I am using dup2(), pipe(), and fork() to process commands with input of another. The output from the ls is correctly passed into cat, and the terminal displays output, but it does not stop receiving input. In other words cat does not terminate, so I can continue typing.
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
int main() {
int pipefd[2], child_pid, grand_child;
pipe(pipefd);
child_pid = fork();
if (child_pid) {
waitpid(child_pid, NULL, 0);
/* Parent */
grand_child = fork();
if (!grand_child) {
dup2(pipefd[0], STDIN_FILENO);
close(pipefd[0]);
close(pipefd[1]);
execlp("cat", "cat", NULL);
} else {
waitpid(grand_child, NULL, 0);
}
} else {
/* Child */
dup2(pipefd[1], STDOUT_FILENO);
close(pipefd[1]);
close(pipefd[0]);
execlp("ls", "ls", NULL);
}
return 0;
}
The parent still has the write side of the pipe open. cat is waiting for the parent to close it, and the parent is waiting for cat to terminate. You should close both sides of the pipe in the parent before you wait for the grand child.
Related
I have taken a look at this and also this stack overflow links.
I am having trouble understanding the process for closing write ends of pipes. In the code below, I have 3 processes, one parent, a child of the parent, and a child of the child. I am trying to simulate a pipe for the command - cat xxx | grep 28 | sort. I have written some code for this, and it essentially creates the sorts, "grips"/filters and prints my input, but it hangs at the end. I have to ctrl + c to exit. My code is a little messy, but if you can help me spot the problem that would be nice.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
/**
* Executes the command "cat scores | grep Lakers". In this quick-and-dirty
* implementation the parent doesn't wait for the child to finish and
* so the command prompt may reappear before the child terminates.
*
*/
int main(int argc, char **argv)
{
int pipefd[2];
int pipefd2[2];
int pid;
char *cat_args[] = {"cat", "scores", NULL};
char *grep_args[] = {"grep", "28", NULL};
char *sort_args[] = {"sort", NULL};
// make a pipe (fds go in pipefd[0] and pipefd[1])
if (pipe(pipefd) != 0){
return 1;
}
if (pipe(pipefd2) != 0){
return 1;
}
pid = fork();
if (pid < 0)
{
fprintf(stderr, "fork Failed" );
exit(EXIT_FAILURE);
}
else if (pid == 0)
{
int pid2;
pid2 = fork();
if (pid2 < 0){
fprintf(stderr, "fork Failed" );
return 1;
}
else if (pid2 == 0){
// replace standard input with input part of pipe
// close(0);
// close(pipefd[1]);
// close(pipefd2[1]);
dup2(pipefd2[0], 0);
// close unused hald of pipe
close(pipefd2[0]);
close(pipefd[1]);
close(pipefd2[1]);
close(pipefd[0]);
// execute grep
execvp("sort", sort_args);
close(pipefd[1]);
close(pipefd2[1]);
exit(0);
}else{
// replace standard input with input part of pipe
// close(pipefd[1]);
// close(pipefd2[1]);
dup2(pipefd[0], 0);
dup2(pipefd2[1], 1);
// close unused hald of pipe
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd[1]);
close(pipefd2[0]);
// execute grep
execvp("grep", grep_args);
waitpid(pid2, NULL, 0);
close(pipefd[1]);
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd2[0]);
exit(0);
waitpid(pid2, NULL, 0);
}
}
else
{
// close(pipefd[1]);
// close(pipefd2[1]);
dup2(pipefd[1], 1);
// close unused unput half of pipe
close(pipefd[1]);
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd2[0]);
// execute cat
execvp("cat", cat_args);
exit(0);
waitpid(pid, NULL, 0);
}
close(pipefd[1]);
close(pipefd[0]);
close(pipefd2[1]);
close(pipefd2[0]);
}
here is the output I am getting. Not sure it is relevant but as you can see, the result is sorted by team name. It just doesn't terminate.
Houston 44 28 .611
Indiana 45 28 .616
Oklahoma City 44 28 .611
Utah 44 28 .611
^C
Calling execvp replaces the current process image with a new process image. If no error occured, your code will never reach any line after that, so your close() and waitpid() function calls are useless.
EDIT
Here's a fully functional code to your problem. The comments should be self explanatory. Notice that the command executing order is different and I'm waiting for processes to finish.
