Grabbing output from exec
Solution 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;
}
Solution 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
Solution 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;
}
TrewTzu
Updated on July 16, 2022Comments
-
TrewTzu almost 2 years
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.