writing stdin into a file with fwrite()

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 f开发者_Python百科ile...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()?

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.

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;
  }
}

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.