Segmentation fault in comparing the second byte of two files

Question

I was testing the commands fread() and fseek, and I made a programme to compare the second byte of both the programme. The programme gave segmentation fault at the line indicated (the last fread()).

int main(int argc, char** argv)
{
    FILE *file1, *file2;
    long size1, size2;
    long size1a, size2a;
    char *name1, *name2;
    char *temp1, *temp2;

    if (argc != 3)
    {
        cout<<"Enter two file names"<<endl;
        exit(1);
    }

    name1 = argv[1];
    name2 = argv[2];

    //open files
    file1 = fopen(name1, "r");
    file2 = fopen(name2, "r");

    //get file size
    fseek(file1, 0, SEEK_END);
    size1 = ftell(file1);
    rewind(file1);

    fseek (file2,0,SEEK_END);
    size2 = ftell(file2);
    rewind(file2);

    fseek(file1,1,SEEK_SET);
    fseek(file2,1,SEEK_SET);
    cout<<"1"<<endl; //----cout worked
    fread(temp1,1,1,file1);
    cout<<"2"<<temp1<<endl; //---cout worked. 2nd byte of file1 was printed.
    fread(temp2,1,1,file2); //SEGMENTATION FAULT AT THIS LINE

    if(*temp1==*temp2)
        cout<<"same"<<endl;
    else
        cout<<"different"<<endl;    

    return 0;
}

Although I corrected the programme by defining

char temp1, temp2;

and writing

fread(&temp1,1,1,file1);
fread(&temp2,1,1,file2);

I still do not know what gave the seg fault at the second fread whereas the first one ran correctly. What might be the reason for the segmentation fault?

Answer 1

1) open files in binary mode `fp = fopen(name, "rb");

2) check the result of file openning if(fp != NULL)

3) allocate memory for data: char *temp1; is not enough, it is just pointer, you need something like:

  temp = (char*) malloc(sizeof(char) * N); // N number of bytes

EDIT:

4) do not forget to close the file (even if it is not a cause of segmentation fault).

I just make changes and the following code works without problems:

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
using namespace std;

int main(int argc, char** argv)
{
    FILE *file1, *file2;
    long size1, size2;
    long size1a, size2a;
    long greater;
    char *name1, *name2;
    char *temp1, *temp2;
    // memory allocation (two lines added)
    temp1 = (char*) malloc(sizeof(char));
    temp2 = (char*) malloc(sizeof(char));

    if (argc != 3)
    {
        cout<<"Enter two file names"<<endl;
        exit(1);
    }

    name1 = argv[1];
    name2 = argv[2];

    //open files
    file1 = fopen(name1, "rb");
    if(file1 == NULL)
    {
        cout<<"File "<< name1 << " cannot be read" <<endl;
        exit(1);
    }
    file2 = fopen(name2, "rb");
    if(file2 == NULL)
    {
        cout<<"File "<< name2 << " cannot be read" <<endl;
        exit(1);
    }
    //get file size
    fseek(file1, 0, SEEK_END);
    size1 = ftell(file1);
    rewind(file1);

    fseek (file2,0,SEEK_END);
    size2 = ftell(file2);
    rewind(file2);

    if(size1>size2) greater = size1;
    else greater = size2;

    fseek(file1,1,SEEK_SET);
    fseek(file2,1,SEEK_SET);
    cout<<"1"<<endl; //----cout worked
    fread(temp1,1,1,file1);
    cout<<"2"<<temp1<<endl; //---cout worked. 2nd byte of file1 was printed.
    fread(temp2,1,1,file2); //SEGMENTATION FAULT AT THIS LINE

    if(*temp1==*temp2)
        cout<<"same"<<endl;
    else
        cout<<"different"<<endl;    

    fclose(file1);
    fclose(file2);
    // memory de-allocation (two lines added)
    free(temp1);
    free(temp2);
    return 0;
}

EDIT2:

5) de-allocate memory after usage (two more lines added)