Reverse file content

file_rev.cpp

#include <stdio.h>

size_t filesize(FILE *f)
{
    fseek(f, 0, SEEK_END);
    size_t size = ftell(f);
    fseek(f, 0, SEEK_SET);
    return size;
}

int main(int argc, char *argv[])
{
    if (argc < 3) {
        printf("Requited arguments: <input_file> <output_file>\n");
        return -1;
    }
    char *inname = argv[1];
    char *outname = argv[2];
    FILE *fin = fopen(inname,"rb");
    if (fin == NULL) {
        printf("[ERROR] Cannot input output file\n");
        return -1;
    }
    size_t fsize = filesize(fin);
    printf("file size: %x\n",fsize);

    unsigned char buffer[fsize];
    fread(buffer, 1, fsize, fin);
    fclose(fin);
    fin = NULL;

    FILE *fout = fopen(outname, "wb");
    if (fout == NULL) {
        printf("[ERROR] Cannot open output file\n");
        return -1;
    }
    const size_t lastchar = fsize - 1;
    for (int i = 0; i <= fsize/2; i++) {
        unsigned char swapc = buffer[i];
        buffer[i] = buffer[lastchar - i];
        buffer[lastchar - i] = swapc;
    }
    fwrite(buffer, 1, fsize, fout);
    fclose(fout);
    
    printf("File %s reversed and saved to: %s\n", inname, outname);
    return 0;
}