c - Program stuck on Pipe (exec ls grep sort) -


i'm trying make program executes following commands connecting output of 1 input of next using pipes , taking 2 arguments dir (directory) , arg (filetype, example: jpg).

ls dir -lar | grep arg | sort

here's code:

int main(int argc, char *argv[]) {      if (argc != 3) {         printf("invalid arguments. <dir> <arg>\n");         exit(1);     }      int pipe_fd1[2];     int pipe_fd2[2];     pid_t ls_pid, grep_pid;     int status;      pipe(pipe_fd1);     pipe(pipe_fd2);      ls_pid = fork();     if (ls_pid == 0) { //first child ls dir -lar         dup2(pipe_fd1[1], stdout_fileno);         close(pipe_fd1[0]);          execlp("ls", "ls", argv[1], "-lar", null);      } else if (ls_pid > 0) {         grep_pid = fork();         if (grep_pid == 0) { //second child grep arg             dup2(pipe_fd1[0], stdin_fileno);             dup2(pipe_fd2[1], stdout_fileno);                    close(pipe_fd1[1]);             close(pipe_fd2[0]);              waitpid(ls_pid, &status, 0);                 execlp("grep", "grep", argv[2], null);          } else if (grep_pid > 0) { //parent sort             dup2(pipe_fd2[0], stdin_fileno);             close(pipe_fd2[1]);              waitpid(grep_pid, &status, 0);             execlp("sort", "sort", null);         }      }      return 0; }

it seems stuck? not sure why?

you never close pipe_fd1 on parent, grep , sort doen't know when stop reading input: because pipe read , write ends never closed on parent, reader blocks waiting more input never arrive. need close it.

also, don't need waitpid(): way pipes work ensures input flows linearly , in order throughout pipe.

here's working version these issues addressed:

#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <sys/wait.h>  int main(int argc, char *argv[]) {      if (argc != 3) {         printf("invalid arguments. <dir> <arg>\n");         exit(1);     }      int pipe_fd1[2];     int pipe_fd2[2];     pid_t ls_pid, grep_pid;      pipe(pipe_fd1);      ls_pid = fork();     if (ls_pid == 0) { //first child ls dir -lar         dup2(pipe_fd1[1], stdout_fileno);         close(pipe_fd1[0]);         execlp("ls", "ls", argv[1], "-lar", null);      } else if (ls_pid > 0) {         dup2(pipe_fd1[0], stdin_fileno);         close(pipe_fd1[1]);          pipe(pipe_fd2);         grep_pid = fork();          if (grep_pid == 0) { //second child grep arg             dup2(pipe_fd2[1], stdout_fileno);             close(pipe_fd2[0]);               execlp("grep", "grep", argv[2], null);          } else if (grep_pid > 0) { //parent sort             dup2(pipe_fd2[0], stdin_fileno);             close(pipe_fd2[1]);             execlp("sort", "sort", null);         }      }      return 0; }

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