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

int main(int argc, char **argv) {
    FILE *fp;
    char line1[128], line2[128];
    fpos_t pos;

    if(argc < 2){
        printf("Usage : %s <file>", argv[0]);
        exit(0);
    }
    fp = fopen(argv[1], "r+");
    if (fp == NULL) {
        fprintf(stderr, "Cannot open file %s\n", argv[1]);
        return 1;
    }

    while (fgetpos(fp, &pos) == 0 &&
           fgets(line1, sizeof line1, fp) != NULL &&
           fgets(line2, sizeof line2, fp) != NULL) {

        fsetpos(fp, &pos);
        fputs(line2, fp);
        fputs(line1, fp);
        fflush(fp);
    }

    fclose(fp);
    return 0;
}