fshuffle.c

fshuffle.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int fswap(FILE *file, off64_t x, off64_t y, size_t size) {
    unsigned char i[size], o[size];

    return fseeko64(file, x * size, SEEK_SET) || fread(i,  size, 1, file) != 1 ||
           fseeko64(file, y * size, SEEK_SET) || fread(o,  size, 1, file) != 1 ||
           fseeko64(file, y * size, SEEK_SET) || fwrite(i, size, 1, file) != 1 ||
           fseeko64(file, x * size, SEEK_SET) || fwrite(o, size, 1, file) != 1 ||
                                              || fflush(file);
}

int main(int argc, char **argv) {
    size_t size;
    if (argc <= 1 || sscanf(argv[1], "%zu", &size) != 1) {
        puts("Usage:  ./app filename");
    	return EXIT_FAILURE;
    }
    
    FILE *file = fopen(argv[2], "rb+");
    if (file == NULL) {
        puts("Error opening file...");
        return EXIT_FAILURE;
    }
    
    off64_t position = 0;
    for (;;) {
    	unsigned char entry[size];
        if (fread(entry, sizeof entry, 1, file) != 1 || fswap(file, position, position ? rand() % position : 0, sizeof entry)) {
    	    return ferror(file) ? EXIT_FAILURE : 0;
    	}
    	
    	fwrite(entry, sizeof entry, 1, stdout);
        position++;
    }
}