Why won't Ubuntu show section 3 of signal manual?

Question

The title says it all. I'm running Ubuntu 20.04, and I would like to read section 3 of the documentation of signal.

However, when I type the following the my terminal window:

$ man 3 signal

I get:

No manual entry for signal in section 3

However, I have found it somewhere else online. What am I missing here?

Answer 1

I think it's because - in Linux - signal is implemented as a system call rather than a library function, and therefore belongs in Section 2 of the manual rather than Section 3.

You can add a Section 3 man page for the POSIX signal implementation by installing the manpages-posix-dev package:

$ dpkg -S $(man -w 3 signal)
manpages-posix-dev: /usr/share/man/man3/signal.3posix.gz

For compatibility reasons, glibc also provides a signal library function that is documented in Section 3 of the manpages as bsd_signal:

BSD_SIGNAL(3)              Linux Programmer's Manual             BSD_SIGNAL(3)

NAME
       bsd_signal - signal handling with BSD semantics

SYNOPSIS
       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t bsd_signal(int signum, sighandler_t handler);

See also:

• What is the difference between system call and library call?