-2

When executing my program, I have this error:

*** Error in `./xorcipher': double free or corruption (!prev): 0x00000000024a1010 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7fe869b317e5]
/lib/x86_64-linux-gnu/libc.so.6(+0x8037a)[0x7fe869b3a37a]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7fe869b3e53c]
/lib/x86_64-linux-gnu/libc.so.6(fclose+0x103)[0x7fe869b27363]
./xorcipher[0x4009de]
./xorcipher[0x400d68]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7fe869ada830]
./xorcipher[0x4007b9]
======= Memory map: ========
00400000-00402000 r-xp 00000000 fd:01 1056628                            /home/user1/Dropbox/SCOLARITY/L2Informatique/S3/Projet/Projet xorcipher/Code/programme final/test/xorcipher
00601000-00602000 r--p 00001000 fd:01 1056628                            /home/user1/Dropbox/SCOLARITY/L2Informatique/S3/Projet/Projet xorcipher/Code/programme final/test/xorcipher
00602000-00603000 rw-p 00002000 fd:01 1056628                            /home/user1/Dropbox/SCOLARITY/L2Informatique/S3/Projet/Projet xorcipher/Code/programme final/test/xorcipher
024a1000-024c2000 rw-p 00000000 00:00 0                                  [heap]
7fe864000000-7fe864021000 rw-p 00000000 00:00 0 
7fe864021000-7fe868000000 ---p 00000000 00:00 0 
7fe8698a4000-7fe8698ba000 r-xp 00000000 fd:01 136064                     /lib/x86_64-linux-gnu/libgcc_s.so.1
7fe8698ba000-7fe869ab9000 ---p 00016000 fd:01 136064                     /lib/x86_64-linux-gnu/libgcc_s.so.1
7fe869ab9000-7fe869aba000 rw-p 00015000 fd:01 136064                     /lib/x86_64-linux-gnu/libgcc_s.so.1
7fe869aba000-7fe869c7a000 r-xp 00000000 fd:01 137277                     /lib/x86_64-linux-gnu/libc-2.23.so
7fe869c7a000-7fe869e7a000 ---p 001c0000 fd:01 137277                     /lib/x86_64-linux-gnu/libc-2.23.so
7fe869e7a000-7fe869e7e000 r--p 001c0000 fd:01 137277                     /lib/x86_64-linux-gnu/libc-2.23.so
7fe869e7e000-7fe869e80000 rw-p 001c4000 fd:01 137277                     /lib/x86_64-linux-gnu/libc-2.23.so
7fe869e80000-7fe869e84000 rw-p 00000000 00:00 0 
7fe869e84000-7fe869eaa000 r-xp 00000000 fd:01 137058                     /lib/x86_64-linux-gnu/ld-2.23.so
7fe86a089000-7fe86a08c000 rw-p 00000000 00:00 0 
7fe86a0a6000-7fe86a0a9000 rw-p 00000000 00:00 0 
7fe86a0a9000-7fe86a0aa000 r--p 00025000 fd:01 137058                     /lib/x86_64-linux-gnu/ld-2.23.so
7fe86a0aa000-7fe86a0ab000 rw-p 00026000 fd:01 137058                     /lib/x86_64-linux-gnu/ld-2.23.so
7fe86a0ab000-7fe86a0ac000 rw-p 00000000 00:00 0 
7ffea9648000-7ffea9669000 rw-p 00000000 00:00 0                          [stack]
7ffea96b5000-7ffea96b7000 r--p 00000000 00:00 0                          [vvar]
7ffea96b7000-7ffea96b9000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted

The problem seems to be in the xor function but I can't figure what is the problem. According to this, my problem seems to be caused by a dereferenced pointer. But according to valgrind, it is a open no cancel error (which I don't know what it is and can't find anything relevant on google). On the other hand, I have no compilation errors.

What is wrong with my code?

Here is the program:

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

void xor(char* name_file_in,char* name_file_out,char* name_key)
{
    FILE *file_in;//file to encrypt
    FILE *file_out;//encrypted file
    unsigned char read_char;//character of the file
    int count_char=0;//count number of characters in file
    file_in=fopen(name_file_in,"r");
    file_out=fopen(name_file_out,"w");

    while ( fscanf(file_in,"%c",&read_char) != EOF)//each character of in file is read
    {
        read_char=read_char^name_key[count_char%strlen(name_key)];//read_char is encrypted according to key's length's modulo
        fputc(read_char,file_out);//writing character to out file
        count_char++;
    }
    fclose(file_in);
    fclose(file_in);
}

int main(int argc,char* argv[])
{   
    xor(argv[1],argv[2],argv[3]);
    return(0);
}
LinuxCrusher
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2 Answers2

2

You are closing your file twice:

fclose(file_in);
fclose(file_in);

This is undefined behavior.

Note: I only take 5s with valgrind to see that... use -g when you compile for debugging !

Stargateur
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  • I already use -g, see the compilation code line at top of my post (If I understand correctly). Thanks for the double fclose(file_in)! – LinuxCrusher Dec 30 '17 at 08:19
0

You must read and write the file in binary, even if it is text file. This is because XOR operator can result in zero bytes, this breaks the read routine in text mode.

file_in = fopen(name_file_in, "rb");
file_out = fopen(name_file_out, "wb");

In addition you have to check to make sure input is valid

if (!name_file_in) return;
if (!name_file_out) return;
if (!name_key) return;

And don't proceed if file handle is invalid

void xor(char* name_file_in, char* name_file_out, char* name_key)
{
    if (!name_file_in) return;
    if (!name_file_out) return;
    if (!name_key) return;

    FILE *file_in;
    FILE *file_out;
    unsigned char read_char;
    int count_char = 0;

    file_in = fopen(name_file_in, "rb");
    file_out = fopen(name_file_out, "wb");

    if(file_in && file_out)
    {
        while(fscanf(file_in, "%c", &read_char) > 0)
        {
            read_char ^= name_key[count_char%strlen(name_key)];
            fputc(read_char, file_out);//writing character to out file
            count_char++;
        }
    }

    if (file_in)
        fclose(file_in);

    if (file_out)
        fclose(file_out);
}

You can change the loop to for(int i = 0; i < argc - 1; i++){...} because you are accessing argv[i+1] in each iteration.

Barmak Shemirani
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