和 calloc 之间的区别？
What is the difference between doing:
ptr = (char **) malloc (MAXELEMS * sizeof(char *));
or:
ptr = (char **) calloc (MAXELEMS, sizeof(char*));
When is it a good idea to use calloc over malloc or vice versa?
转载于:https://*.com/questions/1538420/difference-between-malloc-and-calloc
calloc() zero-initializes the buffer, while malloc() leaves the memory uninitialized.
EDIT:
Zeroing out the memory may take a little time, so you probably want to use malloc() if that performance is an issue. If initializing the memory is more important, use calloc(). For example, calloc() might save you a call to memset().
A less known difference is that in operating systems with optimistic memory allocation, like Linux, the pointer returned by malloc isn't backed by real memory until the program actually touches it.
calloc does indeed touch the memory (it writes zeroes on it) and thus you'll be sure the OS is backing the allocation with actual RAM (or swap). This is also why it is slower than malloc (not only does it have to zero it, the OS must also find a suitable memory area by possibly swapping out other processes)
See for instance this SO question for further discussion about the behavior of malloc
One often-overlooked advantage of calloc is that (conformant implementations of) it will help protect you against integer overflow vulnerabilities. Compare:
size_t count = get_int32(file);
struct foo *bar = malloc(count * sizeof *bar);
vs.
size_t count = get_int32(file);
struct foo *bar = calloc(count, sizeof *bar);
The former could result in a tiny allocation and subsequent buffer overflows, if count is greater than SIZE_MAX/sizeof *bar. The latter will automatically fail in this case since an object that large cannot be created.
Of course you may have to be on the lookout for non-conformant implementations which simply ignore the possibility of overflow... If this is a concern on platforms you target, you'll have to do a manual test for overflow anyway.