Capturing output from execv() [duplicate] - c

Question

I'm trying to write a C program that grabs command output and then i'll be passing that to another program.
I'm having an issue, I cant work out how to get the command output and store it. Below is a sample of what I have
if(fork() == 0){
execl("/bin/ls", "ls", "-1", (char *)0);
/* do something with the output here */
}
else{
//*other stuff goes here*
}
so basically im wondering if there is any way i can get the output from the "execl" and pass it to some thing else (e.g. via storing it in some kind of buffer).
Suggestions would be great.

Answer 1

You have to create a pipe from the parent process to the child, using pipe().
Then you must redirect standard ouput (STDOUT_FILENO) and error output (STDERR_FILENO) using dup or dup2 to the pipe, and in the parent process, read from the pipe.
It should work.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);
int main() {
int link[2];
pid_t pid;
char foo[4096];
if (pipe(link)==-1)
die("pipe");
if ((pid = fork()) == -1)
die("fork");
if(pid == 0) {
dup2 (link[1], STDOUT_FILENO);
close(link[0]);
close(link[1]);
execl("/bin/ls", "ls", "-1", (char *)0);
die("execl");
} else {
close(link[1]);
int nbytes = read(link[0], foo, sizeof(foo));
printf("Output: (%.*s)\n", nbytes, foo);
wait(NULL);
}
return 0;
}

Answer 2

Open a pipe, and change stdout to match that pipe.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int pipes[2];
pipe(pipes); // Create the pipes
dup2(pipes[1],1); // Set the pipe up to standard output
After that, anything which goes to stdout,(such as through printf), comes out pipe[0].
FILE *input = fdopen(pipes[0],"r");
Now you can read the output like a normal file descriptor. For more details, look at this

Answer 3

Thanks Jonathan Leffler, and i optimize the above code for it can't read all response for one time.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);
int main() {
int link[2];
pid_t pid;
char foo[4096 + 1];
memset(foo, 0, 4096);
if (pipe(link)==-1)
die("pipe");
if ((pid = fork()) == -1)
die("fork");
if(pid == 0) {
dup2 (link[1], STDOUT_FILENO);
close(link[0]);
close(link[1]);
execl("/bin/ls", "ls", "-1", (char *)0);
die("execl");
} else {
close(link[1]);
int nbytes = 0;
std::string totalStr;
while(0 != (nbytes = read(link[0], foo, sizeof(foo)))) {
totalStr = totalStr + foo;
printf("Output: (%.*s)\n", nbytes, foo);
memset(foo, 0, 4096);
}
wait(NULL);
}
return 0;
}

Answer 4

If you want the output in a string (char *), here's an option (for Linux at least):
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/uio.h>
#include <sys/wait.h>
#include <unistd.h>
char* qx(char** cmd, int inc_stderr) {
int stdout_fds[2];
pipe(stdout_fds);
int stderr_fds[2];
if (!inc_stderr) {
pipe(stderr_fds);
}
const pid_t pid = fork();
if (!pid) {
close(stdout_fds[0]);
dup2(stdout_fds[1], 1);
if (inc_stderr) {
dup2(stdout_fds[1], 2);
}
close(stdout_fds[1]);
if (!inc_stderr) {
close(stderr_fds[0]);
dup2(stderr_fds[1], 2);
close(stderr_fds[1]);
}
execvp(*cmd, cmd);
exit(0);
}
close(stdout_fds[1]);
const int buf_size = 4096;
char* out = malloc(buf_size);
int out_size = buf_size;
int i = 0;
do {
const ssize_t r = read(stdout_fds[0], &out[i], buf_size);
if (r > 0) {
i += r;
}
if (out_size - i <= 4096) {
out_size *= 2;
out = realloc(out, out_size);
}
} while (errno == EAGAIN || errno == EINTR);
close(stdout_fds[0]);
if (!inc_stderr) {
close(stderr_fds[1]);
do {
const ssize_t r = read(stderr_fds[0], &out[i], buf_size);
if (r > 0) {
i += r;
}
if (out_size - i <= 4096) {
out_size *= 2;
out = realloc(out, out_size);
}
} while (errno == EAGAIN || errno == EINTR);
close(stderr_fds[0]);
}
int r, status;
do {
r = waitpid(pid, &status, 0);
} while (r == -1 && errno == EINTR);
out[i] = 0;
return out;
}
int main() {
char* argv[3];
argv[0] = "ls";
argv[1] = "-la";
argv[2] = NULL;
char* out = qx(argv, 0);
printf("%s", out);
free(out);
}