Reading a File

Once a file is open, we might want to read input from it. There are three common functions in C for reading from a file: fgetc, fgets, and fread. Here’s a breakdown of how fgetc works:

Prototype:

int fgetc(FILE * stream);

• Although you might expect fgetc to return a char, it actually returns an int. This allows it to return all possible character values, plus a distinct value to indicate the end of the file (EOF). The EOF value is defined as a constant in stdio.h.
• Each time you call fgetc, it reads a character and advances the stream’s position.

Common Issue in Loops:

When reading characters in a loop, beginners often encounter issues because fgetc advances the stream’s position with each call. Consider this incorrect code:

//broken
FILE * f = fopen(inputfilename, "r");
if (f == NULL) { /* error handling code omitted */ }
while (fgetc(f) != EOF) {
  char c = fgetc(f);
  printf("%c", c);
}

• The code reads one character, checks if it’s EOF, then reads the next character and prints it. This results in printing every other character from the file, missing half of the content.

Correct Approach:

To fix this, we can leverage the fact that assignment is also an expression, meaning it both assigns a value and evaluates to that value. This allows us to combine the assignment and the EOF check in one line:

//fixed
FILE * f = fopen(inputfilename, "r");
if (f == NULL) { /* error handling code omitted */ }
int c;
while ( (c = fgetc(f)) != EOF ) {
  printf("%c", c);
}

Here’s what happens:

• We assign the value from fgetc(f) to c inside the loop condition. This assignment is wrapped in parentheses to ensure the correct order of operations.
• We then compare the assigned value with EOF to check when to stop reading.

Why int Instead of char?

The variable c is declared as an int, not a char. If we used char, our code would introduce a subtle bug:

• fgetc returns an int so it can represent all possible character values plus the EOF value. If you store the result in a char, which can only hold 256 different values, you might lose information.
• On most systems, EOF is represented by 1, but a char can only hold values from 0 to 255. This means if your file contains a character with the value 255, your program would misinterpret it as EOF and stop reading prematurely.