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struct a { struct b { int i; float j; }x; struct c { int k; float l; }y; }z; Can anybody explain me how to find the offset of int k so that we can find the address of int i?
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Description. The C library macro offsetof(type, member-designator) results in a constant integer of type size_t which is the offset in bytes of a structure member from the beginning of the structure. The member is given by member-designator, and the name of the structure is given in type.
The offsetof() macro returns the offset of the element name within the struct or union composite. This provides a portable method to determine the offset. At this point, your eyes start to glaze over, and you move on to something that's more understandable and useful.
Use offsetof() to find the offset from the start of z or from the start of x.
offsetof() - offset of a structure member
SYNOPSIS
#include <stddef.h> size_t offsetof(type, member); offsetof() returns the offset of the field member from the start of the structure type.
EXAMPLE
#include <stddef.h> #include <stdio.h> #include <stdlib.h> int main(void) { struct s { int i; char c; double d; char a[]; }; /* Output is compiler dependent */ printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n", (long) offsetof(struct s, i), (long) offsetof(struct s, c), (long) offsetof(struct s, d), (long) offsetof(struct s, a)); printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s)); exit(EXIT_SUCCESS); } You will get the following output on a Linux, if you compile with GCC:
offsets: i=0; c=4; d=8 a=16 sizeof(struct s)=16 If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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