reads the page size for a virtual memory address of a given pid

pagesize.c

/* pagesize: reads the page size for a virtual memory address of a given pid */

#include <errno.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char** argv) {
    if (argc != 3) {
        printf("Usage: %s <pid> <page>\n", argv[0]);
        exit(1);
    }

    unsigned long page_addr;
    if (!sscanf(argv[2], "%lx", &page_addr))
        errx(1, "can't parse page address");

    long page_size = sysconf(_SC_PAGESIZE);
    if (page_addr % page_size != 0)
        errx(1, "page address must be a multiple of the page size");

    char pmap_path[32];
    snprintf(pmap_path, sizeof(pmap_path), "/proc/%s/pagemap", argv[1]);
    FILE* pagemap = fopen(pmap_path, "r");
    if (pagemap == NULL)
        err(1, "%s", pmap_path);

    /* pagemaps contains 64-bit entries per page */
    unsigned long page_offset = page_addr / page_size * 8;
    lseek64(fileno(pagemap), page_offset, SEEK_SET);

    uint64_t page;
    fread(&page, sizeof(page), 1, pagemap);
    fclose(pagemap);

    /* bits 55-60 contain the page shift */
    int page_shift = page >> 55;
    page_shift &= 0x40-1;
    printf("page size = %ld bytes\n", 1 << page_shift);
}