writing stdin into a file with fwrite()

Question

I have to capture the stdout in a program and write that into a file...so I created a pipe. In the parent process, I captured the stdout in the pipe using dup() and I need to get this into a file...so I did a dup() in the child to get the captured file descriptor into the stdin. Now, how do I write this stdin into a file using fwrite()?

Answer 1

You should capture into a byte or char buffer and the send that ot the fwrite. When I say a buffer I mean an array or dynamically allocated block of bytes/chars.

Answer 2

Isn't that doing things the hard way? All you need to do in the parent is use freopen() to connect stdout to the file of your choosing.

FILE *fp = freopen("/tmp/mylogfile", "w", stdout);

if (fp == 0)
    error("...something went wrong opening the log file...\n");

---

The direct answer to your question is:

char buffer[32768];
ssize_t nbytes;
FILE *fp = fopen("/tmp/mylogfile", "w");

if (fp == 0)
    error("....something went wrong opening my log file...\n");

while ((nbytes = fread(buffer, sizeof(char), sizeof(buffer), stdin)) > 0)
    if (fwrite(buffer, sizeof(char), nbytes, fp) != nbytes)
        error("...something went wrong writing to standard output...\n");

However, this is hardly necessary. You can improve the error handling in all sorts of ways; I'm simply assuming that 'error()' reports a message and does not return.

Answer 3

The easiest way is just to open the file and provide that as the child's stdout:

#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <unistd.h>

#include <stdio.h>

int main() {
  pid_t pid = fork();
  switch (pid) {
  case -1:
    perror("fork");
    return 1;

  case 0:;
    int new_out = open("output.txt", O_WRONLY | O_CREAT, 0666);
    if (new_out == -1) {
      perror("open");
      return 1;
    }
    if (dup2(new_out, 1) == -1) {
      perror("dup2");
      return 1;
    }
    char* args[] = {"/bin/echo", "test output", 0};
    execv(args[0], args);
    perror("exec");
    return 1;

  default:;
    int s;
    if (waitpid(pid, &s, 0) == -1) {
      perror("waitpid");
      return 1;
    }
    if (WIFEXITED(s)) {
      return WEXITSTATUS(s);
    }
    return 1;
  }
}