filemap.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>

#define BUFFER_SIZE	1000
#define ALLOC_SIZE	(100*1024*1024)

static char command[BUFFER_SIZE];
static char file_contents[BUFFER_SIZE];
static char overwrite_data[] = "HELLO";

int main(void)
{
	pid_t pid;

	pid = getpid();
	snprintf(command, BUFFER_SIZE, "cat /proc/%d/maps", pid);

	puts("*** memory map before mapping file ***");
	fflush(stdout);
	system(command);

	int fd;
	fd = open("testfile", O_RDWR);
	if (fd == -1)
		err(EXIT_FAILURE, "open() failed");

	char * file_contents;
	file_contents = mmap(NULL, ALLOC_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (file_contents == (void *) -1) {
		warn("mmap() failed");
		goto close_file;
	}

	puts("");
	printf("*** succeeded to map file: address = %p; size = 0x%x ***\n",
		file_contents, ALLOC_SIZE);

	puts("");
	puts("*** memory map after mapping file ***");
	fflush(stdout);
	system(command);

	puts("");
	printf("*** file contents before overwrite mapped region: %s", file_contents);

	// overwrite mapped region
	memcpy(file_contents, overwrite_data, strlen(overwrite_data));

	puts("");
	printf("*** overwritten mapped region with: %s\n", file_contents);

	if (munmap(file_contents, ALLOC_SIZE) == -1)
		warn("munmap() failed");
close_file:
	if (close(fd) == -1)
		warn("close() failed");
	exit(EXIT_SUCCESS);
}