How to correctly check the size passed to malloc() calloc() realloc() as well as their return value?
allocation, c, pointers, return-value, size
Solution
It is correct to do the checks, some would say pedantic. Some thoughts
#include <stdint.h>
...
// MAX_CHAIN > SIZE_MAX/sizeof(char*)
MAX_CHAIN > (SIZE_MAX - sizeof(char*) +1)/sizeof(char*)
// This deals with SIZE_MAX/sizeof(char*) rounding toward 0
A return value of NULL is valid if memory allocation size is 0.
Watch for attempted negative memory allocation requests as many requests are formed with `int` (signed) even though the request takes `size_t` (unsigned).
Memory allocation is limited to `SIZE_MAX` as in typical systems cannot allocate `(size_t)-1` or any where new that. Look at the C standard definition of `RSZIE_MAX` which is closely related to your concerns.
C11dr §K.3.4 3 Extremely large object sizes are frequently a sign that an object’s size was calculated incorrectly. For example, negative numbers appear as very large positive numbers when converted to an unsigned type like size_t. Also, some implementations do not support objects as large as the maximum value that can be represented by type size_t.
Problem
If I have understood correctly, it is also important to check that the desired space passed to malloc(), calloc() or realloc() does not exceed the SIZE_MAX constant after the multiplication with the sizeof(desired_type). In order to make sure that does not happen I started to do something like this in my codes: ``` #define MAX_CHAIN (50) #ifndef SIZE_MAX #define SIZE_MAX (~(size_t)0) #endif int main(int argc, char const *argv[]){ char **parsed_string; if(MAX_CHAIN > SIZE_MAX/sizeof(char*) || (parsed_string=(char**)malloc(MAX_CHAIN * sizeof(char*)))==NULL){ printf("%s\n", "Error, cannot allocate memory"); exit(1); } /*Otherwise pointer is valid and the execution continues*/ } ``` So it basically checks for two things: - That the requested memory is not bigger that SIZE_MAX - That the pointer returned by malloc() is valid. My questions are: Is it a correct to do these checks? Is this considered as good practice or should I use something else instead? Also, can someone explain why the allocation is limited by SIZE_MAX?