Reading from an empty pipe will block until there is some data to read or an error occured, so this is not the only solution.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
static void die (const char *msg) {
perror (msg);
exit (EXIT_FAILURE);
}
int main (int argc, char** argv) {
int pipefd[2];
int pid;
char *cat_args[] = {"cat", "scores", NULL};
char *grep_args[] = {"grep", "28", NULL};
char *sort_args[] = {"sort", NULL};
//make a pipe (file descriptor to read is pipefd[0], fd to write is pipefd[1])
if (pipe (pipefd) < 0)
die ("creating a pipe failed");
pid = fork();
if (pid < 0)
die ("fork");
else if (pid == 0) {
//child process
int pipefd2[2]; //only visible to the affected processes
if (pipe (pipefd2) < 0)
die ("pipe");
int pid2;
pid2 = fork();
if (pid2 < 0)
die ("fork");
else if (pid2 == 0) {
//child of child will execute cat command
close (pipefd2[0]); //we only need to write to the second pipe. close its reading end
//first pipe is for communication between parent and grandparent only
close (pipefd[0]);
close (pipefd[1]);
dup2 (pipefd2[1], STDOUT_FILENO); //write the output to the second pipe instead of the standard output
close (pipefd2[1]); //close writing end of second pipe
execvp("cat", cat_args); //execute cat command
die ("execvp should never return");
}
else {
//child process will execute the grep command
close (pipefd2[1]); // we only need to read from the second pipe. close its writing end
close(pipefd[0]); //we won't read from the first pipe
waitpid (pid2, NULL, 0); //wait for cat command to finish
dup2 (pipefd2[0], STDIN_FILENO); //read from the second pipe instead of the stdin
close (pipefd2[0]); //child finished. close reading end of second pipe
dup2 (pipefd[1], STDOUT_FILENO); //write the results of grep command to first pipe instead of standard output
close (pipefd[1]); //we dup2 the output, close the writing end of first pipe
execvp ("grep", grep_args);
die ("execvp should never return");
}
} else {
//parent process will execute the sort command
close (pipefd[1]); //we won't write to the first pipe
waitpid (pid, NULL, 0); //wait for child to write grep output to the first pipe
dup2 (pipefd[0], STDIN_FILENO); //read from the first pipe instead of stdin
close (pipefd[0]); //child finished. close reading end of first pipe
execvp ("sort", sort_args); //execute command
die ("execvp should never return");
}
//exit (EXIT_SUCCESS); we don't need this. the programm will never reach this line
}
I'm trying to execute grep -o colour colourfile.txt | wc -w > newfile.txt through a program in C, instead of using the command line.
This is what I have so far:
#include <stdlib.h>
#include <unistd.h>
int main (void) {
int fd[2];
pipe(fd);
if (fork()) {
// Child process
dup2(fd[0], 0); // wc reads from the pipe
close(fd[0]);
close(fd[1]);
execlp("wc", "wc", "-w", ">", "newfile.txt", NULL);
} else {
// Parent process
dup2(fd[1], 1); // grep writes to the pipe
close(fd[0]);
close(fd[1]);
execlp("grep", "grep", "-o", "colour", "colourfile.txt", NULL);
}
exit(EXIT_FAILURE);
}
if (fork()) { means parent process not child process, see http://man7.org/linux/man-pages/man2/fork.2.html
You should handle > like | use open()
The following code could work:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
int main (void) {
int pipefd[2];
pipe(pipefd);
if (fork()) {
// Child process
dup2(pipefd[0], 0); // wc reads from the pipe
close(pipefd[0]);
close(pipefd[1]);
int fd = open("newfile.txt", O_CREAT|O_TRUNC|O_WRONLY, S_IRUSR|S_IWUSR);
dup2(fd, 1);
close(fd);
execlp("wc", "wc", "-w", NULL);
} else {
// Parent process
dup2(pipefd[1], 1); // grep writes to the pipe
close(pipefd[0]);
close(pipefd[1]);
execlp("grep", "grep", "-o", "colour", "colourfile.txt", NULL);
}
exit(EXIT_FAILURE);
}
I'm trying to execute the Linux command "ls -l | tail -n 2" with a simple pipe in a c code.
I added your tips and now this works but the output isn't exactly as it should be. It prints the output in a single line instead of two and waits for a user input to close.
here is the new code:
#include "stdio.h"
#include "unistd.h"
#include "stdlib.h"
#include "sys/wait.h"
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
void main()
{
char line[100];
pid_t pid;
int fd[2];
int status;
char* ls_arguments[] = {"ls", "-l", NULL};
char* tail_arguments[] = {"tail", "-n", "2", NULL};
pipe(fd);
pid = fork();
if(pid == 0)//ls client
{
close(1);
dup(fd[1]);
close(fd[0]);
execvp("ls", ls_arguments);
}
pid = fork();
if(pid == 0)//tail client
{
close(0);
close(fd[1]);
dup(fd[0]);
execvp("tail", tail_arguments);
}
wait(pid, 0, WNOHANG);
close(fd[0]);
close(fd[1]);
}
this should run the "ls -l" command and output to the pipe and the next "tail" client would get it as input and run the "tail -n 2" command and print out the final output but the terminal prints nothing. Any help?
First of all, there is not such wait function, here is what the man says:
#include <sys/types.h>
#include <sys/wait.h>
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);
I think you meant to use waitpid.
