I'm trying to understand getchar() != EOF

Question

I'm reading The C Programming Language and have understood everything so far. However when I came across the getchar() and putchar(), I failed to understand what is their use, and more specifically, what the following code does.

main() {     int c;     while ((c = getchar()) != EOF)        putchar(c); } 

I understand the main() function, the declaration of the integer c and the while loop. Yet I'm confused about the condition inside of the while loop. What is the input in this C code, and what is the output.

Sorry if this is a basic and stupid question, but I'm just looking for a simple explanation before I move on in the book and become more confused.

Answer 1

This code can be written more clearly as:

main() {     int c;     while (1) {         c = getchar();            // Get one character from the input         if (c == EOF) { break; }  // Exit the loop if we receive EOF ("end of file")         putchar(c);               // Put the character to the output     } } 

The EOF character is received when there is no more input. The name makes more sense in the case where the input is being read from a real file, rather than user input (which is a special case of a file).

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[As an aside, generally the main function should be written as int main(void).]

Answer 2

getchar() is a function that reads a character from standard input. EOF is a special character used in C to state that the END OF FILE has been reached.

Usually you will get an EOF character returning from getchar() when your standard input is other than console (i.e., a file).

If you run your program in unix like this:

$ cat somefile | ./your_program 

Then your getchar() will return every single character in somefile and EOF as soon as somefile ends.

If you run your program like this:

$ ./your_program 

And send a EOF through the console (by hitting CTRL+D in Unix or CTRL+Z in Windows), then getchar() will also returns EOF and the execution will end.