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’:
_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