< c‎ | types
Defined in header <stddef.h>
typedef /*implementation-defined*/ ptrdiff_t;

ptrdiff_t is the signed integer type of the result of subtracting two pointers.

The bit width of ptrdiff_t is not less than 17.

(since C99)
(until C23)

The bit width of ptrdiff_t is not less than 16.

(since C23)


[edit] Notes

ptrdiff_t is used for pointer arithmetic and array indexing, if negative values are possible. Programs that use other types, such as int, may fail on, e.g. 64-bit systems when the index exceeds INT_MAX or if it relies on 32-bit modular arithmetic.

Only pointers to elements of the same array (including the pointer one past the end of the array) may be subtracted from each other.

If an array is so large (greater than PTRDIFF_MAX elements, but equal to or less than SIZE_MAX bytes), that the difference between two pointers may not be representable as ptrdiff_t, the result of subtracting two such pointers is undefined.

For char arrays shorter than PTRDIFF_MAX, ptrdiff_t acts as the signed counterpart of size_t: it can store the size of the array of any type and is, on most platforms, synonymous with intptr_t).

[edit] Possible implementation

typedef typeof((int*)nullptr - (int*)nullptr) ptrdiff_t; // valid since C23

[edit] Example

#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
int main(void)
    const size_t N = 100;
    int numbers[N];
    printf("PTRDIFF_MAX = %ld\n", PTRDIFF_MAX);
    int *p1 = &numbers[18], *p2 = &numbers[23];
    ptrdiff_t diff = p2 - p1;
    printf("p2-p1 = %td\n", diff);

Possible output:

PTRDIFF_MAX = 9223372036854775807
p2-p1 = 5

[edit] References

  • C23 standard (ISO/IEC 9899:2023):
  • 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>

[edit] See also

unsigned integer type returned by the sizeof operator
(typedef) [edit]
byte offset from the beginning of a struct type to specified member
(function macro) [edit]
C++ documentation for ptrdiff_t