#define offsetof(type, member) /*implementation-defined*/
The macro offsetof expands to an integer constant expression of type size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified subobject, including padding if any.
Given an object o of type type with static storage duration, &(o.member) shall be an address constant expression and point to a subobject of o. Otherwise, the behavior is undefined.
If the type name specified in type contains a comma not between matching parentheses, the behavior is undefined.
If offsetof is applied to a bit-field member, the behavior is undefined, because the address of a bit-field cannot be taken.
member is not restricted to a direct member. It can denote a subobject of a given member, such as an element of an array member.
Even though it is specified in C23 that specifying a new type containing an unparenthesized comma in offsetof is undefined behavior, such usage is generally not supported even in earlier modes: offsetof(struct Foo { int a, b; }, a) generally fails to compile.
typeof can be used to avoid the bad effect of commas in the definition of a new type, e.g. offsetof(typeof(struct { int i, j; }), i) is well-defined.
(since C23) [edit] Example#include <stdio.h>
#include <stddef.h>
struct S {
char c;
double d;
};
int main(void)
{
printf("the first element is at offset %zu\n", offsetof(struct S, c));
printf("the double is at offset %zu\n", offsetof(struct S, d));
}
Possible output:
the first element is at offset 0 the double is at offset 8[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C standards.
DR Applied to Behavior as published Correct behavior DR 496 C89 only structs and struct members were mentioned unions and other subobjects are also supported [edit] See alsoRetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4