返回值：
1、如果 system()在調用/bin/sh 時失敗則返回127, 其他失敗原因返回-1.。
2、若參數string 爲空指針(NULL), 則返回非零值.
3、如果system()調用成功則最後會返回執行shell 命令後的返回值, 但是此返回值也有可能爲system()調用/bin/sh 失敗所返回的127, 因此最好能再檢查errno 來確認執行成功.
附加說明：在編寫具有 SUID/SGID 權限的程序時請勿使用system(), system()會繼承環境變量, 通過環境變量可能會造成系統安全的問題.
linux版system函數的源碼：
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>
#include <unistd.h>
int system(const char * cmdstring)
{
pid_t pid;
int status;
if(cmdstring == NULL){
return (1);
}
if((pid = fork())<0){
status = -1;
}
else if(pid = 0){
execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
-exit(127); //子進程正常執行則不會執行此語句
}
else{
while(waitpid(pid, &status, 0) < 0){
if(errno != EINTER){
status = -1;
break;
}
}
}
return status;
}
man system
SYSTEM(3) Linux Programmer’s Manual SYSTEM(3)
NAME
system - execute a shell command
SYNOPSIS
#include <stdlib.h>
int system(const char *command);
DESCRIPTION
system() executes a command specified in command by calling /bin/sh -c command, and returns after the command
has been completed. During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be
ignored.
RETURN VALUE
The value returned is -1 on error (e.g. fork() failed), and the return status of the command otherwise. This
latter return status is in the format specified in wait(2). Thus, the exit code of the command will be WEXIT-
STATUS(status). In case /bin/sh could not be executed, the exit status will be that of a command that does
exit(127).
If the value of command is NULL, system() returns non-zero if the shell is available, and zero if not.
system() does not affect the wait status of any other children.
CONFORMING TO
C89, C99, POSIX..1-2001.
NOTES
If the _XOPEN_SOURCE feature test macro is defined, then the macros described in wait(2) (WEXITSTATUS(), etc.)
are made available when including <stdlib.h>.
As mentioned, system() ignores SIGINT and SIGQUIT. This may make programs that call it from a loop uninter-
ruptible, unless they take care themselves to check the exit status of the child. E.g.
while(something) {
int ret = system("foo");
if (WIFSIGNALED(ret) &&
(WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
break;
}
Do not use system() from a program with set-user-ID or set-group-ID privileges, because strange values for some
environment variables might be used to subvert system integrity. Use the exec(3) family of functions instead,
but not execlp(3) or execvp(3). system() will not, in fact, work properly from programs with set-user-ID or
set-group-ID privileges on systems on which /bin/sh is bash version 2, since bash 2 drops privileges on
startup. (Debian uses a modified bash which does not do this when invoked as sh.)
In versions of glibc before 2.1.3, the check for the availability of /bin/sh was not actually performed if com-
mand was NULL; instead it was always assumed to be available, and system() always returned 1 in this case.
Since glibc 2.1.3, this check is performed because, even though POSIX.1-2001 requires a conforming implementa-
tion to provide a shell, that shell may not be available or executable if the calling program has previously
called chroot(2) (which is not specified by POSIX.1-2001).
It is possible for the shell command to return 127, so that code is not a sure indication that the execve()
call failed.
SEE ALSO
sh(1), signal(2), wait(2), exec(3)
2004-12-20 SYSTEM(3)
