c - execve grep process never exit -

the following code simulate pipe , grep operation forking process , using execve system call. output seems fine, however, grep process seems never exit (still running in back) until whole process ends. what's problem? abnormal since using grep in shell exit.

#include <stdio.h> #include <string.h> #include <unistd.h> #include <stdlib.h> #include <sys/wait.h> #include <fcntl.h> int main(int argc, char *argv[], char *env[]) { char ch[32]; while (1) {     scanf("%s", ch);     int pd[2];     if (pipe(pd) == -1) {         perror("pipe failed");         exit(1);     }      int childpid, pid;     if ((childpid = fork()) < 0) {         perror("fork failed\n");         exit(1);     } else {         // parent process         if (childpid) {             int status;             wait(&status);             // print exit code of child process             printf("exit code %d\n", status);         }         // child process, execute command         else {             // fork child             if ((pid = fork()) < 0) {                 perror("fork failed\n");                 exit(1);             }             if (pid) {   // parent pipe writer                 close(pd[0]);                 close(1);                 // replace input pipe                 dup(pd[1]);                 char* cmds[] = { "/bin/cat", "aa", 0 };                 execve(cmds[0], cmds, env);                  exit(0);             } else {   // child pipe reader                 close(pd[1]);                 close(0); // close read end                 dup(pd[0]);                  char* cmds[] = { "/bin/grep", "rw", 0 };                 execve(cmds[0], cmds, env);                 exit(0); // never been here             }         }     } } return 0; } 

here output monitor process before , after running program once.

hengstar@ubuntu:~$ ps -ef | grep "grep" hengstar 58073 58038  0 01:43 pts/26   00:00:00 grep --color=auto grep hengstar@ubuntu:~$ ps -ef | grep "grep" hengstar 58075  1886  0 01:43 pts/11   00:00:00 /bin/grep drw hengstar 58077 58038  0 01:43 pts/26   00:00:00 grep --color=auto grep 

the exec family of functions never returns. purpose load new program replace current program running in process.

if exec function returns, means there error.