Question

This can be a good question for finding bugs.
No? Okay for beginners at least.

#define SIZE 4
int main(void){
  int chars_read = 1;
  char buffer[SIZE + 1] = {0};  
  setvbuf(stdin, (char *)NULL, _IOFBF, sizeof(buffer)-1);  
  while(chars_read){
    chars_read = fread(buffer, sizeof('1'), SIZE, stdin);
    printf("%d, %s\n", chars_read, buffer);
  }
  return 0;
}

Using the above code, I am trying to read from a file using redirection ./a.out < data. Contents of input file:

1line
2line
3line
4line

But I am not getting the expected output, rather some graphical characters are mixed in.
What is wrong?

---

Hint: (Courtesy Alok)

• sizeof('1') == sizeof(int)
• sizeof("1") == sizeof(char)*2

So, use 1 instead 🙂

Take a look at this post for buffered IO example using fread.

Answer 1

The type of '1' is int in C, not char, so you are reading SIZE*sizeof(int) bytes in each fread. If sizeof(int) is greater than 1 (on most modern computers it is), then you are reading past the storage for buffer. This is one of the places where C and C++ are different: in C, character literals are of type int, in C++, they are of type char.

So, you need chars_read = fread(buffer, 1, SIZE, stdin); because sizeof(char) is 1 by definition.

In fact, I would write your loop as:

while ((chars_read = fread(buffer, 1, sizeof buffer - 1)) > 0) {
    buffer[chars_read] = 0; /* In case chars_read != sizeof buffer - 1.
                               You may want to do other things in this case,
                               such as check for errors using ferror. */
    printf("%d, %s\n", chars_read, buffer);
}

To answer your another question, '\0' is the int 0, so {'\0'} and {0} are equivalent.

For setvbuf, my documentation says:

The size argument may be given as zero to obtain deferred optimal-size buffer allocation as usual.

Why are you commenting with \\ instead of // or /* */? 🙂

Edit: Based upon your edit of the question, sizeof("1") is wrong, sizeof(char) is correct.

sizeof("1") is 2, because "1" is a char array containing two elements: '1' and 0.