nptl(7)

✨ nptl — Native POSIX Threads Library ✨

okay bestie, welcome to the nptl(7) page 💖 this is from the Miscellaneous Information Manual and it's all about the threading situation on your Linux box. it's giving "backbone of everything you've ever run" energy, fr fr.

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🌸 NAME 🌸

nptl — Native POSIX Threads Library

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🎀 DESCRIPTION 🎀

NPTL (Native POSIX Threads Library) is the GNU C library POSIX threads implementation that is used on modern Linux systems. that's it, that's the whole vibe — she's the threading queen and she runs your entire system, no cap. 👑

💅 NPTL and signals 💅

so here's the tea, bestie: NPTL makes internal use of the first two real-time signals — that's signal numbers 32 and 33. one of these signals is used to support thread cancelation and POSIX timers (peep timer_create(2) for that one); the other is used as part of a mechanism that ensures all threads in a process always have the same UIDs and GIDs, as required by POSIX. that's a whole lot of responsibility for two little signals tbh.

⚠️ warning bestie: these signals cannot be used in applications. periodt. not yours to touch.

to prevent accidental use of these signals in applications — which might interfere with the operation of the NPTL implementation — various glibc library functions and system call wrapper functions attempt to hide these signals from applications, as follows:

• SIGRTMIN is defined with the value 34 (rather than 32). sneaky!! 🙈
• the sigwaitinfo(2), sigtimedwait(2), and sigwait(3) interfaces silently ignore requests to wait for these two signals if they are specified in the signal set argument of these calls.
• the sigprocmask(2) and pthread_sigmask(3) interfaces silently ignore attempts to block these two signals.
• the sigaction(2), pthread_kill(3), and pthread_sigqueue(3) interfaces fail with the error EINVAL (indicating an invalid signal number) if these signals are specified.
• sigfillset(3) does not include these two signals when it creates a full signal set.

like… it's a whole protective-big-sister situation and honestly? iconic. 🫶

🦋 NPTL and process credential changes 🦋

ngl this part is lowkey fascinating. at the Linux kernel level, credentials (user and group IDs) are a per-thread attribute. but POSIX said "absolutely not, bestie" and requires that all of the POSIX threads in a process have the same credentials. slay, standards.

to accommodate this requirement, the NPTL implementation wraps all of the system calls that change process credentials with functions that, in addition to invoking the underlying system call, arrange for all other threads in the process to also change their credentials. so like, no thread gets left behind. 💖

here's how it goes down, mechanically speaking:

the implementation of each of these system calls involves the use of a real-time signal that is sent (using tgkill(2)) to each of the other threads that must change its credentials. before sending these signals, the thread that is changing credentials saves the new credential(s) and records the system call being employed in a global buffer. then a signal handler in the receiving thread(s) fetches this information and then uses the same system call to change its credentials. chef's kiss coordination honestly 👑

wrapper functions employing this technique are provided for:

• setgid(2)
• setuid(2)
• setegid(2)
• seteuid(2)
• setregid(2)
• setreuid(2)
• setresgid(2)
• setresuid(2)
• setgroups(2)

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⭐ STANDARDS ⭐

for details of the conformance of NPTL to the POSIX standard, see pthreads(7). (yes bestie, we're citing sources, we're being academic about it 💜)

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🌈 NOTES 🌈

POSIX says that any thread in any process with access to the memory containing a process-shared (PTHREAD_PROCESS_SHARED) mutex can operate on that mutex. sounds great, right?

BUT — and this is the plot twist — on 64-bit x86 systems, the mutex definition for x86-64 is incompatible with the mutex definition for i386, meaning that 32-bit and 64-bit binaries can't share mutexes on x86-64 systems. 💀 so keep that in mind, fr fr.

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🔥 SEE ALSO 🔥

• credentials(7)
• pthreads(7)
• signal(7)
• standards(7)

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and that's the whole page, bestie!! 💖 this was nptl(7) from Linux man-pages 6.20, dated 2026-02-08. go forth and thread responsibly, you absolute main character 🌸🦋✨