size_t

typedef /*implementation-defined*/ 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 unsignedint, 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

• C23 standard (ISO/IEC 9899:2024):
• 7.19 Common definitions <stddef.h> (p: TBD)
• 7.20.3 Limits of other integer types (p: TBD)
• C17 standard (ISO/IEC 9899:2018):
• 7.19 Common definitions <stddef.h> (p: 211)
• 7.20.3 Limits of other integer types (p: 215)
• C11 standard (ISO/IEC 9899:2011):
• 7.19 Common definitions <stddef.h> (p: 288)
• 7.20.3 Limits of other integer types (p: 293)
• C99 standard (ISO/IEC 9899:1999):
• 7.17 Common definitions <stddef.h> (p: 253)
• 7.18.3 Limits of other integer types (p: 258)
• C89/C90 standard (ISO/IEC 9899:1990):
• 4.1.6 Common definitions <stddef.h>

See also