initgroups(3)

💖 initgroups — the glibc manual page, but make it cute 💖

okay bestie, so this one lives in the Library Functions Manual (section 3, the libc girlies 🌸). It's called initgroups, and it's the little function that sorts out your supplementary group access list. Yes it's giving sysadmin. Yes it's important. Let's go 🫶

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✨ NAME ✨

initgroups — initialize the supplementary group access list

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📚 LIBRARY 📚

Standard C library (libc, -lc)

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🌈 SYNOPSIS 🌈

#include <sys/types.h>
#include <grp.h>

int initgroups(const char *user, gid_t group);

🎀 Feature Test Macro Requirements for glibc 🎀

(see feature_test_macros(7))

initgroups():

• Since glibc 2.19: _DEFAULT_SOURCE
• glibc 2.19 and earlier: _BSD_SOURCE

periodt, those are the macros you need, no cap 💅

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🦋 DESCRIPTION 🦋

okay so here's the vibe. initgroups() initializes the group access list. The way it does that is it reads the group database — that's /etc/group, iconic file, we love her — and it uses all the groups of which user is a member. So it's basically doing your group homework for you 📝✨

And then, as a lil bonus treat, the additional group group is also added to the list. Extra. Very main character of it, honestly 💅

⚠️ heads up bestie: the user argument must be non-NULL. Like, must. Don't be sending a NULL in there, that's not the move 🚫

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💀 RETURN VALUE 💀

• On success: initgroups() returns 0 🎉 slay
• On error: it returns -1, and errno gets set to tell you what went wrong (iykyk, always check your errno)

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🙈 ERRORS 🙈

• ENOMEM — not enough memory to allocate the group information structure 😭 the malloc girlies are struggling
• EPERM — the calling process has insufficient privilege. Peep the underlying system call setgroups(2) for the full tea ☕

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📁 FILES 📁

• /etc/group — the group database file, the source of truth, the blueprint 💖

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⭐ ATTRIBUTES ⭐

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

• Interface: initgroups()
• Attribute: Thread safety
• Value: MT-Safe locale

no table bestie, we do bullets here, but the facts are the facts: initgroups() is thread-safe with respect to locale 🧚

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👑 STANDARDS 👑

None. Literally none. It's just out here doing its own thing, unstandardized and unbothered 💅

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🌸 HISTORY 🌸

SVr4, 4.3BSD. Ancient. Vintage. A true icon of the ages 🏛️✨

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🔥 SEE ALSO 🔥

• getgroups(2)
• setgroups(2)
• credentials(7)

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and that's the whole page bestie, go forth and initialize those groups 🫶🌈⭐

signed off with love: this is from Linux man-pages 6.20, dated 2026-02-08 — catch you in the next manpage, mwah 💖🎀