offsetof(3) Library Functions Manual offsetof(3) offsetof - (libc -lc) #include size_t offsetof(type, member); offsetof() member type. . . member ( ). offsetof() member type . C11, POSIX.1-2008. POSIX.1-2001 C89. Linux/i386 gcc(1) : $ ./a.out offsets: i=0; c=4; d=8 a=16 sizeof(struct s)=16 #include #include #include int main(void) { struct s { int i; char c; double d; char a[]; }; /* Output is compiler dependent */ printf("offsets: i=%zu; c=%zu; d=%zu a=%zu\n", offsetof(struct s, i), offsetof(struct s, c), offsetof(struct s, d), offsetof(struct s, a)); printf("sizeof(struct s)=%zu\n", sizeof(struct s)); exit(EXIT_SUCCESS); } 3 . . : . 6.18 8 2026 offsetof(3)