How to write a signal handler in amd64 assembly

main.c

#include <time.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <limits.h>

extern int var;
extern void pew(int);

int main(int argc, char **argv){
                if(signal(10, pew) == SIG_ERR){ /* sigusr1 */
                                fprintf(stderr, "Cannot register signal handler\n");
                                exit(2);
                }
                struct timespec st = {0, 100000}; // 100µs
                for(;;){
                                if(nanosleep(&st, NULL))
                                                printf("Interrupted.\n");
                                if(var == 1){
                                                printf("Signal handler called.\n");
                                }
                                var = 0;
                }
                return 0;
}

Makefile

all:
        gcc -Wall -o foo main.c test.S

test.S

.text 
.comm var,4
.global pew
pew:
        push %rbp

        movl $0x1, var

        pop %rbp
        ret

.data