STDERR output is not being captured when running invalid command - c

Question

I am trying to catch the output from running the command /bin/lsx -lah /.
The output should be: bash: /bin/lsx: no such file or directory
However, I am just not getting anything in the printf().
This is the code
#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/lsx", "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;
}
I am just wondering why the output is not being captured and printed in the bottom printf().

Answer 1

As John pointed out, you are only capturing stdout, not stderr, and well-formed programs usually send error messages to stderr (the die macro you have shown is an example of this).
A quick solution is to redirect the child processes' stderr to its stdout with another call to dup2
dup2(link[1], STDOUT_FILENO);
dup2(STDOUT_FILENO, STDERR_FILENO);
which will redirect both output streams to the single pipe.
Alternatively, capture both streams with two pipes to retain their separation as they propagate.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);
ssize_t push_through(const char *prefix,
FILE *out, int in,
char *buf, size_t bufsz)
{
ssize_t bytes = read(in, buf, bufsz);
if (bytes > 0)
fprintf(out, "%s: (%.*s)\n", prefix, (int) bytes, buf);
return bytes;
}
int main(void)
{
pid_t pid;
int stdout_pipe[2];
int stderr_pipe[2];
char buffer[4096];
if (pipe(stdout_pipe) == -1 || pipe(stderr_pipe) == -1)
die("pipe");
if ((pid = fork()) == -1)
die("fork");
if (pid == 0) {
close(stdout_pipe[0]);
close(stderr_pipe[0]);
dup2(stdout_pipe[1], STDOUT_FILENO);
dup2(stderr_pipe[1], STDERR_FILENO);
close(stdout_pipe[1]);
close(stderr_pipe[1]);
execl("/bin/lsx", "ls", "-1", (char *) NULL);
perror("execl failed because");
die("execl");
} else {
ssize_t outbytes, errbytes;
close(stdout_pipe[1]);
close(stderr_pipe[1]);
do {
outbytes = push_through("out", stdout, stdout_pipe[0],
buffer, sizeof buffer);
errbytes = push_through("err", stderr, stderr_pipe[0],
buffer, sizeof buffer);
} while (outbytes > 0 || errbytes > 0);
wait(NULL);
}
}
In the event that execl fails, it might be useful to call perror to print a more detailed error message.
Note that expecting the error
bash: /bin/lsx: no such file or directory
is possibly misguided, as exec* functions only defer to a shell under certain conditions.
The only mention of a shell in man 3 exec is with regards to execlp, execvp, and execvpe:
If the header of a file isn't recognized (the attempted execve(2)
failed with the error ENOEXEC), these functions will execute the
shell (/bin/sh) with the path of the file as its first argument.
(If this attempt fails, no further searching is done.)