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      1 /* Variable-sized buffer with on-stack default allocation.
      2    Copyright (C) 2017-2022 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 /* Return a copy of *BUFFER's first SIZE bytes as a heap-allocated block,
    102    deallocating *BUFFER if it was heap-allocated.  SIZE must be at
    103    most *BUFFER's size.  Return NULL (setting errno) on memory
    104    exhaustion.  */
    105 #if 0
    106 extern void *scratch_buffer_dupfree (struct scratch_buffer *buffer,
    107                                      size_t size);
    108 #endif
    109 
    110 
    111 /* The implementation is imported from glibc.  */
    112 
    113 /* Avoid possible conflicts with symbols exported by the GNU libc.  */
    114 #define __libc_scratch_buffer_dupfree gl_scratch_buffer_dupfree
    115 #define __libc_scratch_buffer_grow gl_scratch_buffer_grow
    116 #define __libc_scratch_buffer_grow_preserve gl_scratch_buffer_grow_preserve
    117 #define __libc_scratch_buffer_set_array_size gl_scratch_buffer_set_array_size
    118 
    119 #ifndef _GL_LIKELY
    120 /* Rely on __builtin_expect, as provided by the module 'builtin-expect'.  */
    121 # define _GL_LIKELY(cond) __builtin_expect ((cond), 1)
    122 # define _GL_UNLIKELY(cond) __builtin_expect ((cond), 0)
    123 #endif
    124 
    125 #include <malloc/scratch_buffer.gl.h>
    126 
    127 #endif /* _GL_SCRATCH_BUFFER_H */
    128