C malloc
When you use a pointer point to a malloc allocated memory space, then you can’t use the sizeof to get the size of the memory. Instead of using sizeof, you can use the malloc_usable_size function to get the number of usable bytes in the block pointed to by ptr, a pointer to a block of memory allocated by malloc(3) or a related function. Note that The value returned by malloc_usable_size() may be greater than the requested size of the allocation because of alignment and minimum size constraints. Although the excess bytes can be overwritten by the application without ill effects, this is programming practice: the number of excess bytes in an allocation depends on the underlying implementation. For example:
#include <malloc.h>
char* a = malloc(1024);
printf("The usable malloc size is %lu\n", malloc_usable_size(a)); // The output is not 1024 but 1032
malloc, realloc and calloc
From the linux manual about these API, we can get the following information.
SYNOPSIS
#include <stdlib.h>
void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
void *reallocarray(void *ptr, size_t nmemb, size_t size);
DESCRIPTION
The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be success‐
fully passed to free().
The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc(), or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.
The calloc() function allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory. The memory is set to zero. If nmemb or size is 0, then calloc() returns either NULL, or a unique pointer value that can later be successfully passed to free(). If the multiplication of nmemb and size would result in integer overflow, then calloc() returns an error. By contrast, an integer overflow would not be detected in the following call to malloc(), with the result that an incorrectly sized block of memory would be allocated:
malloc(nmemb * size);
The realloc() function changes the size of the memory block pointed to by ptr to size bytes. The contents will be unchanged in the range from the start of the region up to the minimum of the old and new sizes. If the new size is larger than the old size, the added memory will not be initialized. If ptr is NULL, then the call is equivalent to malloc(size), for all values of size; if size is equal to zero, and ptr is not NULL, then the call is equivalent to free(ptr). Unless ptr is NULL, it must have been returned by an earlier call to malloc(), calloc(), or realloc(). If the area pointed to was moved, a free(ptr) is done.
The reallocarray() function changes the size of the memory block pointed to by ptr to be large enough for an array of nmemb elements, each of which is size bytes. It is equivalent to the call
realloc(ptr, nmemb * size);
However, unlike that realloc() call, reallocarray() fails safely in the case where the multiplication would overflow. If such an overflow occurs, reallocarray() returns NULL, sets errno to ENOMEM, and leaves the original block of memory unchanged.
RETURN VALUE
The malloc() and calloc() functions return a pointer to the allocated memory, which is suitably aligned for any built-in type. On error, these functions return NULL. NULL may also be returned by a successful call to malloc() with a size of zero, or by a successful call to calloc() with nmemb or size equal to zero.
The free() function returns no value.
The realloc() function returns a pointer to the newly allocated memory, which is suitably aligned for any built-in type, or NULL if the request failed. The returned pointer may be the same as ptr if the allocation was not moved (e.g., there was room to expand the allocation in-place), or different from ptr if the allocation was moved to a new address. If size was equal to 0, either NULL or a pointer suitable to be passed to free() is returned. If realloc() fails, the original block is left untouched; it is not freed or moved.
On success, the reallocarray() function returns a pointer to the newly allocated memory. On failure, it returns NULL and the original block of memory is left untouched.
2020-02-09
Reference introduction: Overview of malloc