Question

I use below code to read a char from file and replace it with another,
but I have an error.loop in going to end of file.

What is wrong?

I tested this code on linux (netbeans IDE) and it was correct and worked beautiful but when I tried to use VS 2008 in windows , I found a non end loop.

//address = test.txt

FILE *fp;
fp=fopen(address,"r+");
if(fp == 0)
{
    printf("can not find!!");
}
else
{
    char w = '0';  /// EDIT : int w;
    while(1)
    {
        if((w = fgetc(fp)) != EOF)
        {
            if((w = fgetc(fp)) != EOF)
            {
                fseek(fp,-2,SEEK_CUR);
                fprintf(fp,"0");
            }
        }
        else
        {
            break;
        }
    }
} 
fclose(fp);

Answer 1

The fopen documentation on cplusplus.com says:

For the modes where both read and
writing (or appending) are allowed
(those which include a “+” sign), the
stream should be flushed (fflush) or
repositioned (fseek, fsetpos, rewind)
between either a reading operation
followed by a writing operation or a
writing operation followed by a
reading operation.

We can add an fflush call after the fprintf to satisfy that requirement.

Here is my working code. It creates a file named example.txt and after the program exits that file’s contents will be 000000000000n.

#include <stdio.h>

int main(int argc, char **argv)
{
    FILE * fp;
    int w;

    fp = fopen("example.txt","w");
    fprintf(fp, "David Grayson");
    fclose(fp);

    fp = fopen("example.txt","r+");
    while(1)
    {
        if((w = fgetc(fp)) != EOF)
        {
            if((w = fgetc(fp)) != EOF)
            {
                fseek(fp,-2,SEEK_CUR);
                fprintf(fp,"0");
                fflush(fp);  // Necessary!
            }
        }
        else
        {
            break;
        }
    }
    fclose(fp);
}

This was tested with MinGW in Windows.