size_t

 

C

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Concurrency support (C11)

Technical Specifications

Symbol index

 

Type support

size_t

ptrdiff_t

nullptr_t (C23)

NULL

max_align_t (C11)

offsetof

Numeric limits

Fixed width integer types (C99)

 

Defined in header <stddef.h>
Defined in header <stdio.h>
Defined in header <stdlib.h>
Defined in header <string.h>
Defined in header <time.h>
Defined in header <uchar.h>		(since C11)
Defined in header <wchar.h>		(since C95)
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.

The bit width of size_t is not less than 16.

(since C99)

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

Run this code

#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);
}

Possible output:

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

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