Question

I’ve got a bit problem. My program throws segmentation fault when returning zero in main.

The main function looks like this:

int main(int argc, char* argv[]){
    ifstream fs("test.dat", ios::binary);
    cSendStream sendstr(&fs,20);

    char *zomg=sendstr.data();
    //zomg[20]=0;

    sendstr.read(20);

    cout<<"Buffer: "<<sendstr.data()<<endl;
    cout<<"Remaining: "<<sendstr.dataAvailable()<<endl;

    sendstr.read(2);
    cout<<"Buffer: "<<zomg<<endl;
    cout<<"Remaining: "<<sendstr.dataAvailable()<<endl;

    sendstr.read(10);
    cout<<"Buffer: "<<zomg<<endl;
    cout<<"Remaining: "<<sendstr.dataAvailable()<<endl;
    cout<<"end..."<<endl;
    return 0;
}

The commented zomg part is the point which makes the program crash. zomg is pointing on char[20]. My point on that line is to set the end of the array because somehow if I don’t do that the stream reads more data than 20 bytes, but it prints just one unnecessary symbol though.

Funny thing is that even if I write there some additional code between this and return 0 it throws fault first when returning.

Just for the case you wanted to see the cSendStream class:

cSendStream.h:

class cSendStream{
  public:
    cSendStream(std::istream*, int streamsize);
    int read(int);
    int dataAvailable();
    char* data();
  private:
    void shift(int);

    std::istream *source;
    int streamsize;
    char* buffer;
};

and the cSendStream.cpp:

#include "cSendStream.h"

cSendStream::cSendStream(std::istream *src, int size){
    source=src;
    streamsize=size;
    buffer=new char[streamsize];
    memset(buffer,0,streamsize);
}

int cSendStream::read(int i){
    if(dataAvailable()<1 || i<=0){
        return 0;
    }
    if(i>dataAvailable()){
        i=dataAvailable()-1;
    }
    if(i>streamsize){
        i=streamsize;
    }

    shift(i);
    source->read(&(buffer[streamsize-i]),i);
    return i;
}

int cSendStream::dataAvailable(){
    int current=source->tellg();
    source->seekg (0, std::ios::end);
    int available = (int)(source->tellg())-current;
    source->seekg (current);

    return available;
}

char* cSendStream::data(){
    return buffer;
}

void cSendStream::shift(int i){
    char tmp[2048];
    memcpy(tmp,buffer,streamsize);
    memcpy(&(buffer[0]),&(tmp[i]),streamsize-i);
}

Answer 1

zomg[20]=0 is writing one past the end of the allocated array, but it is difficult to guess why the segfault is occurring. My guess is that your clever compiler is using alloca for the allocation and you are scribbling on the return address.

It might be fun to look at the assembly (usually -S) to see what’s happening.