CODEWITHSUNDEEP
c++stringsegmentation-faultundefined-behaviorstring-literals
WannabeArchitect
WannabeArchitect

Reputation: 1166

Segmentation Fault before reaching main - C++

I'm writing a code to find a proper input that would generate a certain output for the SHA-1 hash function.

The problem I'm running into is that my code raises a segmentation fault, but gdb finds that it raises the following error upon entering main() and befor execution of any other code:

Program received signal SIGSEGV, Segmentation fault.
__strncpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S:636
636 ../sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S: No such file or directory.

This is my code:

#include <iostream>
#include <cstdlib>
#include <cstring>

#include "sha1.hpp"

int main() {
    char *prefix = "SHA1sha1";
    char *suffix = "chicken and beer";
    std::string hashvalue = "nonzero";
    char *line = "just some dummy string";
    int loop_number = 0;

    while (hashvalue.c_str()[0] != '0' || hashvalue.c_str()[1] != '0') {
        // change prefix
        strncpy(prefix, hashvalue.c_str(), 8);
        // hash the concatenated string of prefix and suffix
        strncpy(line, prefix, 8);
        strncat(line, suffix, strlen(suffix));
        hashvalue = sha1(line);

        loop_number++;
        if (loop_number % 1000 == 0) std::cout << loop_number << "th loop, hash value: " << hashvalue << std::endl;
    }

    std::cout << "Found prefix: " << prefix << " with hash value: " << hashvalue << std::endl;

    return 0;
}

The sha1.hpp was not implemented by me, but was taken from here: http://www.zedwood.com/article/cpp-sha1-function

I have changed sha1.h to sha1.hpp, though, but this is probably not what's causing the segmentation fault.

Now I have tried searching for a solution to this problem with the error message and also with the keywords "segmentation fault before main", and this post seems to be going through similar problems: Segmentation Fault before main

However, I have looked into the two suggested solutions, but couldn't find one that fits me.

  1. I don't think my code has too many variables in the stack. In fact, I have tried commenting out using the function sha1() just in case, but the same problem occurred.

  2. I have initialized all char* and std::string in my code before usage.

FYI, I'm using g++ to compile my C++ codes.

Any help or push in the right direction would be greatly appreciated.

Upvotes: 3

Views: 305

Answers (2)

kiran Biradar
kiran Biradar

Reputation: 12732

You are modifying the immutable contents.

// change prefix
strncpy(prefix, hashvalue.c_str(), 8);
// hash the concatenated string of prefix and suffix
strncpy(line, prefix, 8);
strncat(line, suffix, strlen(suffix));

Try to change the declaration as below.

char prefix[100] = "SHA1sha1";
char suffix[200] = "chicken and beer";
char line[200] = "just some dummy string

Also, I guess

while (hashvalue.c_str()[0] != '0' || hashvalue.c_str()[1] != '0') {

should be

while (hashvalue.c_str()[0] != '0' && hashvalue.c_str()[1] != '0') {

Update:

De Morgan's law states,

not (A and B) = not A or not B

Again it is individual's choice to use whatever form they want

Upvotes: 4

Vlad from Moscow
Vlad from Moscow

Reputation: 310950

String literals have types of constant character arrays in C++, and any attempt to modify a string literal results in undefined behavior. You are trying to modify a string literal at least in this statement:

strncpy(prefix, hashvalue.c_str(), 8);

Instead of string literals you should use character arrays or objects of the type std::string.

Pay attention to that; for example this if statement

while (hashvalue.c_str()[0] != '0' || hashvalue.c_str()[1] != '0') {

could be written much simpler:

while (hashvalue[0] != '0' || hashvalue[1] != '0') {

though it seems the condition does not make sense. Maybe you mean the following

while ( hashvalue.size() > 1 ) {

Also you need to include the header <string>

#include <string>

Upvotes: 4

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