How to write a signal handler to catch SIGSEGV?

I want to write a signal handler to catch SIGSEGV. I protect a block of memory for read or write using

char *buffer;
char *p;
char a;
int pagesize = 4096;

mprotect(buffer,pagesize,PROT_NONE)

This protects pagesize bytes of memory starting at buffer against any reads or writes.

Second, I try to read the memory:

p = buffer;
a = *p 

This will generate a SIGSEGV, and my handler will be called. So far so good. My problem is that, once the handler is called, I want to change the access write of the memory by doing

mprotect(buffer,pagesize,PROT_READ);

and continue normal functioning of my code. I do not want to exit the function. On future writes to the same memory, I want to catch the signal again and modify the write rights and then record that event.

Here is the code:

#include <signal.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

char *buffer;
int flag=0;

static void handler(int sig, siginfo_t *si, void *unused)
{
    printf("Got SIGSEGV at address: 0x%lx\n",(long) si->si_addr);
    printf("Implements the handler only\n");
    flag=1;
    //exit(EXIT_FAILURE);
}

int main(int argc, char *argv[])
{
    char *p; char a;
    int pagesize;
    struct sigaction sa;

    sa.sa_flags = SA_SIGINFO;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = handler;
    if (sigaction(SIGSEGV, &sa, NULL) == -1)
        handle_error("sigaction");

    pagesize=4096;

    /* Allocate a buffer aligned on a page boundary;
       initial protection is PROT_READ | PROT_WRITE */

    buffer = memalign(pagesize, 4 * pagesize);
    if (buffer == NULL)
        handle_error("memalign");