Then, you child process doesn't finish because the pipe is still opened somewhere: in the parent. Indeed you should first close the descriptors and then wait for your childs process. I would write:
close(fd[0]);
close(fd[1]);
wait(NULL); // Wait for the first child to finish
wait(NULL); // Wait fot the second one
return 0;
}
Instead of:
wait(pid, 0, WNOHANG);
close(fd[0]);
close(fd[1]);
}
I'm trying to create two child processes and pipe them, but the second child is not sorting the output produced by the first child which does ls. What am I doing wrong?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
int pipefd[2];
pid_t ls_pid, wc_pid;
pipe(pipefd);
if ((ls_pid = fork()) == 0) {
dup2(pipefd[1],STDOUT_FILENO);
close(pipefd[0]);
execl("/bin/ls", "ls", 0);
perror("exec ls failed");
exit(EXIT_FAILURE);
}
if ((wc_pid = fork()) == 0) {
dup2(pipefd[0], STDIN_FILENO);
close(pipefd[1]);
execl("/usr/bin/sort", "sort", NULL);
perror("exec wc failed");
exit(EXIT_FAILURE);
}
return EXIT_SUCCESS;
}
The sort should work, but there are 2 caveats in your code, first, make sure close fd in all the processes that holds references to the fd, otherwise the fd won't close, and that's why the sort process hangs there when done, because it does not receive the EOF from stdin, and that's because the pipefd in the parent process were not closed. The other one is make sure wait the children to exit and check their exit status. Add the following to the send of main function:
close(pipefd[0]);
close(pipefd[1]);
int status;
int pid = waitpid(ls_pid, &status, 0);
pid = waitpid(wc_pid, &status, 0);
You have to pass NULL as the third parameter of the firtexecl just like you do in the second one. What happens is that execl executes correctly (that's why you dont get an error) but the ls command does not work as you give it an invalid command.
Btw, you should make error control on all OS requests, like in fork()
Combining all comments, and tested:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
int pipefd[2];
pid_t ls_pid, wc_pid;
int status;
pipe(pipefd);
if ((ls_pid = fork()) == 0) {
dup2(pipefd[1],STDOUT_FILENO);
close(pipefd[0]);
close(pipefd[1]);
execlp("ls", "ls", NULL);
perror("exec ls failed");
exit(EXIT_FAILURE);
}
if ((wc_pid = fork()) == 0) {
dup2(pipefd[0], STDIN_FILENO);
close(pipefd[0]);
close(pipefd[1]);
execlp("sort", "sort", NULL);
perror("exec sort failed");
exit(EXIT_FAILURE);
}
close (pipefd[0]);
close (pipefd[1]);
/* wait for two children to finish */
wait(&status);
wait(&status);
return EXIT_SUCCESS;
}
I'm creating various processes (3 to be precise) and making them do different things.
So far so good. I'm trying to wait in the parent until all children are completed. I've played around with many options (such as the one listed below) but either the parent waits but I have to press enter to return to the shell (meaning that some child completes after the parent?) or the parent never returns to the shell. Any ideas? pointers to where to look for more help? Thanks
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#define READ_END 0
#define WRITE_END 1
int main (int argc, char **argv)
{
pid_t pid;
int fd[2];
int fd2[2];
pipe(fd);
pipe(fd2);
for (int i=0; i<3; i++) {
pid=fork();
if (pid==0 && i==0) {
//never uses fd2, so close both descriptors
close(fd2[READ_END]);
close(fd2[WRITE_END]);
printf("i'm the child used for ls \n");
close(fd[READ_END]); /*close read end since I don't need it */
dup2(fd[WRITE_END], STDOUT_FILENO);
close(fd[WRITE_END]);
execlp("ls", "ls", "-hal", NULL);
break; /*exit for loop to end child's code */
}
else if (pid==0 && i==1) {
printf("i'm in the second child, which will be used to run grep\n");
close(fd[WRITE_END]);
dup2(fd[READ_END], STDIN_FILENO);
close(fd[READ_END]);
close(fd2[READ_END]);
dup2(fd2[WRITE_END], STDOUT_FILENO);
close(fd2[WRITE_END]);
execlp("grep", "grep","p",NULL);
break;
}
else if (pid==0 && i==2) {
//never uses fd so close both descriptors
close(fd[READ_END]);
close(fd[WRITE_END]);
printf("i'm in the original process which will be replaced with wc\n");
close(fd2[WRITE_END]);
dup2(fd2[READ_END], STDIN_FILENO);
close(fd2[READ_END]);
printf("going to exec wc\n");
execlp("wc","wc","-w",NULL);
break;
}
else {
//do parenty things
}
}
wait(NULL);
while (1){
wait(NULL);
if(errno== ECHILD) {
printf("all children ended\n");
break;
}
}
close(fd[READ_END]);
close(fd[WRITE_END]);
close(fd2[READ_END]);
close(fd2[WRITE_END]);
return 0;
}
grep and wc never exit.
Why? They never receive an EOF on stdin.
Why? Because, even though ls has exited and closed the write end of pipe(fd), the main process still has the write end of pipe(fd) open, thus the read end of pipe(fd) is still waiting for more data. Similar thing goes for fd2: even if grep exited, wc wouldn't get an EOF on stdin.
Solution: close all the pipe fds in the main process before you wait.