How can I read this C code? [duplicate]

Question

I am quite new to C and I can not successfully understand the following code:

#include <signal.h> //1
typedef void (*sighandler_t)(int); //2
sighandler_t signal (int signo, sighandler_t handler); //3

Line 3 is:

signal is a function that takes 2 arguments, one being an integer and the other being a sighandler_t and returns a sighandler_t?

But what is sighandler_t?

Is it a pointer to a function, where the function that is being pointed is a function that takes an argument of type int and returns void?

Can you give an example on how I can use it?

signal() is depreciated these days. "man signal" says: 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. — Yetti99
– Yetti99, Commented Jun 4, 2015 at 20:55
If you plan to use function pointers in C consider using raw pointers instead. — Alexandre Severino
– Alexandre Severino, Commented Jun 4, 2015 at 20:57
@AlexandreSeverino This is from a book really. Linux System Programming. What do you mean use raw pointers? — Koray Tugay
– Koray Tugay, Commented Jun 4, 2015 at 20:58

haccks · Accepted Answer · 2015-06-04 21:11:54Z

3

Line

typedef void (*sighandler_t)(int);

define a new type sighandler_t which is pointer to a function which expects an argument of type int and returns nothing.

Line

sighandler_t signal (int signo, sighandler_t handler);

declares signal a function which expects its first argument of type int and second argument of type sighandler_t and returns a type sighandler_t.
sighandler_t is user defined type which is ultimately pointer to a function which expects an argument of type int and returns nothing.

Without typedef it would be written as

void (*signal(int signo, void (*handler)(int)))(int);

which is more confusing.

edited Jun 4, 2015 at 21:11

answered Jun 4, 2015 at 20:57

haccks

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6 Comments

Koray Tugay Over a year ago

I am still confused. Is sighandler_t defined in standard C, or is it just the book I am reading?

Koray Tugay Over a year ago

Why would ´signal´ return a ´pointer to a function´ ?

Koray Tugay Over a year ago

Thanks for the great answer, the only thing I do not understand is, how is the return value of signal useful?

haccks Over a year ago

The return value may be used to call a function of type sighandler_t.

Koray Tugay Over a year ago

so like handle a signal, and then run a different function?

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