What is the difference between sigaction and signal?

Use sigaction() unless you've got very compelling reasons not to do so.

The signal() interface has antiquity (and hence availability) in its favour, and it is defined in the C standard. Nevertheless, it has a number of undesirable characteristics that sigaction() avoids - unless you use the flags explicitly added to sigaction() to allow it to faithfully simulate the old signal() behaviour.

1. The signal() function does not (necessarily) block other signals from arriving while the current handler is executing; sigaction() can block other signals until the current handler returns.
2. The signal() function (usually) resets the signal action back to SIG_DFL (default) for almost all signals. This means that the signal() handler must reinstall itself as its first action. It also opens up a window of vulnerability between the time when the signal is detected and the handler is reinstalled during which if a second instance of the signal arrives, the default behaviour (usually terminate, sometimes with prejudice - aka core dump) occurs.
3. The exact behaviour of signal() varies between systems — and the standards permit those variations.

These are generally good reasons for using sigaction() instead of signal(). However, the interface of sigaction() is undeniably more fiddly.

Whichever of the two you use, do not be tempted by the alternative signal interfaces such as sighold(), sigignore(), sigpause() and sigrelse(). They are nominally alternatives to sigaction(), but they are only barely standardized and are present in POSIX for backwards compatibility rather than for serious use. Note that the POSIX standard says their behaviour in multi-threaded programs is undefined.

Multi-threaded programs and signals is a whole other complicated story. AFAIK, both signal() and sigaction() are OK in multi-threaded applications.

Cornstalks observes:

The Linux man page for signal() says:

  The effects of signal() in a multi-threaded process are unspecified.

Thus, I think sigaction() is the only that can be used safely in a multi-threaded process.

That's interesting. The Linux manual page is more restrictive than POSIX in this case. POSIX specifies for signal():

If the process is multi-threaded, or if the process is single-threaded and a signal handler is executed other than as the result of:

• The process calling abort(), raise(), kill(), pthread_kill(), or sigqueue() to generate a signal that is not blocked
• A pending signal being unblocked and being delivered before the call that unblocked it returns

the behavior is undefined if the signal handler refers to any object other than errno with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t, or if the signal handler calls any function defined in this standard other than one of the functions listed in Signal Concepts.

So POSIX clearly specifies the behaviour of signal() in a multi-threaded application.

Nevertheless, sigaction() is to be preferred in essentially all circumstances — and portable multi-threaded code should use sigaction() unless there's an overwhelming reason why it can't (such as "only use functions defined by Standard C" — and yes, C11 code can be multi-threaded). Which is basically what the opening paragraph of this answer also says.

In short:

sigaction() (see here and here) is good and well-defined, but is a POSIX function and so it works only on Linux or POSIX systems. signal() (see here and here) is bad and poorly-defined, but is a C standard function and so it works on anything.

What do the Linux man pages have to say about it?

man 2 signal (see it online here) states:

The behavior of signal() varies across UNIX versions, and has also varied historically across different versions of Linux. Avoid its use: use sigaction(2) instead. See Portability below.

Portability

The only portable use of signal() is to set a signal's disposition to SIG_DFL or SIG_IGN. The semantics when using signal() to establish a signal handler vary across systems (and POSIX.1 explicitly permits this variation); do not use it for this purpose.

In other words: do not use signal(). Use sigaction() instead!

This position is reiterated in the next line which states (emphasis added):

POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit control of the semantics when a signal handler is invoked; use that interface instead of signal().

What does GCC think?

From https://www.gnu.org/software/libc/manual/html_node/Basic-Signal-Handling.html#Basic-Signal-Handling (emphasis added):

Compatibility Note: As said above for signal, this function should be avoided when possible. sigaction is the preferred method.

So, if both Linux and GCC say not to use signal(), but to use sigaction() instead, that begs the question: how the heck do we use this confusing sigaction() thing!?

