bsort source code

bsort.c

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

static int recordwidth;

static int comparer(const void *a, const void *b) {
    return memcmp(a, b, recordwidth);
}

int main(int argc, char *argv[]) {
    if (sizeof(off_t) != 8) {
        fprintf(stderr, "off_t is %lu bytes but should be 8\n", sizeof(off_t));
        return 1;
    }
    if (argc != 3) {
        fprintf(stderr, "usage: %s <recordwidth> <filename>\n", argv[0]);
        return 1;
    }
    recordwidth = atoi(argv[1]);

    int fd = open(argv[2], O_RDWR);
    if (fd < 0) { perror("failed to open file"); return 1; }

    off_t len = lseek(fd, 0, SEEK_END);
    if (len < 0) { perror("failed to seek to end"); return 1; }

    off_t start = lseek(fd, 0, SEEK_SET);
    if (start != 0) { perror("failed to seek to start"); return 1; }

    void *addr = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED) {
        perror("mmap failed");
        return 1;
    }

    qsort(addr, len / recordwidth, recordwidth, comparer);

    munmap(addr, len);
    close(fd);

    return 0;
}