lib/scratch_buffer.h

   1 /* Variable-sized buffer with on-stack default allocation.
   2    Copyright (C) 2017-2024 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation; either version 2.1 of the
   7    License, or (at your option) any later version.
   8
   9    This file 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
  12    GNU Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* Written by Paul Eggert, 2017.  */
  18
  19 #ifndef _GL_SCRATCH_BUFFER_H
  20 #define _GL_SCRATCH_BUFFER_H
  21
  22 /* Scratch buffers with a default stack allocation and fallback to
  23    heap allocation.  It is expected that this function is used in this
  24    way:
  25
  26      struct scratch_buffer tmpbuf;
  27      scratch_buffer_init (&tmpbuf);
  28
  29      while (!function_that_uses_buffer (tmpbuf.data, tmpbuf.length))
  30        if (!scratch_buffer_grow (&tmpbuf))
  31          return -1;
  32
  33      scratch_buffer_free (&tmpbuf);
  34      return 0;
  35
  36    The allocation functions (scratch_buffer_grow,
  37    scratch_buffer_grow_preserve, scratch_buffer_set_array_size) make
  38    sure that the heap allocation, if any, is freed, so that the code
  39    above does not have a memory leak.  The buffer still remains in a
  40    state that can be deallocated using scratch_buffer_free, so a loop
  41    like this is valid as well:
  42
  43      struct scratch_buffer tmpbuf;
  44      scratch_buffer_init (&tmpbuf);
  45
  46      while (!function_that_uses_buffer (tmpbuf.data, tmpbuf.length))
  47        if (!scratch_buffer_grow (&tmpbuf))
  48          break;
  49
  50      scratch_buffer_free (&tmpbuf);
  51
  52    scratch_buffer_grow and scratch_buffer_grow_preserve are guaranteed
  53    to grow the buffer by at least 512 bytes.  This means that when
  54    using the scratch buffer as a backing store for a non-character
  55    array whose element size, in bytes, is 512 or smaller, the scratch
  56    buffer only has to grow once to make room for at least one more
  57    element.
  58 */
  59
  60 /* Scratch buffer.  Must be initialized with scratch_buffer_init
  61    before its use.  */
  62 struct scratch_buffer;
  63
  64 /* Initializes *BUFFER so that BUFFER->data points to BUFFER->__space
  65    and BUFFER->length reflects the available space.  */
  66 #if 0
  67 extern void scratch_buffer_init (struct scratch_buffer *buffer);
  68 #endif
  69
  70 /* Deallocates *BUFFER (if it was heap-allocated).  */
  71 #if 0
  72 extern void scratch_buffer_free (struct scratch_buffer *buffer);
  73 #endif
  74
  75 /* Grow *BUFFER by some arbitrary amount.  The buffer contents is NOT
  76    preserved.  Return true on success, false on allocation failure (in
  77    which case the old buffer is freed).  On success, the new buffer is
  78    larger than the previous size.  On failure, *BUFFER is deallocated,
  79    but remains in a free-able state, and errno is set.  */
  80 #if 0
  81 extern bool scratch_buffer_grow (struct scratch_buffer *buffer);
  82 #endif
  83
  84 /* Like scratch_buffer_grow, but preserve the old buffer
  85    contents on success, as a prefix of the new buffer.  */
  86 #if 0
  87 extern bool scratch_buffer_grow_preserve (struct scratch_buffer *buffer);
  88 #endif
  89
  90 /* Grow *BUFFER so that it can store at least NELEM elements of SIZE
  91    bytes.  The buffer contents are NOT preserved.  Both NELEM and SIZE
  92    can be zero.  Return true on success, false on allocation failure
  93    (in which case the old buffer is freed, but *BUFFER remains in a
  94    free-able state, and errno is set).  It is unspecified whether this
  95    function can reduce the array size.  */
  96 #if 0
  97 extern bool scratch_buffer_set_array_size (struct scratch_buffer *buffer,
  98                                            size_t nelem, size_t size);
  99 #endif
 100
 101
 102 /* The implementation is imported from glibc.  */
 103
 104 /* Avoid possible conflicts with symbols exported by the GNU libc.  */
 105 #define __libc_scratch_buffer_grow gl_scratch_buffer_grow
 106 #define __libc_scratch_buffer_grow_preserve gl_scratch_buffer_grow_preserve
 107 #define __libc_scratch_buffer_set_array_size gl_scratch_buffer_set_array_size
 108
 109 #ifndef _GL_LIKELY
 110 /* Rely on __builtin_expect, as provided by the module 'builtin-expect'.  */
 111 # define _GL_LIKELY(cond) __builtin_expect ((cond), 1)
 112 # define _GL_UNLIKELY(cond) __builtin_expect ((cond), 0)
 113 #endif
 114
 115 #include <malloc/scratch_buffer.gl.h>
 116
 117 #endif /* _GL_SCRATCH_BUFFER_H */