CODEWITHSUNDEEP
c++segmentation-faultifstream
user1106106
user1106106

Reputation: 277

Segmentation fault while using "ifstream"

I'm trying to get a part of text in a file. I used "ifstream":

#include <fstream>

void func(){
    char *myString;

    ifstream infile;
    infile.open("/proc/cpuinfo");

    while (infile >> myString){
        if  (!strcmp(myString,"MHz"))
        break;
    }
}

and I get a "Segmentation fault". does anyone know why?

Upvotes: 1

Views: 12549

Answers (5)

Software_Designer
Software_Designer

Reputation: 8587

char myString[256] compiles fine just as well too.

#include <iostream>
#include <fstream>
#include <string>
using namespace std;


 void func()
 {
    char myString[256] ;

    ifstream infile;
    infile.open("/proc/cpuinfo");

    while ( ! infile.eof() )
    {
          infile >> myString;
          cout<<myString<<" \n"; 

        if  (! strcmp(myString,"MHz") )
        {
            infile.close();
            break;
        }
    }
     infile.close();
     cout<<" \n";  
}

int main()
{

    func();
    return 0;
}

Upvotes: 0

Stack Overflow is garbage
Stack Overflow is garbage

Reputation: 247999

There's a reason why C++ has a string class, you know. It's because using char pointers is cumbersome and error-prone.

infile >> myString

will read from the file into *wherever myString points to. And it is an uninitialized pointer, it points to some random garbage address.

If you absolutely do want to use char pointers instead of strings, you'll have to allocate a buffer you can read data into.

But the sane solution is to replace it entirely by std::string.

Upvotes: 1

Paul Manta
Paul Manta

Reputation: 31577

Because you did not allocate memory for myString. The quick solution to this is to use std::string instead of the C-style char* strings, which does memory management so you don't have to.

Here's why your error occurs:

When you declare char *myString you are creating a pointer to a character. However you do not initialize this pointer to anything, so it can point absolutely anywhere in memory. When you do infile >> myString you are going to write a bunch of characters at an unspecified location in memory. It just so happens that this location was a vital part of your program, which caused it to crash.

Upvotes: 0

James Kanze
James Kanze

Reputation: 153919

Because the target value should be:

std::string myString;

and not char*. It's possible to use char*, but you have to ensure that it points to something big enough first. (In your case, it doesn't point anywhere—you forgot to initialize it.) And defining “big enough” is non-trivial, given that you don't know the size of the next word until you've read it.

Upvotes: 2

kan
kan

Reputation: 28951

You have not allocated memory for the myString. Use std::string. Or better any other language, python, perl, or unix utils such as grep, awk, sed.

Upvotes: 8

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