What is the difference between sigaction and signal?

Question

I was about to add an extra signal handler to an app we have here and I noticed that the author had used sigaction() to set up the other signal handlers. I was

Answer 1

In short:

sigaction() is good and well-defined, but is a Linux function and so it works only on Linux. signal() 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: don't use signal(). Use sigaction() instead!

What does GCC think?

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

Source: https://www.gnu.org/software/libc/manual/html_node/Basic-Signal-Handling.html#Basic-Signal-Handling

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
#include  // sigaction()
#include   // printf()
#include  // strerror()

#define LOG_LOCATION __FILE__, __LINE__, __func__ // Format: const char *, unsigned int, const char *
#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;\n"
                   "  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 

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
   1. Termination Signals: https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html#Termination-Signals
2. 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
3. Official GCC sigaction() usage example: https://www.gnu.org/software/libc/manual/html_node/Sigaction-Function-Example.html
4. 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. TutorialsPoint C++ Signal Handling [with excellent demo code]: https://www.tutorialspoint.com/cplusplus/cpp_signal_handling.htm
2. https://www.tutorialspoint.com/c_standard_library/signal_h.htm