NULL(3const)

💖 NULL(3const) 💖

okay so bestie. today we're talking about NULL, from the <stddef.h> corner of the Standard C library (that's libc, mwah 😘). this is a section 3const page, which is man-speak for "it's a constant, not a function, don't @ me". she's a legend. she's been here since before any of us were born. let's get into it 🫶

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✨ NAME ✨

NULL — the null pointer constant 🕳️✨

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📚 LIBRARY 📚

Your one and only Standard C library, aka libc. the blueprint. the originator. 💅

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🦋 SYNOPSIS 🦋

#include <stddef.h>

#define NULL  ((void *) 0)

that's it. that's the whole thing. one macro, infinite power. 🎀

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🌸 DESCRIPTION 🌸

NULL is a null pointer constant, which is a fancy way of saying: it's a pointer that literally does not point at anything. it's pointing into the void. it's at the function but nobody's home. 🫥

not "points at zero bytes", not "points at nothingness the place" — it just isn't pointing. periodt. 💜

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⭐ STANDARDS ⭐

• C11 💖
• POSIX.1-2024 💖

both said yes, both put a ring on it. 👑

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🌈 HISTORY 🌈

• C89 🌱
• POSIX.1-1988 🌱

so she's been famous since the 80s. vintage. iconic. genuinely older than your parents' playlist. 😭

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🍓 NOTES 🍓

NULL isn't gatekeeping, okay? she hangs out in so many headers besides <stddef.h>. these ones all provide her too:

• <locale.h>
• <stdio.h>
• <stdlib.h>
• <string.h>
• <time.h>
• <unistd.h>
• <wchar.h>

so if your code includes any of those and suddenly NULL just... works? yeah. that's why. iykyk 🧚

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💀 CAVEATS 💀

okay bestie, this is the part where we sit down and have a serious little chat. ☕

dereferencing a null pointer is undefined behavior, and in practice that usually means a segmentation fault. so like. don't. obviously.

it's also undefined behavior to do pointer arithmetic on it. so NULL + 1? that's not cute. that's not a vibe. that's UB.

and NULL - NULL? undefined behavior according to ISO C 😭 — but it's defined to be 0 in C++. yes, the two languages have beef about this. classic.

to avoid confusing the poor humans who have to read your code later (hi, it's me, i'm the human 🙈), don't compare pointer variables to 0, and don't assign 0 to them either. just always use NULL. literally always. no cap. 👑

and please — do not confuse NULL with NUL. different girl completely. NUL is the ascii(7) character, and in C she's written as '\0'. NULL is a pointer. NUL is a character. they are not the same. they are not sister cities. they are not even mutuals. 🌸

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🐛 BUGS 🐛

okay so say you need to set a pointer variable to a null pointer. would you reach for memset(3) to zero it out? (this is usually what people do when they're zeroing a whole struct that happens to contain pointers. very common. very understandable. very 🚩.)

don't. because ISO C and POSIX do not guarantee that a bit pattern of all 0s represents a null pointer. like. it might. it might not. it's giving gambling. 💀

want an example program that does this correctly? go look at the EXAMPLES section in getaddrinfo(3). go. now. 🔥

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👑 SEE ALSO 👑

• void(3type)

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that's the whole page, bestie!! 🎀💖 if you take nothing else from this: use NULL, never deref it, and do not let anyone tell you NUL is the same girl. 💅✨

this page came to you from Linux man-pages 6.20, dated 2026-01-05 — and the entry itself is NULL(3const). mwah, ily, go write some safe code. 🫶🌸💜