Question

I’m writing a program that encrypts a file by adding 10 to each character. Somehow a portion of the programs working directory is being printed to the file, and I have no idea why.

#include <stdio.h>
    int main(void){
        FILE *fp;
        fp=fopen("tester.csv","r+");
        Encrypt(fp);      
        fclose(fp);
    }

    int Encrypt(FILE *fp){
        int offset=10;
        Shift(fp, offset);
    }
    int Decrypt(FILE *fp){
        int offset= -10;
        Shift(fp, offset);
    }
    int Shift(FILE *fp, int offset){
        char line[50],tmp[50], character;
        long position;
        int i;
        position = ftell(fp);
        while(fgets(line,50,fp) != NULL){
            for(i=0;i<50;i++){
                character = line[i];
                character = (offset+character)%256;
                tmp[i] = character; 
                if(character=='\n' || character == 0){break;}                
            }
            fseek(fp,position,SEEK_SET);
            fputs(tmp,fp);
            position = ftell(fp);
            fseek(stdin,0,SEEK_END);
        }
      }

the file originally reads

this, is, a, test
i, hope, it, works!

after the program is run:

~rs}6*s}6*k6*~o}~
/alexio/D~6*y|u}+
k6*~o}~
/alexio/D

where users/alexio/Desktop is part of the path. How does this happen???

Answer 1

Because you “encode” the string, it won’t be null terminated (that’s your case), or it will contain a null even before the end of the string (character+offset % 256 == 0). Later you try to write it as a string, which overruns your buffer, and outputs part of your program arguments.

Use fread and fwrite.