rm_/lib/read-file.c

/* read-file.c -- read file contents into a string
   Copyright (C) 2006, 2009-2020 Free Software Foundation, Inc.
   Written by Simon Josefsson and Bruno Haible.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3, or (at your option)
   any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, see <https://www.gnu.org/licenses/>.  */

#include <config.h>

#include "read-file.h"

/* Get fstat.  */
#include <sys/stat.h>

/* Get ftello.  */
#include <stdio.h>

/* Get PTRDIFF_MAX.  */
#include <stdint.h>

/* Get malloc, realloc, free. */
#include <stdlib.h>

/* Get errno. */
#include <errno.h>

/* Read a STREAM and return a newly allocated string with the content,
   and set *LENGTH to the length of the string.  The string is
   zero-terminated, but the terminating zero byte is not counted in
   *LENGTH.  On errors, *LENGTH is undefined, errno preserves the
   values set by system functions (if any), and NULL is returned.  */
char *
fread_file (FILE *stream, size_t *length)
{
  char *buf = NULL;
  size_t alloc = BUFSIZ;

  /* For a regular file, allocate a buffer that has exactly the right
     size.  This avoids the need to do dynamic reallocations later.  */
  {
    struct stat st;

    if (fstat (fileno (stream), &st) >= 0 && S_ISREG (st.st_mode))
      {
        off_t pos = ftello (stream);

        if (pos >= 0 && pos < st.st_size)
          {
            off_t alloc_off = st.st_size - pos;

            /* '1' below, accounts for the trailing NUL.  */
            if (PTRDIFF_MAX - 1 < alloc_off)
              {
                errno = ENOMEM;
                return NULL;
              }

            alloc = alloc_off + 1;
          }
      }
  }

  if (!(buf = malloc (alloc)))
    return NULL; /* errno is ENOMEM.  */

  {
    size_t size = 0; /* number of bytes read so far */
    int save_errno;

    for (;;)
      {
        /* This reads 1 more than the size of a regular file
           so that we get eof immediately.  */
        size_t requested = alloc - size;
        size_t count = fread (buf + size, 1, requested, stream);
        size += count;

        if (count != requested)
          {
            save_errno = errno;
            if (ferror (stream))
              break;

            /* Shrink the allocated memory if possible.  */
            if (size < alloc - 1)
              {
                char *smaller_buf = realloc (buf, size + 1);
                if (smaller_buf != NULL)
                  buf = smaller_buf;
              }

            buf[size] = '\0';
            *length = size;
            return buf;
          }

        {
          char *new_buf;

          if (alloc == PTRDIFF_MAX)
            {
              save_errno = ENOMEM;
              break;
            }

          if (alloc < PTRDIFF_MAX - alloc / 2)
            alloc = alloc + alloc / 2;
          else
            alloc = PTRDIFF_MAX;

          if (!(new_buf = realloc (buf, alloc)))
            {
              save_errno = errno;
              break;
            }

          buf = new_buf;
        }
      }

    free (buf);
    errno = save_errno;
    return NULL;
  }
}

static char *
internal_read_file (const char *filename, size_t *length, const char *mode)
{
  FILE *stream = fopen (filename, mode);
  char *out;
  int save_errno;

  if (!stream)
    return NULL;

  out = fread_file (stream, length);

  save_errno = errno;

  if (fclose (stream) != 0)
    {
      if (out)
        {
          save_errno = errno;
          free (out);
        }
      errno = save_errno;
      return NULL;
    }

  return out;
}

/* Open and read the contents of FILENAME, and return a newly
   allocated string with the content, and set *LENGTH to the length of
   the string.  The string is zero-terminated, but the terminating
   zero byte is not counted in *LENGTH.  On errors, *LENGTH is
   undefined, errno preserves the values set by system functions (if
   any), and NULL is returned.  */
char *
read_file (const char *filename, size_t *length)
{
  return internal_read_file (filename, length, "r");
}

/* Open (on non-POSIX systems, in binary mode) and read the contents
   of FILENAME, and return a newly allocated string with the content,
   and set LENGTH to the length of the string.  The string is
   zero-terminated, but the terminating zero byte is not counted in
   the LENGTH variable.  On errors, *LENGTH is undefined, errno
   preserves the values set by system functions (if any), and NULL is
   returned.  */
char *
read_binary_file (const char *filename, size_t *length)
{
  return internal_read_file (filename, length, "rb");
}