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][1]:

    #include &lt;signal.h&gt;
    #include &lt;stdio.h&gt;
    #include &lt;malloc.h&gt;
    #include &lt;stdlib.h&gt;
    #include &lt;errno.h&gt;
    #include &lt;sys/mman.h&gt;
    
    #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(&quot;Got SIGSEGV at address: 0x%lx\n&quot;,(long) si-&gt;si_addr);
    	printf(&quot;Implements the handler only\n&quot;);
    	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(&amp;sa.sa_mask);
    	sa.sa_sigaction = handler;
    	if (sigaction(SIGSEGV, &amp;sa, NULL) == -1)
    		handle_error(&quot;sigaction&quot;);
    
    	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(&quot;memalign&quot;);
    
    	printf(&quot;Start of region:        0x%lx\n&quot;, (long) buffer);
    	printf(&quot;Start of region:        0x%lx\n&quot;, (long) buffer+pagesize);
    	printf(&quot;Start of region:        0x%lx\n&quot;, (long) buffer+2*pagesize);
    	printf(&quot;Start of region:        0x%lx\n&quot;, (long) buffer+3*pagesize);
    	//if (mprotect(buffer + pagesize * 0, pagesize,PROT_NONE) == -1)
    	if (mprotect(buffer + pagesize * 0, pagesize,PROT_NONE) == -1)
    		handle_error(&quot;mprotect&quot;);
    
    	//for (p = buffer ; ; )
    	if(flag==0)
    	{
    		p = buffer+pagesize/2;
    		printf(&quot;It comes here before reading memory\n&quot;);
    		a = *p; //trying to read the memory
    		printf(&quot;It comes here after reading memory\n&quot;);
    	}
    	else
    	{
    		if (mprotect(buffer + pagesize * 0, pagesize,PROT_READ) == -1)
    		handle_error(&quot;mprotect&quot;);
    		a = *p;
    		printf(&quot;Now i can read the memory\n&quot;);
    
    	}
    /*	for (p = buffer;p&lt;=buffer+4*pagesize ;p++ ) 
    	{
    		//a = *(p);
    		*(p) = &#39;a&#39;;
    		printf(&quot;Writing at address %p\n&quot;,p);
    
    	}*/
    
    	printf(&quot;Loop completed\n&quot;);     /* Should never happen */
    	exit(EXIT_SUCCESS);
    }

The problem is that only the signal handler runs and I can&#39;t return to the main function after catching the signal.

  [1]: http://man7.org/linux/man-pages/man2/mprotect.2.html

All Mprotect Solutions on Gang of Coders

Total of 1 Mprotect Solutions

How to write a signal handler to catch SIGSEGV?