sysdeps/posix/preadv.c

   1 /* Copyright (C) 2009-2014 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <stdlib.h>
  20 #include <unistd.h>
  21 #include <string.h>
  22 #include <limits.h>
  23 #include <stdbool.h>
  24 #include <sys/param.h>
  25 #if __WORDSIZE == 64 && !defined PREADV
  26 /* Hide the preadv64 declaration.  */
  27 # define preadv64 __redirect_preadv64
  28 #endif
  29 #include <sys/uio.h>
  30 #include <bits/wordsize.h>
  31
  32 #ifndef PREADV
  33 # define PREADV preadv
  34 # define PREAD __pread
  35 # define OFF_T off_t
  36 #endif
  37
  38
  39 static void
  40 ifree (char **ptrp)
  41 {
  42   free (*ptrp);
  43 }
  44
  45
  46 /* Read data from file descriptor FD at the given position OFFSET
  47    without change the file pointer, and put the result in the buffers
  48    described by VECTOR, which is a vector of COUNT 'struct iovec's.
  49    The buffers are filled in the order specified.  Operates just like
  50    'pread' (see <unistd.h>) except that data are put in VECTOR instead
  51    of a contiguous buffer.  */
  52 ssize_t
  53 PREADV (int fd, const struct iovec *vector, int count, OFF_T offset)
  54 {
  55   /* Find the total number of bytes to be read.  */
  56   size_t bytes = 0;
  57   for (int i = 0; i < count; ++i)
  58     {
  59       /* Check for ssize_t overflow.  */
  60       if (SSIZE_MAX - bytes < vector[i].iov_len)
  61         {
  62           __set_errno (EINVAL);
  63           return -1;
  64         }
  65       bytes += vector[i].iov_len;
  66     }
  67
  68   /* Allocate a temporary buffer to hold the data.  We should normally
  69      use alloca since it's faster and does not require synchronization
  70      with other threads.  But we cannot if the amount of memory
  71      required is too large.  */
  72   char *buffer;
  73   char *malloced_buffer __attribute__ ((__cleanup__ (ifree))) = NULL;
  74   if (__libc_use_alloca (bytes))
  75     buffer = (char *) __alloca (bytes);
  76   else
  77     {
  78       malloced_buffer = buffer = (char *) malloc (bytes);
  79       if (buffer == NULL)
  80         return -1;
  81     }
  82
  83   /* Read the data.  */
  84   ssize_t bytes_read = PREAD (fd, buffer, bytes, offset);
  85   if (bytes_read < 0)
  86     return -1;
  87
  88   /* Copy the data from BUFFER into the memory specified by VECTOR.  */
  89   bytes = bytes_read;
  90   for (int i = 0; i < count; ++i)
  91     {
  92       size_t copy = MIN (vector[i].iov_len, bytes);
  93
  94       (void) memcpy ((void *) vector[i].iov_base, (void *) buffer, copy);
  95
  96       buffer += copy;
  97       bytes -= copy;
  98       if (bytes == 0)
  99         break;
 100     }
 101
 102   return bytes_read;
 103 }
 104 #if __WORDSIZE == 64 && defined preadv64
 105 # undef preadv64
 106 strong_alias (preadv, preadv64)
 107 #endif