bi_popen/bi_popen.c

/* - | Copyright / About | ----------------------------------------------------
   Copyright (c) 2026 Randy Jordan

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Creates two pipes, forks, and runs the given command.  One pipe is
connected between the given *in and the standard input stream of the child;
the other pipe is connected between the given *out and the standard output
stream of the child.

Returns the pid of the child on success, -1 otherwise.  On error, errno
will be set accordingly.

-----------------------------------------------------------------------------*/
#define DEBUG
#include "bi_popen.h"

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>

static const int READ_END = 0;
static const int WRITE_END = 1;
static const int INVALID_FD = -1;

/* The front fell of, which is quite unusual. */
static int
bi_popen_bail(FILE** const in, FILE** const out, int child[2], int parent[2])
{
  /* Save and log the error */
  const int old_errno = errno;
  fprintf(stderr, "bi_popen: %s\n", strerror(old_errno));
  fflush(stderr);

  /* Close the file pointers */
  if (*in != NULL) {
    fclose(*in);
  }

  if (*out != NULL) {
    fclose(*out);
  }

  /* Close all parent and child file descriptors */
  for (int i = 0; i < 2; ++i) {
    if (child[i] != INVALID_FD) {
      close(child[i]);
    }
    if (parent[i] != INVALID_FD) {
      close(parent[i]);
    }
  }

  errno = old_errno;
  return -1;
}

/* Setup pipe redirection, and call exec process. */
static int
bi_popen_child(FILE** const in,
               FILE** const out,
               int child[2],
               int parent[2],
               const char* command)
{
  /* Redirect child's stdin to the read end of to_child. */
  if (dup2(child[READ_END], STDIN_FILENO) < 0) {
    perror("dup2");
    exit(1);
  }
  /* Close original pipe fds in the child after duplicating. */
  close(child[READ_END]);
  close(child[WRITE_END]);

  /* Redirect child's stdout to the write end of to_parent */
  if (dup2(parent[WRITE_END], STDOUT_FILENO) < 0) {
    perror("dup2");
    exit(1);
  }
  /* Close original pipe fds in the child after duplicating. */
  close(parent[READ_END]);
  close(parent[WRITE_END]);

  /* Replace the child process image with the requested command. */
  execlp(command, command, NULL);

  /* If execlp returns, it failed. */
  perror("execlp");
  exit(1);
}

/* Convert remaining fd's to streams and return pid_t to wait */
static int
bi_popen_parent(FILE** const in,
                FILE** const out,
                int child[2],
                int parent[2],
                pid_t pid)
{
  /* Parent process: Close ends that the parent does not use. */
  close(child[READ_END]);
  close(parent[WRITE_END]);
  parent[WRITE_END] = INVALID_FD;
  child[READ_END] = INVALID_FD;

  /* Convert remaining pipe fds into stdio streams */
  *out = fdopen(parent[READ_END], "r");
  if (*out == NULL) {
    return bi_popen_bail(in, out, child, parent);
  }
  parent[READ_END] = INVALID_FD;

  *in = fdopen(child[WRITE_END], "w");
  if (*in == NULL) {
    return bi_popen_bail(in, out, child, parent);
  }
  child[WRITE_END] = INVALID_FD;

  /* Make writes to *in unbuffered (so data is sent immediately) */
  setvbuf(*in, NULL, _IONBF, BUFSIZ);

  /* Success: return the child's pid, so the caller can use to wait. */
  return pid;
}

int
bi_popen(const char* const command, FILE** const in, FILE** const out)
{
  /* Initialize file descriptors and pointers for safe cleanup. */
  int to_child[2] = { INVALID_FD, INVALID_FD };
  int to_parent[2] = { INVALID_FD, INVALID_FD };
  *in = NULL;
  *out = NULL;

  /* Validate inputs. */
  if (command == NULL || in == NULL || out == NULL) {
    errno = EINVAL;
    return bi_popen_bail(in, out, to_child, to_parent);
  }

  /* Create pipe for sending data to the child (stdin of child). */
  if (pipe(to_child) < 0) {
    return bi_popen_bail(in, out, to_child, to_parent);
  }

  /* Create pipe for recieving data from the child (stdout of child). */
  if (pipe(to_parent) < 0) {
    return bi_popen_bail(in, out, to_child, to_parent);
  }

  /* Fork the process and have the child setup stdio redirection and exec */
  const pid_t pid = fork();
  if (pid < 0) {
    return bi_popen_bail(in, out, to_child, to_parent);
  }

  if (pid == 0) { // Child Process - execlp shouldn't return .
    return bi_popen_child(in, out, to_child, to_parent, command);
  }

  /* Parent process,  convert the fd's to streams and return pid to wait */
  return bi_popen_parent(in, out, to_child, to_parent, pid);
}

int
main(int argc, char* argv[])
{
  (void)argc;
  (void)argv;

  FILE* in = NULL;
  FILE* out = NULL;
  char* line = NULL;
  size_t size = 0;

  const int pid = bi_popen("/bin/bash", &in, &out);
  if (pid < 0) {
    perror("bi_popen");
    return 1;
  }

  fprintf(in, "date\n");
  getline(&line, &size, out);
  printf("-> %s", line);
  free(line);

  // Since in this case we can tell the child to terminate, we'll do so
  // and wait for it to terminate before we close down.
  fprintf(in, "exit\n");
  waitpid(pid, NULL, 0);

  fclose(in);
  fclose(out);

  return EXIT_SUCCESS;
}