source: azure_iot_hub/trunk/musl-1.1.18/src/stdio/freopen.c@ 389

#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;
}

LFS64(freopen);