putenv(3)

💖 putenv(3) — the "change or add an environment variable" era 💖

okay bestie, we're in putenv(3) territory, which lives in the Library Functions Manual (that's the manual name, iykyk) and it's giving chaotic-but-iconic. Let's get into it 🫶

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✨ NAME ✨

putenv — change or add an environment variable. that's it. that's the whole job. 💅

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📚 LIBRARY 📚

Standard C library (libc, -lc). the blueprint, the moment, the classic. 🎀

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🌸 SYNOPSIS 🌸

#include <stdlib.h>

int putenv(char *string);

🦋 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

putenv():
    _XOPEN_SOURCE
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE

yes gorgeous, you gotta declare your vibe (I mean your feature test macro) before glibc will let you have this one. slots open, no cap ✨

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💜 DESCRIPTION 💜

so putenv() adds or changes the value of environment variables. lowkey iconic behaviour.

the argument string is of the form name=value. classic.

here's the tea ☕:

• if name does not already exist in the environment, then string is added to the environment. new entry, who dis 💖
• if name does exist, then the value of name in the environment is changed to value. makeover time 💅

and now the part that makes this function spicy: the string pointed to by string becomes part of the environment, so altering the string changes the environment. not a copy, bestie. it's your string now. no take-backs, no backsies 🙈

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⭐ RETURN VALUE ⭐

putenv() returns zero on success. slay.

on failure, it returns a nonzero value, and errno is set to indicate the error. sad girl hours 😭

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💀 ERRORS 💀

• ENOMEM — Insufficient space to allocate new environment. the environment said "no room, I'm booked" 💀

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👑 ATTRIBUTES 👑

for an explanation of the terms used in this section, see attributes(7).

• Interface: putenv()
• Attribute: Thread safety
• Value: MT-Unsafe const:env

so like... thread safety? not it. she's MT-Unsafe const:env, which is the technical way of saying "please don't share her across threads while she's mid-glow-up" 🫠

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🌈 STANDARDS 🌈

POSIX.1-2008. certified, periodt ✨

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🧚 HISTORY 🧚

POSIX.1-2001, SVr2, 4.3BSD-Reno. ancient lineage, main character energy since forever 👑

putenv() is not required to be reentrant, and the one in glibc 2.0 is not, but the glibc 2.1 version is. glow-up confirmed 💅

Since glibc 2.1.2, the glibc implementation conforms to SUSv2: the pointer string given to putenv() is used. In particular, this string becomes part of the environment; changing it later will change the environment.

⚠️ ok so this is the DRAMATIC part: (Thus, it is an error to call putenv() with an automatic variable as the argument, then return from the calling function while string is still part of the environment.) like, if you put a stack variable in there and then yeet out of the function, the environment is holding a pointer to something that no longer exists. that's undefined behaviour-flavoured chaos. do not. 🙈

However, from glibc 2.0 to glibc 2.1.1, it differs: a copy of the string is used. on the one hand this causes a memory leak, and on the other hand it violates SUSv2. baby glibc was messy fr fr 😭

The 4.3BSD-Reno version, like glibc 2.0, uses a copy; this is fixed in all modern BSDs. growth! healing! 🌱

SUSv2 removes the const from the prototype, and so does glibc 2.1.3. bye const 👋

✨ the nonstandard bonus round ✨

The GNU C library implementation provides a nonstandard extension. If string does not include an equal sign:

putenv("NAME");

then the named variable is removed from the caller's environment. surprise! she can also ghost variables 💅✨

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🎀 SEE ALSO 🎀

• clearenv(3)
• getenv(3)
• setenv(3)
• unsetenv(3)
• environ(7)

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and that's the whole page, bestie 🫶 this was Linux man-pages 6.20, dated 2026-02-08, and we read putenv(3) together like the icons we are. go forth and not pass it a stack variable okay? mwah 💖🌸🦋