atoi(3)

✨ atoi, atol, atoll — bestie edition ✨

okay so you want to turn a string into an integer. say less 💅 this is atoi(3), it lives in the Standard C library (libc, link it with -lc), and honestly? she's giving "easy to use, hard to trust." let's get into it 🫶

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🌸 NAME 🌸

atoi, atol, atoll — convert a string to an integer. that's it, that's the whole vibe ✨

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📚 LIBRARY 📚

Standard C library (libc, -lc) — yeah you gotta link her in, no cap.

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🎀 SYNOPSIS 🎀

#include <stdlib.h>

int atoi(const char *nptr);
long atol(const char *nptr);
long long atoll(const char *nptr);

🦋 Feature Test Macro Requirements for glibc 🦋

(if you're confused what this is, go read feature_test_macros(7), she explains everything 💜)

for atoll():

_ISOC99_SOURCE
    || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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💖 DESCRIPTION 💖

so atoi() takes the initial portion of the string that nptr points at and converts it to an int. and the behavior is literally identical to:

strtol(nptr, NULL, 10);

except — and this is the part, this is the part — atoi() does not detect errors. not even a little. she's not checking anything. she's just vibing 😭

atol() and atoll() do the exact same thing as atoi(), the only difference is they convert that initial portion into their own return type — long or long long respectively. same energy, bigger container 🍓

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🔥 RETURN VALUE 🔥

the converted value. or 0 on error. (yes, that ambiguity is a whole thing, we'll get to it in BUGS, bestie, don't worry 👀)

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⭐ ATTRIBUTES ⭐

for what all these terms even mean, peep attributes(7) — she's the source of truth 🧚

here's the rundown for our girlies:

• Interface: atoi(), atol(), atoll()
  • Attribute: Thread safety
  • Value: MT-Safe locale

so yeah, thread-safe, for the locale. slay 💅

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🌈 VERSIONS 🌈

POSIX.1 leaves the return value of atoi() on error unspecified — like, do whatever, we're not checking 🤷‍♀️ but on glibc, musl libc, and uClibc? 0 is returned on error. consistent queens 👑

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👑 STANDARDS 👑

C11, POSIX.1-2008. periodt.

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📜 HISTORY 📜

C99, POSIX.1-2001, SVr4, 4.3BSD.

fun little lore drop: C89 and POSIX.1-1996 include the functions atoi() and atol() only. atoll() wasn't even invited yet 💀

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💀 BUGS 💀

okay bestie, sit down, this is the part where it gets real 🙈

• errno is not set on error, so there is literally no way to distinguish between 0 as an error and 0 as the converted value. she said "same thing" and meant it 😭
• No checks for overflow or underflow are done. none. zero. nada. it'll just do whatever 🔥
• Only base-10 input can be converted. that's the whole menu, babe.
• the recommendation: for new programs, use the strtol() and strtoul() family of functions instead. this is them telling you to upgrade, fr fr 🫶

ngl the "recommended to instead use strtol() and strtoul()" line is doing a lot of heavy lifting in this section. listen to her. she knows. 👑

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🦋 SEE ALSO 🦋

• atof(3)
• strtod(3)
• strtol(3)
• strtoul(3)

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and that's the whole page, bestie 💖 this was pulled straight from Linux man-pages 6.20, dated 2026-02-08. go forth, convert some strings, and please please please use strtol() okay? ily, stay safe 🫶✨🎀