Thread: malloc - verifying amount of memory allocated

How can I view the number of bytes that have been allocated by using the malloc function?

I tried:

mem = (float*)malloc(num*sizeof(float));

printf("The amount of memory allocated using malloc is %d.", mem);

Note: The variable "num" in my program is equal to 7.

But every time I run the program, this value changes.

The size is precisely the size you requested. If it was unable to be allocated in full, malloc returns NULL.

The program is printing the address of the allocated memory, not the size. malloc() doesn't return the size of the actual amount of memory allocated. malloc() uses internal structures to keep track of the amount of allocated memory, but those structures will be different depending on the operating system and/or the compiler being used. A program wouldn't normally be able to access those internal structures, so there's no real way to verify the size requested, other than to trust that malloc() worked if it returns a non-NULL pointer.

Originally Posted by rcgldr

The program is printing the address of the allocated memory, not the size. malloc() doesn't return the size of the actual amount of memory allocated. malloc() uses internal structures to keep track of the amount of allocated memory, but those structures will be different depending on the operating system and/or the compiler being used. A program wouldn't normally be able to access those internal structures, so there's no real way to verify the size requested, other than to trust that malloc() worked if it returns a non-NULL pointer.

I think you're confusing the op now. Perhaps this will make things clear:

Code:

char * p;
size_t sz = 7 * sizeof *p;

if (p = malloc(sz))
  fprintf(stderr, "Successfully allocated _at least_ %lu bytes at pointer %p\n", sz, p);
else
  fprintf(stderr, "Failed to allocate %lu bytes\n", sz);

It may be confusing but the op should start to learn that a pointer is an address and that valgrind is life. And that tcmalloc also exists and that google is da best!

@mutantjohn: google iz 4 kidz, u got 2 write ur own web crawler!

@nonpuz: n u got 2 cast sz to unsigned long in call 2 fprintf!

I just felt like using a different writing style. ^_^

Originally Posted by std10093

@Barney: Hey, probably some letters in your keyboard have jammed or something... Consider changing your keyboard as soon as possible, if that is the case.

Personally I thought it looked kewl.

google iz 4 kidz, u got 2 write ur own web crawler!

XD

I heard tcmalloc has multithreaded performance comparable to hoard which compared to malloc is like 8x better. Granted, that's for multithreaded allocations but those might be more common than you think.