Fast SHA1 calculation on files

sha1sum.c

/* cc -Wpedantic sha1sum.c -osha1sum $(shell pkg-config --cflags --libs libcrypto) */

#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/mman.h>

#include <openssl/sha.h>
#include <stdio.h>

int main(int args, char* argv[]) {
    int fd = open(argv[1], O_RDONLY);
    if (fd == -1) {
        perror(argv[1]);
        return 1;
    }

    struct stat st;
    fstat(fd, &st);
    printf("File size: %lld\n", st.st_size);
    SHA_CTX ctx;
    SHA1_Init(&ctx);

    unsigned char *data = mmap(0, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);

    if (!data) {
        perror("mmap");
        return 1;
    }

    SHA1_Update(&ctx, data, st.st_size);
    close(fd);

    unsigned char md[SHA_DIGEST_LENGTH];
    SHA1_Final(md, &ctx);

    for (int i = 0; i < SHA_DIGEST_LENGTH; ++i) {
        printf("%.02x", md[i]);
    }
    printf("\n");
}