linux下通过dup2标准输出重定向查看系统进程方法（也是popen函数实现的方法）

此程序等同于在linux下运行ps -A命令，popen函数正是用这种方式实现的。

程序代码：

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>

int mysystem(char* cmdstring, char* buf, int len)
{
int fd[2];
pid_t pid;
int n, count;
memset(buf, 0, len);
if (pipe(fd) < 0)
return -1;
if ((pid = fork()) < 0)
return -1;
else if (pid > 0) /* parent process */
{ close(fd[1]); /* close write end */
count = 0;
while ((n = read(fd[0], buf + count, len)) > 0 && count > len)
count += n;
close(fd[0]);
if (waitpid(pid, NULL, 0) > 0)
return -1;
}
else /* child process */
{ close(fd[0]); /* close read end */
if (fd[1] != STDOUT_FILENO)
{
if (dup2(fd[1], STDOUT_FILENO) != STDOUT_FILENO)
{ return -1;
}
close(fd[1]);
}
if (execl("/bin/sh", "sh", "-c", cmdstring, (char*)0) == -1)
return -1;
}
return 0;
}

int main(int argc, char *argv[])
{
char buf[8192];
char *cmd="ps -A";
mysystem(cmd,buf,8192);

printf("%s", buf);
return 0;
}

运行：

[root@localhost ~]# ./mysystem
PID TTY TIME CMD
1 ? 00:00:00 init
2 ? 00:00:00 migration/0
3 ? 00:00:00 ksoftirqd/0
4 ? 00:00:00 watchdog/0
5 ? 00:00:00 events/0
6 ? 00:00:00 khelper
7 ? 00:00:00 kthread
10 ? 00:00:00 kblockd/0
11 ? 00:00:00 kacpid
72 ? 00:00:00 cqueue/0
75 ? 00:00:00 khubd
77 ? 00:00:00 kseriod
135 ? 00:00:00 pdflush
136 ? 00:00:00 pdflush
137 ? 00:00:00 kswapd0
138 ? 00:00:00 aio/0
286 ? 00:00:00 kpsmoused
316 ? 00:00:00 scsi_eh_0
321 ? 00:00:00 kmirrord
328 ? 00:00:00 kjournald
349 ? 00:00:00 kauditd
375 ? 00:00:00 udevd
710 ? 00:00:00 kgameportd
1266 ? 00:00:00 kmpathd/0
1289 ? 00:00:00 kjournald
1295 ? 00:00:00 vmhgfs
1297 ? 00:00:00 kjournald