size_t

Defined in header `<stddef.h>`
Defined in header `<stdio.h>`
Defined in header `<string.h>`
Defined in header `<time.h>`
typedef /implementation-defined/ size_t;

size_t is the unsigned integer type of the result of sizeof , alignof (since C11) and offsetof.

[edit] 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.

[edit] Example

Run this code

#include <stdio.h>
#include <stddef.h>
#include <stdint.h>
 
int main(void)
{
    const size_t N = 100;
    int numbers[N];
    for (size_t ndx = 0; ndx < N; ++ndx)
        numbers[ndx] = ndx;
    printf("SIZE_MAX = %lu\n", SIZE_MAX);
    size_t size = sizeof(numbers);
    printf("size = %zu\n", size);
 
    return 0;
}

Possible output:

SIZE_MAX = 18446744073709551615
size = 400

[edit] See also

ptrdiff_t	signed integer type returned when subtracting two pointers (typedef)
offsetof	byte offset from the beginning of a struct type to specified member (function macro)
C++ documentation for size_t

Language
headers
Type support
Dynamic memory management
Error handling
Program utilities
Variadic function support
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Thread support (C11)
Atomic operations (C11)
Technical Specifications