offsetof - cppreference.com (original) (raw)
| | | | | ----------------------------------------------------------- | | | | #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. | (since C23) |
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[edit] Notes
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) |
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[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 |