libc/bionic/tmpfile.cpp

/*-
 * Copyright (c) 1990, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Chris Torek.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

class ScopedSignalBlocker {
 public:
  ScopedSignalBlocker() {
    sigset_t set;
    sigfillset(&set);
    sigprocmask(SIG_BLOCK, &set, &old_set_);
  }

  ~ScopedSignalBlocker() {
    sigprocmask(SIG_SETMASK, &old_set_, NULL);
  }

 private:
  sigset_t old_set_;
};

static FILE* __tmpfile_dir(const char* tmp_dir) {
  char buf[PATH_MAX];
  int path_length = snprintf(buf, sizeof(buf), "%s/tmp.XXXXXXXXXX", tmp_dir);
  if (path_length >= sizeof(buf)) {
    return NULL;
  }

  int fd;
  {
    ScopedSignalBlocker ssb;
    fd = mkstemp(buf);
    if (fd == -1) {
      return NULL;
    }

    // Unlink the file now so that it's removed when closed.
    unlink(buf);

    // Can we still use the file now it's unlinked?
    // File systems without hard link support won't have the usual Unix semantics.
    struct stat sb;
    int rc = fstat(fd, &sb);
    if (rc == -1) {
      int old_errno = errno;
      close(fd);
      errno = old_errno;
      return NULL;
    }
  }

  // Turn the file descriptor into a FILE*.
  FILE* fp = fdopen(fd, "w+");
  if (fp != NULL) {
    return fp;
  }

  // Failure. Clean up. We already unlinked, so we just need to close.
  int old_errno = errno;
  close(fd);
  errno = old_errno;
  return NULL;
}

FILE* tmpfile() {
  // TODO: get this app's temporary directory from the framework ("/data/data/app/cache").

  // $EXTERNAL_STORAGE turns out not to be very useful because it doesn't support hard links.
  // This means we can't do the usual trick of calling unlink before handing the file back.

  FILE* fp = __tmpfile_dir("/data/local/tmp");
  if (fp == NULL) {
    // P_tmpdir is "/tmp/", but POSIX explicitly says that tmpdir(3) should try P_tmpdir before
    // giving up. This is potentially useful for bionic on the host anyway.
    fp = __tmpfile_dir(P_tmpdir);
  }
  return fp;
}