offsetof(3)

✨ offsetof(3) — Library Functions Manual ✨

okay bestie, gather round, we're talking about offsetof(3), your new fave little macro from the Library Functions Manual 💖

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🌸 NAME 🌸

offsetof — the offset of a structure member. that's it. that's the whole vibe. 💅

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🎀 LIBRARY 🎀

Standard C library (libc, -lc). you know her, you love her 🫶

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✨ SYNOPSIS ✨

#include <stddef.h>

size_t offsetof(type, member);

just one include and you're in, no cap 🌈

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💜 DESCRIPTION 💜

so what does offsetof() actually do, bestie? it's a macro that returns the offset of the field member from the start of the structure type. like, how many bytes into the struct is that field sitting. iconic behavior honestly ✨

and the reason this macro is SO useful (main character energy fr) is that the sizes of the fields that compose a structure can vary across implementations, and compilers may insert different numbers of padding bytes between fields. 😳 so because of all that padding drama, an element's offset is not necessarily given by just adding up the sizes of the previous elements. do the math yourself and you WILL get played. use the macro. periodt. 👑

🚨 heads up: a compiler error will result if member is not aligned to a byte boundary — i.e., if it's a bit field. bit fields? not invited to this party. sorry babe 💀

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🌟 RETURN VALUE 🌟

offsetof() returns the offset of the given member within the given type, in units of bytes. simple, clean, slay 🦋

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📚 STANDARDS 📚

C11, POSIX.1-2008. certified, stamped, valid 💅

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🕰️ HISTORY 🕰️

POSIX.1-2001, C89. she's been around since forever and she's still THAT girl 🌸

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🧚 EXAMPLES 🧚

okay so on a Linux/i386 system, when compiled using the default gcc(1) options, the program below produces the following output:

$ ./a.out
offsets: i=0; c=4; d=8 a=16
sizeof(struct s)=16

look at those numbers. gorgeous. 🎀

🍓 Program source 🍓

#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=%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);
}

💡 lowkey important: that comment says Output is compiler dependent — so don't @ me if your numbers hit different on your machine. it's giving implementation defined, fr fr. 🙈

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and that's the whole page bestie!! this one's from Linux man-pages 6.20, dated 2026-02-08, and it lives at offsetof(3). go forth and measure your structs correctly, you absolute queen 👑💖 bye ily!! ✨🦋🌸