Getting stdout from an executed application

Question

Does anyone know how to catch the output (I think its stdout) from execvp instead of the system printing it (in c on linux) in the terminal?

Answer 1

execvp replaces the current running process in memory. There's no "catching" the output.

I suspect you're trying to run an external process from an existing process, and parse its output. For that you need to use popen() which does a fork() then an exec(), returning a FILE * to read (which will be the stdout of the process you just ran).

Answer 2

I distrust popen/pclose, as I've worked on too many systems where SIGCHLD was handled slightly differently. And I distrust the sh-shell parsing used by popen, since I rarely use it.

The short 22-year-old O'Reilly book Using C on the UNIX System, by Dave Curry is still a very good reference for this sort of stuff.

Anyway, here is some code. It is a bit lengthy, as it parses the sample string "/bin/ls /etc" into the array {"/bin/ls", "/etc", 0}. But I find using the string format easier and shorter 98% of the time, although this example belies that.

This code generates a listing of /etc. You'll need to change some stuff like e.g. NUMBER() which is the same asXtNumber(). And you'll need to decide whether it matches your handling of SIGCHLD.

int main(void) {  // list the files in /etc
   char buf[100];
   FILE *fp;
   int pid = spawnfp("/bin/ls /etc", &fp);
   while (fgets(buf, sizeof buf, fp))
      printf("%s", buf);
   fclose(fp);                    // pclose() replacement
   kill(pid, SIGKILL);            // pclose() replacement
   return 0;
}

The subroutines here are:

static int spawnpipe(const char *argv[], int *fd) // popen() replacement
{
   int pid;
   int pipe_fds[2];

   if (pipe(pipe_fds) < 0)
      FatalError("pipe");

   switch ((pid = fork()))
   {
      case -1:
         FatalError("fork");
      case 0:                     // child
         close(1);
         close(2);
         dup(pipe_fds[0]);
         dup(pipe_fds[1]);
         close(pipe_fds[0]);
         close(pipe_fds[1]);

         execv(argv[0], (char * const *)argv);
         perror("execv");
         _exit(EXIT_FAILURE);    // sic, not exit()
      default:
         *fd = pipe_fds[0];
         close(pipe_fds[1]);
         return pid;
   }
}

This converts an ascii string to an argv list, which is probably useless to you:

Bool convertStringToArgvList(char *p, const char **argv, int maxNumArgs)
{
   // Break up a string into tokens, on spaces, except that quoted bits, 
   // with single-quotes, are kept together, without the quotes. Such  
   // single-quotes cannot be escaped. A double-quote is just an ordinary char.
   // This is a *very* basic parsing, but ok for pre-programmed strings.
   int cnt = 0;
   while (*p)
   {
      while (*p && *p <= ' ')    // skip spaces
         p++;
      if (*p == '\'')            // single-quote block
      {
         if (cnt < maxNumArgs)
            argv[cnt++] = ++p;   // drop quote
         while (*p && *p != '\'')
            p++;
      }
      else if (*p)               // simple space-delineated token
      {
         if (cnt < maxNumArgs)
            argv[cnt++] = p;
         while (*p > ' ')
            p++;
      }
      if (*p)
         *p++ = 0;               // nul-terminate
   }
   if (cnt < maxNumArgs)
      argv[cnt++] = 0;
   return cnt <= maxNumArgs;     // check for too many tokens (unlikely)
}

This converts the argument string to tokens and, more importantly, the fd to an fp, since the OP requested stdout:

int spawnfp(const char *command, FILE **fpp)
{
   const char *argv[100];
   int fd, pid;
   if (!convertStringToArgvList(strdupa(command), argv, NUMBER(argv)))
      FatalError("spawnfp");
   pid = spawnpipe(argv, &fd);
   *fpp = fdopen(fd, "r");
   return pid;
}

Answer 3

See the documentation of popen, I think it's exactly what you need.

Answer 4

As others have said, popen is what you want to use. Something like this...

#include <iomanip>
#include <iostream>

using namespace std;

const int MAX_BUFFER = 255;

int main()
{
        string cmd;
        cout << "enter cmd: ";
        cin >> cmd;
        cout << endl << "running " << cmd << "…" << endl;


        string stdout;
        char buffer[MAX_BUFFER];
        FILE *stream = popen(cmd.c_str(), "r");
        while ( fgets(buffer, MAX_BUFFER, stream) != NULL )
        stdout.append(buffer);
        pclose(stream);


        cout << endl << "output: " << endl << stdout << endl;
}