NPTL and signals¶

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).
• 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.

NPTL and process credential changes¶

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. A signal handler in the receiving thread(s) fetches this information and then uses the same system call to change its credentials.

Wrapper functions employing this technique are provided for setgid(2), setuid(2), setegid(2), seteuid(2), setregid(2), setreuid(2), setresgid(2), setresuid(2), and setgroups(2).