal_r  _signal_r
         _kill_r         __sigtramp_r
         _raise_r

    Stdlib functions:

         _calloc_r       _mblen_r        _setenv_r
         _dtoa_r         _mbstowcs_r     _srand_r
         _free_r         _mbtowc_r       _strtod_r
         _getenv_r       _memalign_r     _strtol_r
         _mallinfo_r     _mstats_r       _strtoul_r
         _malloc_r       _putenv_r       _system_r
         _malloc_r       _rand_r         _wcstombs_r
         _malloc_stats_r _realloc_r      _wctomb_r

    String functions:

         _strdup_r       _strtok_r

    System functions:

         _close_r        _link_r         _unlink_r
         _execve_r       _lseek_r        _wait_r
         _fcntl_r        _open_r         _write_r
         _fork_r         _read_r
         _fstat_r        _sbrk_r
         _gettimeofday_r _stat_r
         _getpid_r       _times_r

    Time function:

         _asctime_r