common/include/iszero.h

   1 /* nbdkit
   2  * Copyright (C) 2018 Red Hat Inc.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are
   6  * met:
   7  *
   8  * * Redistributions of source code must retain the above copyright
   9  * notice, this list of conditions and the following disclaimer.
  10  *
  11  * * Redistributions in binary form must reproduce the above copyright
  12  * notice, this list of conditions and the following disclaimer in the
  13  * documentation and/or other materials provided with the distribution.
  14  *
  15  * * Neither the name of Red Hat nor the names of its contributors may be
  16  * used to endorse or promote products derived from this software without
  17  * specific prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
  20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
  21  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
  22  * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
  23  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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  25  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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  29  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30  * SUCH DAMAGE.
  31  */
  32
  33 #ifndef NBDKIT_ISZERO_H
  34 #define NBDKIT_ISZERO_H
  35
  36 #include <string.h>
  37 #include <stdbool.h>
  38
  39 /* Return true iff the buffer is all zero bytes.
  40  *
  41  * The clever approach here was suggested by Eric Blake.  See:
  42  * https://www.redhat.com/archives/libguestfs/2017-April/msg00171.html
  43  * https://rusty.ozlabs.org/?p=560
  44  *
  45  * See also:
  46  * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69908
  47  */
  48 static inline bool __attribute__((__nonnull__ (1)))
  49 is_zero (const char *buffer, size_t size)
  50 {
  51   size_t i;
  52   const size_t limit = size < 16 ? size : 16;
  53
  54   for (i = 0; i < limit; ++i)
  55     if (buffer[i])
  56       return false;
  57   if (size != limit)
  58     return ! memcmp (buffer, buffer + 16, size - 16);
  59
  60   return true;
  61 }
  62
  63 #endif /* NBDKIT_ISZERO_H */