size_t

size_t is the unsigned integer type of the result of sizeof , offsetof and _Alignof (until C23) alignof (since C23) , depending on the data model .

Notes

size_t can store the maximum size of a theoretically possible object of any type (including array).

size_t is commonly used for array indexing and loop counting. Programs that use other types, such as unsigned int , for array indexing may fail on, e.g. 64-bit systems when the index exceeds UINT_MAX or if it relies on 32-bit modular arithmetic.

Possible implementation

typedef typeof(sizeof(0)) size_t; // valid since C23

Example

#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
 
int main(void)
{
    const size_t N = 101;
    int numbers[N];
    size_t sum = 0;
    for (size_t ndx = 0; ndx < N; ++ndx)
        sum += numbers[ndx] = ndx;
    size_t size = sizeof numbers;
    printf("sum = %zu\n", sum);
    printf("size = %zu\n", size);
    printf("SIZE_MAX = %zu\n", SIZE_MAX);
}

sum = 5050
size = 404
SIZE_MAX = 18446744073709551615

References

See also