Defined in header <stddef.h>  

typedef /*implementationdefined*/ ptrdiff_t; 
ptrdiff_t
is the signed integer type of the result of subtracting two pointers.
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. 64bit systems when the index exceeds INT_MAX
or if it relies on 32bit 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 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
).
#include <stdio.h> #include <stddef.h> #include <stdint.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 = p2p1; printf("p2p1 = %td\n", diff); return 0; }
Possible output:
PTRDIFF_MAX = 9223372036854775807 p2p1 = 5
unsigned integer type returned by the sizeof operator (typedef) 

byte offset from the beginning of a struct type to specified member (function macro) 

C++ documentation for ptrdiff_t 
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