Thread: signal function definited twice with no error: how is it possible?

Hi,
I'm using a library in my code in a file called "my_signal.c".
In this file I've defined this function:

Code:

Sigfunc * signal(int signo, Sigfunc *func){
    struct sigaction    act, oact;
...
}

Since the signal function is also in file signal.h and I included it in "my_sygnal.h" file, I'm wondering why the compiler did not say anything about this "double declaration" of the function with the same name.

Thanks,
Massimo

Did you include signal.h in your C file? If not, the "official" definition isn't seen. (And if you #defined things the same, then even if you did the function matches the prototype, so then you're playing linker roulette as to which version gets called.)

Originally Posted by tabstop

Did you include signal.h in your C file? If not, the "official" definition isn't seen. (And if you #defined things the same, then even if you did the function matches the prototype, so then you're playing linker roulette as to which version gets called.)

So the situation is this one:

In file my_signal.c I have included:

Code:

#include        "basic.h"
#include        "my_signal.h"

while in basic.h I have included:
#include <signal.h>

I'm attaching the source code (it a very simple echo client/server) so not much to read.

Thanks,
M

Anyone, please?
Massimo

Hi, first of all, I'm sorry about my previous posts. I'll be less lazy (I just tried to be coincise, maybe too much). So the significant source code follows (probably it is just necessary to look at my_signal.h source code:

my_signal.h:

Code:

#include <signal.h>
#include <sys/wait.h>

typedef void Sigfunc(int); 

Sigfunc* signal(int , Sigfunc *); 

void manage_zombie (int segnale);

my_signal.c:

Code:

#include        "basic.h"
#include        "my_signal.h"

Sigfunc * signal(int signo, Sigfunc *func){
        struct sigaction        act, oact;

        act.sa_handler = func;
        sigemptyset(&act.sa_mask);
        act.sa_flags = 0;
        if (signo != SIGALRM) {
           act.sa_flags |= SA_RESTART;
        }
        if (sigaction(signo, &act, &oact) < 0)
                return(SIG_ERR);
        return(oact.sa_handler);
}

basic.h

Code:

#include        <signal.h>      
#include        <stdio.h>       
#include        <stdlib.h>

I'm wondering if this is the case described by Grumpy.

In particular, I'm a little confuse in my_signal.h:
- it includes "signal.h"
- it declares a "signal" function

why does not the compile complain about that?

Thanks,
Massimo

Originally Posted by mcanonic

In particular, I'm a little confuse in my_signal.h:
- it includes "signal.h"
- it declares a "signal" function

why does not the compile complain about that?

Thanks,
Massimo

As mentioned above, the function you define and the standard function have the same "type" (function taking int and pointer-to-void-function-with-int-parameter returning pointer-to-void-function-with-int-parameter) so there is no conflict. Note that the standard headers don't have any code in them (because you don't put code in .h files), so the compiler only sees one function of that name and is perfectly happy. Which one the linker calls is up to the linker. (C can't really complain about people defining functions that exist in the standard library, because they keep adding to the standard library all the time, so the names are not absolutely reserved.)

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signal function definited twice with no error: how is it possible?