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@node Reentrancy
@chapter Reentrancy
@cindex 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. Cygnus's 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.
@findex _reent
@findex reent.h
@cindex reentrancy structure
These hooks use the structure @code{_reent} defined in @file{reent.h}.
A variable defined as @samp{struct _reent} is called a @dfn{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 @samp{_} and
appending @samp{_r}, and takes a pointer to the particular reentrancy
structure to use.
For example, the function @code{fopen} takes two arguments, @var{file}
and @var{mode}, and uses the global reentrancy structure. The function
@code{_fopen_r} takes the arguments, @var{struct_reent}, which is a
pointer to an instance of the reentrancy structure, @var{file}
and @var{mode}.
There are two versions of @samp{struct _reent}, a normal one and one
for small memory systems, controlled by the @code{_REENT_SMALL}
definition from the (automatically included) @file{<sys/config.h>}.
@cindex global reentrancy structure
@findex _impure_ptr
Each function which uses the global reentrancy structure uses the global
variable @code{_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 @samp{struct _reent}:
@enumerate
@item
@cindex extra argument, reentrant fns
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.
@item
Ensure that each thread of execution control has a pointer to its own
unique reentrancy structure in the global variable @code{_impure_ptr},
and call the standard library subroutines.
@end enumerate
@cindex list of reentrant functions
@cindex reentrant function list
The following functions are provided in both reentrant
and non-reentrant versions.
@example
@exdent @emph{Equivalent for errno variable:}
_errno_r
@exdent @emph{Locale functions:}
_localeconv_r _setlocale_r
@exdent @emph{Equivalents for stdio variables:}
_stdin_r _stdout_r _stderr_r
@page
@exdent @emph{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
@exdent @emph{Signal functions:}
_init_signal_r _signal_r
_kill_r __sigtramp_r
_raise_r
@exdent @emph{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
@exdent @emph{String functions:}
_strdup_r _strtok_r
@exdent @emph{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
@ifset STDIO64
@exdent @emph{Additional 64-bit I/O System functions:}
_fstat64_r _lseek64_r _open64_r
@end ifset
@exdent @emph{Time function:}
_asctime_r
@end example
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