fusl/src/stdio/freopen.c

#include "stdio_impl.h"
#include <fcntl.h>

/* The basic idea of this implementation is to open a new FILE,
 * hack the necessary parts of the new FILE into the old one, then
 * close the new FILE. */

/* Locking IS necessary because another thread may provably hold the
 * lock, via flockfile or otherwise, when freopen is called, and in that
 * case, freopen cannot act until the lock is released. */

int __dup3(int, int, int);

FILE* freopen(const char* restrict filename,
              const char* restrict mode,
              FILE* restrict f) {
  int fl = __fmodeflags(mode);
  FILE* f2;

  FLOCK(f);

  fflush(f);

  if (!filename) {
    if (fl & O_CLOEXEC)
      __syscall(SYS_fcntl, f->fd, F_SETFD, FD_CLOEXEC);
    fl &= ~(O_CREAT | O_EXCL | O_CLOEXEC);
    if (syscall(SYS_fcntl, f->fd, F_SETFL, fl) < 0)
      goto fail;
  } else {
    f2 = fopen(filename, mode);
    if (!f2)
      goto fail;
    if (f2->fd == f->fd)
      f2->fd = -1; /* avoid closing in fclose */
    else if (__dup3(f2->fd, f->fd, fl & O_CLOEXEC) < 0)
      goto fail2;

    f->flags = (f->flags & F_PERM) | f2->flags;
    f->read = f2->read;
    f->write = f2->write;
    f->seek = f2->seek;
    f->close = f2->close;

    fclose(f2);
  }

  FUNLOCK(f);
  return f;

fail2:
  fclose(f2);
fail:
  fclose(f);
  return NULL;
}