Redirect STDOUT and STDERR to socket in C?

Question

I am trying to redirect STDOUT AND STDERR to a socket.

I did:

if(fork() == 0)
{
   dup2(newsock, STDOUT_FILENO);
   dup2(newsock, STDERR_FILENO);
   execvp();
}

Somehow, it only showed the first little part of the output.

for example, it showed on "mkdir" when I try to execute ls or mkdir.

What's the problem?

I tried the flollowing it works, but I can only redirect one of STDOUT or STDERR

close(1);
dup(newsock);

Thanks a lot.

Answer 1

Your use of dup2() looks fine, so the problem is probably elsewhere. The simple program I threw together to test with does not have the issues you are experiencing, so I'll just go over the core of it (around the fork()/execvp() area) with some error checking omitted for brevity:

int    lsock, /* listening socket */
       csock; /* active connection's socket */
pid_t  cpid;  /* child process ID from fork() */
char   *cmd = "somecommand";
char   *cmd_args[] = { "somecommand",
                       "firstarg",
                       "secondarg",
                       "howevermanyargs",
                       NULL }; /* note: last item is NULL */
/*  ... 
    call socket(), bind(), listen(), etc.
    ... */

for (;;) {  /* loop, accepting connections */
  if ( (csock = accept( lsock, NULL, NULL )) == -1) exit(1);
  cpid = fork();
  if (cpid < 0) exit(1);  /* exit if fork() fails */
  if ( cpid ) {
    /* In the parent process: */
    close( csock ); /* csock is not needed in the parent after the fork */
    waitpid( cpid, NULL, 0 ); /* wait for and reap child process */
  } else {
    /* In the child process: */
    dup2( csock, STDOUT_FILENO );  /* duplicate socket on stdout */
    dup2( csock, STDERR_FILENO );  /* duplicate socket on stderr too */
    close( csock );  /* can close the original after it's duplicated */
    execvp( cmd, cmd_args );   /* execvp() the command */
  }
}

The above is the core of a very basic server (only one client at a time) that, when it receives a connection, forks a new process to run a command and sends its stderr and stdout to the client over the socket. Hopefully you can solve your problem by examining it -- but don't just copy the code without understanding what it does.

Try testing by connecting with a telnet client first... if it works with telnet but not with your client program, then look for problems in your client program.