Usage Examples:

Read GCC's EXCELLENT signal() example here: https://www.gnu.org/software/libc/manual/html_node/Basic-Signal-Handling.html#Basic-Signal-Handling

And their EXCELLENT sigaction() example here: https://www.gnu.org/software/libc/manual/html_node/Sigaction-Function-Example.html

After reading those pages, I came up with the following technique for sigaction():

1. sigaction(), since it's the right way to attach a signal handler, as described above:

#include <errno.h>  // errno
#include <signal.h> // sigaction()
#include <stdio.h>  // printf()
#include <string.h> // strerror()

// Format: const char *, unsigned int, const char *
#define LOG_LOCATION __FILE__, __LINE__, __func__ 
#define LOG_FORMAT_STR "file: %s, line: %u, func: %s: "

/// @brief      Callback function to handle termination signals, such as 
///             Ctrl + C
/// @param[in]  signal  Signal number of the signal being handled by this 
///             callback function
/// @return     None
static void termination_handler(const int signal)
{
    switch (signal)
    {
    case SIGINT:
        printf("\nSIGINT (%i) (Ctrl + C) signal caught.\n", signal);
        break;
    case SIGTERM:
        printf("\nSIGTERM (%i) (default `kill` or `killall`) signal caught.\n", 
            signal);
        break;
    case SIGHUP:
        printf("\nSIGHUP (%i) (\"hang-up\") signal caught.\n", signal);
        break;
    default:
        printf("\nUnk signal (%i) caught.\n", signal);
        break;
    }

    // DO PROGRAM CLEANUP HERE, such as freeing memory, closing files, etc.

    
    exit(signal);
}

/// @brief      Set a new signal handler action for a given signal
/// @details    Only update the signals with our custom handler if they are NOT
///     set to "signal ignore" (`SIG_IGN`), which means they are currently
///     intentionally ignored. GCC recommends this "because non-job-control
///     shells often ignore certain signals when starting children, and it is
///     important for children to respect this." See
///     https://www.gnu.org/software/libc/manual/html_node/Basic-Signal-Handling.html#Basic-Signal-Handling
///     and
///     https://www.gnu.org/software/libc/manual/html_node/Sigaction-Function-Example.html.
///     Note that termination signals can be found here:
///     https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html#Termination-Signals
/// @param[in]  signal  Signal to set to this action
/// @param[in]  action  Pointer to sigaction struct, including the callback 
///     function inside it, to attach to this signal
/// @return     None
static inline void set_sigaction(int signal, const struct sigaction *action)
{
    struct sigaction old_action;

    // check current signal handler action to see if it's set to SIGNAL IGNORE
    sigaction(signal, NULL, &old_action);
    if (old_action.sa_handler != SIG_IGN)
    {
        // set new signal handler action to what we want
        int ret_code = sigaction(signal, action, NULL);
        if (ret_code == -1)
        {
            printf(LOG_FORMAT_STR "sigaction failed when setting signal to "
                   "%i; errno = %i: %s\n", 
                   LOG_LOCATION, signal, errno, strerror(errno));
        }
    }
}

int main(int argc, char *argv[])
{
    //...

    // Register callbacks to handle kill signals; prefer the Linux function
    // `sigaction()` over the C function `signal()`: "It is better to use
    // sigaction if it is available since the results are much more reliable."
    // Source:
    // https://www.gnu.org/software/libc/manual/html_node/Basic-Signal-Handling.html#Basic-Signal-Handling
    // and
    // https://stackoverflow.com/questions/231912/what-is-the-difference-between-sigaction-and-signal/232711#232711.
    // See here for official gcc `sigaction()` demo, which this code is modeled
    // after:
    // https://www.gnu.org/software/libc/manual/html_node/Sigaction-Function-Example.html

    // Set up the structure to specify the new action, per GCC's demo.
    struct sigaction new_action;
    new_action.sa_handler = termination_handler; // set callback function
    sigemptyset(&new_action.sa_mask);
    new_action.sa_flags = 0;

    // SIGINT: ie: Ctrl + C kill signal
    set_sigaction(SIGINT, &new_action);
    // SIGTERM: termination signal--the default generated by `kill` and
    // `killall`
    set_sigaction(SIGTERM, &new_action);
    // SIGHUP: "hang-up" signal due to lost connection
    set_sigaction(SIGHUP, &new_action);

    //...
}

2. And for signal(), even though its not a good way to attach a signal handler, as described above, it's still good to know how to use it.

