Close pipes get Bad file descriptor - c

Question

Here is the code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <unistd.h>
#include <dirent.h>
int main(int argc, char **argv) {
int num = 2;
pid_t pid;
int i;
int p1[num][2], p2[num][2];
for (i = 0; i < num; i++) {
if (pipe(p1[i]) == -1) {
perror("pipe");
exit(1);
}
if (pipe(p2[i]) == -1) {
perror("pipe");
exit(1);
}
}
for (i = 0; i < num; i++) {
if ((pid = fork()) == 0) {
if (close(p1[i][1]) != 0) {
perror("close");
exit(1);
}
if (close(p2[i][0]) != 0) {
perror("close");
exit(1);
}
printf("%d\n", getpid());
exit(0);
} else if (pid > 0) {
if (close(p1[i][0]) != 0) {
perror("close");
exit(1);
}
if (close(p2[i][1]) != 0) {
perror("close");
exit(1);
}
continue;
} else {
perror("fork");
exit(1);
}
}
for (i = 0; i < num; i++) {
if (close(p1[i][0]) != 0) {
perror("close1"); // <----error
}
if (close(p1[i][1]) != 0) {
perror("close");
}
if (close(p2[i][0]) != 0) {
perror("close");
}
if (close(p2[i][1]) != 0) {
perror("close2"); // <----error
}
}
for (i = 0; i < num; i++) {
if (wait(NULL) == -1) {
perror("wait");
exit(1);
}
}
return 0;
}
When I run this, it gives me this output
close1: Bad file descriptor
close2: Bad file descriptor
close1: Bad file descriptor
close2: Bad file descriptor
8798
8799
What I'm trying to do is to create two 2D array of pipes and fork num times.
The creating and running are working well, but some pipes can't close.
It seems that p1[i][0] and p2[i][1] are never closer properly.

Answer 1

The reason you're getting the EBADFD is you're attempting to close the same file descriptor twice.
I added a print statement in this first snippet of code to show/track what file descriptor is being closed. If you compile and run this you'll see that the error message appears right after you try to close the descriptor for a second time.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <unistd.h>
#include <dirent.h>
int main(int argc, char **argv) {
int num = 1;
pid_t pid;
int i;
int p1[num][2], p2[num][2];
for (i = 0; i < num; i++) {
if (pipe(p1[i]) == -1) {
perror("pipe");
exit(1);
}
if (pipe(p2[i]) == -1) {
perror("pipe");
exit(1);
}
}
for (i = 0; i < num; i++) {
if ((pid = fork()) == 0) {
printf("Child closing: Pipe1 %d End %d\n", i, 1);
if (close(p1[i][1]) != 0) {
perror("close");
exit(1);
}
printf("Child closing: Pipe2 %d End %d\n", i, 1);
if (close(p2[i][0]) != 0) {
perror("close");
exit(1);
}
printf("%d\n", getpid());
exit(0);
} else if (pid > 0) {
printf("Parent closing: Pipe1 %d End %d\n", i, 0);
if (close(p1[i][0]) != 0) {
perror("close");
exit(1);
}
printf("Parent closing: Pipe1 %d End %d\n", i, 1);
if (close(p2[i][1]) != 0) {
perror("close");
exit(1);
}
continue;
} else {
perror("fork");
exit(1);
}
}
for (i = 0; i < num; i++) {
printf("Closing: Pipe1: %d End: %d\n", i, 0);
if (close(p1[i][0]) != 0) {
perror("close1"); // <----error
}
printf("Closing: Pipe1: %d End: %d\n", i, 1);
if (close(p1[i][1]) != 0) {
perror("close");
}
printf("Closing: Pipe2: %d End: %d\n", i, 0);
if (close(p2[i][0]) != 0) {
perror("close");
}
printf("Closing: Pipe2: %d End: %d\n", i, 1);
if (close(p2[i][1]) != 0) {
perror("close2"); // <----error
}
}
for (i = 0; i < num; i++) {
if (wait(NULL) == -1) {
perror("wait");
exit(1);
}
}
return 0;
}
Check for the pid and close the ends that weren't closed inside your first loop. This code assumes you are reading and writing on a specific pipe depending on the child's/parent's need. You may need to adjust to however your use case dictates:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <unistd.h>
#include <dirent.h>
int main(int argc, char **argv) {
int num = 1;
pid_t pid;
int i;
int p1[num][2], p2[num][2];
for (i = 0; i < num; i++) {
if (pipe(p1[i]) == -1) {
perror("pipe");
exit(1);
}
if (pipe(p2[i]) == -1) {
perror("pipe");
exit(1);
}
}
for (i = 0; i < num; i++) {
if ((pid = fork()) == 0) {
if (close(p1[i][1]) != 0) {
perror("close");
exit(1);
}
if (close(p2[i][0]) != 0) {
perror("close");
exit(1);
}
printf("%d\n", getpid());
exit(0);
} else if (pid > 0) {
if (close(p1[i][0]) != 0) {
perror("close");
exit(1);
}
if (close(p2[i][1]) != 0) {
perror("close");
exit(1);
}
continue;
} else {
perror("fork");
exit(1);
}
}
for (i = 0; i < num; i++) {
if (pid == 0) {
if (close(p1[i][0]) != 0) {
perror("close1");
}
if (close(p2[i][1]) != 0) {
perror("close");
}
} else {
if (close(p1[i][1]) != 0) {
perror("close");
}
if (close(p2[i][0]) != 0) {
perror("close2");
}
}
}
for (i = 0; i < num; i++) {
if (wait(NULL) == -1) {
perror("wait");
exit(1);
}
}
return 0;
}