Question

I compiled this (gcc compiler) :

#include< stdio.h>

 main() {

    unsigned char ch;
    FILE *fp;
    fp=fopen("trial","r");
    while((ch=getc(fp))!=EOF)
        printf("%c",ch);
    fclose(fp);
}

It gives the follwing:

Warning: comparison is always true due to limited range of the data type

On executing, an endless stream of characters is printed on terminal.
(Assuming I created a file named “trial” before compiling the program and wrote some text in the file.)

Kindly explain the warning…..

Answer 1

The EOF value in C is an int while ch here is a char. The char type is smaller than int and hence can represent less values than int can. EOF is one of the values which char simply can’t ever represent and hence ch will never be equal to EOF.

In this scenario getc actually returns an int so it can represent EOF. But you are immediately shrinking it to a char and losing that extra information.

Here’s a way to properly write this.

int cur;
FILE *fp;
fp=fopen("trial","r");
while((cur = getc(fp))!=EOF) {
  unsigned char ch = cur;
  printf("%c",ch);
}
fclose(fp);