# ptrdiff_t

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< 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)

### 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`).

### Example

Run this code

    #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

### References

- 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

size_t | unsigned integer type returned by the `sizeof` operator (typedef)
offsetof | byte offset from the beginning of a struct type to specified member (function macro)

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