Here's the GCC demonstration code copy-pasted, as it's about as good as it's going to get:

#include <signal.h>

void
termination_handler (int signum)
{
  struct temp_file *p;

  for (p = temp_file_list; p; p = p->next)
    unlink (p->name);
}

int
main (void)
{
  …
  if (signal (SIGINT, termination_handler) == SIG_IGN)
    signal (SIGINT, SIG_IGN);
  if (signal (SIGHUP, termination_handler) == SIG_IGN)
    signal (SIGHUP, SIG_IGN);
  if (signal (SIGTERM, termination_handler) == SIG_IGN)
    signal (SIGTERM, SIG_IGN);
  …
}

The main links to be aware of:

1. Standard Signals: https://www.gnu.org/software/libc/manual/html_node/Standard-Signals.html#Standard-Signals
2. Termination Signals: https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html#Termination-Signals
3. Basic Signal Handling, including official GCC signal() usage example: https://www.gnu.org/software/libc/manual/html_node/Basic-Signal-Handling.html#Basic-Signal-Handling
4. Official GCC sigaction() usage example: https://www.gnu.org/software/libc/manual/html_node/Sigaction-Function-Example.html
5. Signal sets, including sigemptyset() and sigfillset(); I still don't understand these exactly, but know they are important: https://www.gnu.org/software/libc/manual/html_node/Signal-Sets.html

See also:

1. My answer on "How to manually send any signal to any running process" and "How to trap any signal in your program" (ex: in Bash).
2. TutorialsPoint C++ Signal Handling [with excellent demo code]: https://www.tutorialspoint.com/cplusplus/cpp_signal_handling.htm
3. https://www.tutorialspoint.com/c_standard_library/signal_h.htm

They're different interfaces for OS's signal facilities. One should prefer using sigaction to signal if possible as the signal() has implementation-defined (often race prone) behavior and behaves differently on Windows, OS X, Linux and other UNIX systems.

See this security note for details.

From the signal(3) man page:

DESCRIPTION

 This signal() facility is a simplified interface to the more
 general sigaction(2) facility.

Both invoke the same underlying facility. You should presumably not manipulate the response the a single signal with both, but mixing them shouldn't cause anything to break...

I'd use signal() since it's more portable, in theory at least. I'll vote up any commenter who can come up with a modern system that doesn't have a POSIX compatibility layer and supports signal().

Quoting from the GLIBC documentation:

It's possible to use both the signal and sigaction functions within a single program, but you have to be careful because they can interact in slightly strange ways.

The sigaction function specifies more information than the signal function, so the return value from signal cannot express the full range of sigaction possibilities. Therefore, if you use signal to save and later reestablish an action, it may not be able to reestablish properly a handler that was established with sigaction.

To avoid having problems as a result, always use sigaction to save and restore a handler if your program uses sigaction at all. Since sigaction is more general, it can properly save and reestablish any action, regardless of whether it was established originally with signal or sigaction.

On some systems if you establish an action with signal and then examine it with sigaction, the handler address that you get may not be the same as what you specified with signal. It may not even be suitable for use as an action argument with signal. But you can rely on using it as an argument to sigaction. This problem never happens on the GNU system.

So, you're better off using one or the other of the mechanisms consistently within a single program.

Portability Note: The basic signal function is a feature of ISO C, while sigaction is part of the POSIX.1 standard. If you are concerned about portability to non-POSIX systems, then you should use the signal function instead.

Copyright (C) 1996-2008 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

From man page signal(7)

A process-directed signal may be delivered to any one of the threads that does not currently have the signal blocked. If more than one of the threads has the signal unblocked, then the kernel chooses an arbitrary thread to which to deliver the signal.

And I would say this "issue" exists for signal(2) and sigaction(2). So be careful with signals and pthreads.

... and signal(2) seems to call sigaction(2) underneath in Linux with glibc.