void* rv = mmap(0, num_bytes, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
if(rv == MAP_FAILED){
  // handle error
}
if(fork()){
  // we are the parent...
  // do some stuff
}
else {
  // we are the child, drop priviledges irreversibly
  if(mprotect(rv, num_bytes, PROT_READ) == -1){
    // handle error
  }
  // do more child things
}

#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>


static void segv_handler(int sSignal)
{
	if (sSignal == SIGSEGV)
	{
	fprintf(stderr, "SIGSEGV caused by bad memory access!\n");
	signal(sSignal, SIG_DFL);
	raise(sSignal);
	}
}


int main()
{
	int mapping = open("./memory", O_RDWR | O_CREAT | O_TRUNC, 0644);
	if (mapping < 0)
	{
	fprintf(stderr, "mapping error: %s\n", strerror(errno));
	return 1;
	}

	int memory_size = getpagesize();

	ftruncate(mapping, memory_size); //non-standard, but used for convenience

	setlinebuf(stdout);
	signal(SIGSEGV, &segv_handler);


	void *read_only = mmap(NULL, memory_size, PROT_READ, MAP_SHARED, mapping, 0);
	if (!read_only)
	{
	fprintf(stderr, "read error: %s\n", strerror(errno));
	return 1;
	}


	void *read_write = mmap(NULL, memory_size, PROT_READ | PROT_WRITE, MAP_SHARED, mapping, 0);
	if (!read_write)
	{
	fprintf(stderr, "write error: %s\n", strerror(errno));
	return 1;
	}

	close(mapping); //<-- recently added; prevents child from calling 'fstat'


	char *write_to = (char*) read_write;
	snprintf(write_to, memory_size, "hello!");

	char *read_from = (char*) read_only;
	fprintf(stdout, "%s\n", read_from);


	snprintf(read_from, memory_size, "segfault here!");


	return 0;
}