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add some comments to block_split and block_trim_free_leading

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Ivan Grokhotkov
2021-12-13 22:15:44 +01:00
committed by Guillaume Souchere
parent 7d2c0f2264
commit 1322e839d2
1 changed files with 36 additions and 2 deletions
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38
tlsf.c
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@@ -402,10 +402,15 @@ static inline __attribute__((always_inline)) int block_can_split(block_header_t*
/* Split a block into two, the second of which is free. */
static inline __attribute__((always_inline)) block_header_t* block_split(block_header_t* block, size_t size)
{
/* Calculate the amount of space left in the remaining block. */
/* Calculate the amount of space left in the remaining block.
* REMINDER: remaining pointer's first field is `prev_phys_block` but this field is part of the
* previous physical block. */
block_header_t* remaining =
offset_to_block(block_to_ptr(block), size - block_header_overhead);
/* `size` passed as an argument is the first block's new size, thus, the remaining block's size
* is `block_size(block) - size`. However, the block's data must be precedeed by the data size.
* This field is NOT part of the size, so it has to be substracted from the calculation. */
const size_t remain_size = block_size(block) - (size + block_header_overhead);
tlsf_assert(block_to_ptr(remaining) == align_ptr(block_to_ptr(remaining), ALIGN_SIZE)
@@ -418,6 +423,29 @@ static inline __attribute__((always_inline)) block_header_t* block_split(block_h
block_set_size(block, size);
block_mark_as_free(remaining);
/**
* Here is the final outcome of this function:
*
* block remaining (block_ptr + size - BHO)
* + +
* | |
* v v
* +----------------------------------------------------------------------+
* |0000| |xxxxxxxxxxxxxxxxxxxxxx|xxxx| |###########################|
* |0000| |xxxxxxxxxxxxxxxxxxxxxx|xxxx| |###########################|
* |0000| |xxxxxxxxxxxxxxxxxxxxxx|xxxx| |###########################|
* |0000| |xxxxxxxxxxxxxxxxxxxxxx|xxxx| |###########################|
* +----------------------------------------------------------------------+
* | | | |
* + +<------------------------->+ +<------------------------->
* BHO `size` (argument) bytes BHO `remain_size` bytes
*
* Where BHO = block_header_overhead,
* 0: part of the memory owned by a `block`'s previous neighbour,
* x: part of the memory owned by `block`.
* #: part of the memory owned by `remaining`.
*/
return remaining;
}
@@ -495,11 +523,17 @@ static inline __attribute__((always_inline)) block_header_t* block_trim_free_lea
block_header_t* remaining_block = block;
if (block_can_split(block, size))
{
/* We want the 2nd block. */
/* We want to split `block` in two: the first block will be freed and the
* second block will be returned. */
remaining_block = block_split(block, size - block_header_overhead);
/* `remaining_block` is the second block, mark its predecessor (first
* block) as free. */
block_set_prev_free(remaining_block);
block_link_next(block);
/* Put back the first block into the free memory list. */
block_insert(control, block);
}