The simplest possible utility to determine how many pages of a file are in the page cache

fincore.c

#define _BSD_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <stdint.h>
#include <unistd.h>

/*
 * COMPILE:
 *     cc -std=c99 fincore.c -o fincore
 * USE:
 *     ./fincore [FILE]...
 * OUTPUT:
 *     pages_in_file    pages_in_cache    filename
 */

int main(int argc, const char *argv[]) {
    for (int i = 1; i < argc; i++) {
        int fd = open(argv[i], O_RDONLY);
        if (fd != -1) {
            struct stat stats;
            int s = fstat(fd, &stats);
            if (s != -1) {
               void *map = mmap(NULL, stats.st_size, PROT_NONE, MAP_SHARED, fd, 0);
                if (map != MAP_FAILED) {
                    size_t pages = (stats.st_size + sysconf(_SC_PAGESIZE) - 1) / sysconf(_SC_PAGESIZE);
                    char *incore = calloc(pages, sizeof(uint8_t));
                    mincore(map, stats.st_size, incore);
                    size_t cached = 0;
                    for (size_t j = 0; j < pages; j++) {
                        cached += incore[j] & 0x01;
                    }
                    printf("%zu\t%zu\t%s\n", pages, cached, argv[i]);
                    munmap(map, stats.st_size);
                }
            }
            close(fd);
        }
    }
}