C example of mmap and mprotect

main.c

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

// comment to disable mprotect
#define DO_PROTECTION

int main(void) {
    int page_size = getpagesize();

    printf("Page size: %d\n", page_size);

    char *mapped_memory = mmap(NULL, page_size * 10, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

    if (mapped_memory == MAP_FAILED) {
        printf("Mapping memory failed: %s\n", strerror(errno));
        return 1;
    }
    printf("Mapped memory pointer: %p\n", mapped_memory);

    for(size_t i = 0; i < page_size * 10; i++) {
        mapped_memory[i] = 'a';
    }

    printf("Successfully filled mapped memory with 'a'\n");

    #ifdef DO_PROTECTION
    if(mprotect(mapped_memory + (page_size * 8), page_size * 2, PROT_READ) == -1) {
        printf("Failed to mprotect the mapped memory: %s\n", strerror(errno));
        return 1;
    }
    printf("Successfully set protection PROT_READ on last 2 pages of mapped memory\n");
    #endif // DO_PROTECTION

    for(size_t i = 0; i < 10; i++) {
        printf("Writing data to page: %zu...", i);
        for(size_t j = 0; j < page_size; j++) {
            mapped_memory[i*page_size + j] = 'b';
        }
        printf(" OK\n");
    }

    return 0;
}