C system()的返回值

#include <stdlib.h>

int system(const char *command);

* If command is NULL, then a nonzero value if a shell is available, or 0 if no shell is available.

* If a child process could not be created, or its status could not be retrieved, the return value is -1.

* If a shell could not be executed in the child process, then the return value is as though the child shell terminated by calling _exit(2) with the status 127.

* If all system calls succeed, then the return value is the termination status of the child shell used to execute command. (The termination status of a shell is the

termination status of the last command it executes.)

In the last two cases, the return value is a "wait status" that can be examined using the macros described in waitpid(2). (i.e., WIFEXITED(), WEXITSTATUS(), and so on).

system() does not affect the wait status of any other children.

https://stackoverflow.com/questions/808541/any-benefit-in-using-wexitstatus-macro-in-c-over-division-by-256-on-exit-statu

tianyxu@TIANYXU-M-V83Z glibc % git grep __WEXITSTATUS
bits/waitstatus.h:#define       __WEXITSTATUS(status)   (((status) & 0xff00) >> 8)
bits/waitstatus.h:#define       __WSTOPSIG(status)      __WEXITSTATUS(status)
posix/sys/wait.h:# define WEXITSTATUS(status)   __WEXITSTATUS (status)
stdlib/stdlib.h:# define WEXITSTATUS(status)    __WEXITSTATUS (status)

The status stored by waitpid() encodes both the reason that the child process was terminated and the exit code. The reason is stored in the least-significant byte (obtained by status & 0xff), and the exit code is stored in the next byte (masked by status & 0xff00 and extracted by WEXITSTATUS()). When the process terminates normally, the reason is 0 and so WEXITSTATUS is just equivalent to shifting by 8 (or dividing by 256). However, if the process is killed by a signal (such as SIGSEGV), there is no exit code, and you have to use WTERMSIG to extract the signal number from the reason byte.

比如如果system执行grep但是没抓住字符，它的返回值就是256，WEXITSTATUS(rc)就是1。