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Fixed a bug when malloc aligns the data itself - Added comments
#include <stdlib.h> #include <stdio.h> #include "mem_align.h" void *xvid_malloc(size_t size, uint8_t alignment) { uint8_t *mem_ptr; if(!alignment) { /* We have not to satisfy any alignment */ if((mem_ptr = (uint8_t *) malloc(size + 1)) != NULL) { /* Store (mem_ptr - "real allocated memory") in *(mem_ptr-1) */ *mem_ptr = 0; /* Return the mem_ptr pointer */ return (void *) mem_ptr++; } } else { uint8_t *tmp; /* * Allocate the required size memory + alignment so we * can realign the data if necessary */ if((tmp = (uint8_t *) malloc(size + alignment)) != NULL) { /* Align the tmp pointer */ mem_ptr = (uint8_t *)((uint32_t)(tmp + alignment - 1)&(~(uint32_t)(alignment - 1))); /* * Special case where malloc have already satisfied the alignment * We must add alignment to mem_ptr because we must store * (mem_ptr - tmp) in *(mem_ptr-1) * If we do not add alignment to mem_ptr then *(mem_ptr-1) points * to a forbidden memory space */ if(mem_ptr == tmp) mem_ptr += alignment; /* * (mem_ptr - tmp) is stored in *(mem_ptr-1) so we are able to retrieve * the real malloc block allocated and free it in xvid_free */ *(mem_ptr - 1) = (uint8_t)(mem_ptr - tmp); /* Return the aligned pointer */ return (void *) mem_ptr; } } return NULL; } void xvid_free(void *mem_ptr) { /* *(mem_ptr - 1) give us the offset to the real malloc block */ free((uint8_t*)mem_ptr - *((uint8_t*)mem_ptr - 1)); }
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