/*FUNCTION<<system>>---execute command string INDEX	systemINDEX	_system_r SYNOPSIS	#include <stdlib.h>	int system(char *<[s]>); 	int _system_r(void *<[reent]>, char *<[s]>); DESCRIPTION Use <<system>> to pass a command string <<*<[s]>>> to <</bin/sh>> onyour system, and wait for it to finish executing. Use ``<<system(NULL)>>'' to test whether your system has <</bin/sh>>available. The alternate function <<_system_r>> is a reentrant version.  Theextra argument <[reent]> is a pointer to a reentrancy structure. RETURNS<<system(NULL)>> returns a non-zero value if <</bin/sh>> is available, and<<0>> if it is not. With a command argument, the result of <<system>> is the exit statusreturned by <</bin/sh>>. PORTABILITYANSI C requires <<system>>, but leaves the nature and effects of acommand processor undefined.  ANSI C does, however, specify that<<system(NULL)>> return zero or nonzero to report on the existence ofa command processor. POSIX.2 requires <<system>>, and requires that it invoke a <<sh>>.Where <<sh>> is found is left unspecified. Supporting OS subroutines required: <<_exit>>, <<_execve>>, <<_fork_r>>,<<_wait_r>>.*/ #include <_ansi.h>#include <errno.h>#include <stddef.h>#include <stdlib.h>#include <unistd.h>#include <_syslist.h>#include <reent.h> int _system(const char *s); #if defined (unix) || defined (__CYGWIN__)static int do_system (struct _reent *ptr, const char *s);#endif int_system_r (struct _reent *ptr,     const char *s){#if defined(HAVE_SYSTEM)  return _system (s);  ptr = ptr;#elif defined(NO_EXEC)  if (s == NULL)    return 0;  errno = ENOSYS;  return -1;#else   /* ??? How to handle (s == NULL) here is not exactly clear.     If _fork_r fails, that's not really a justification for returning 0.     For now we always return 0 and leave it to each target to explicitly     handle otherwise (this can always be relaxed in the future).  */ #if defined (unix) || defined (__CYGWIN__)  if (s == NULL)    return 1;  return do_system (ptr, s);#else  if (s == NULL)    return 0;  errno = ENOSYS;  return -1;#endif #endif} #ifndef _REENT_ONLY intsystem (const char *s){  return _system_r (_REENT, s);} #endif#if defined (unix) && !defined (__CYGWIN__) && !defined(__rtems__)extern char **environ; /* Only deal with a pointer to environ, to work around subtle bugs with shared   libraries and/or small data systems where the user declares his own   'environ'.  */static char ***p_environ = &environ; static intdo_system (struct _reent *ptr,     const char *s){  char *argv[4];  int pid, status;   argv[0] = "sh";  argv[1] = "-c";  argv[2] = (char *) s;  argv[3] = NULL;   if ((pid = _fork_r (ptr)) == 0)    {      _execve ("/bin/sh", argv, *p_environ);      exit (100);    }  else if (pid == -1)    return -1;  else    {      int rc = _wait_r (ptr, &status);      if (rc == -1)	return -1;      status = (status >> 8) & 0xff;      return status;    }}#endif #if defined (__CYGWIN__)static intdo_system (struct _reent *ptr,     const char *s){  char *argv[4];  int pid, status;   argv[0] = "sh";  argv[1] = "-c";  argv[2] = (char *) s;  argv[3] = NULL;   if ((pid = vfork ()) == 0)    {      /* ??? It's not clear what's the right path to take (pun intended :-).	 There won't be an "sh" in any fixed location so we need each user	 to be able to say where to find "sh".  That suggests using an	 environment variable, but after a few more such situations we may	 have too many of them.  */      char *sh = getenv ("SH_PATH");      if (sh == NULL)	sh = "/bin/sh";      _execve (sh, argv, environ);      exit (100);    }  else if (pid == -1)    return -1;  else    {      extern int _wait (int *);      int rc = _wait (&status);      if (rc == -1)	return -1;      status = (status >> 8) & 0xff;      return status;    }}#endif