How to pipe data to a program which calls scanf() and read() in Linux

Question

I have a C program which looks like this:

#include <stdio.h>
#include <unistd.h>

int main()
{
    int n;
    char str[16];
    scanf("%d", &n);
    printf("n: %d\n", n);

    int count = read(STDIN_FILENO, str, 16);
    printf("str: %s\n", str);
    printf("read %d bytes\n", count);
}

If I pipe data into this program using a command like

(echo -en '45\n'; echo -en 'text\n') | ./program

only scanf() actually reads data. read() simply reads 0 bytes.

In other words, the program outputs

n: 45
str:
read 0 bytes

How can I pipe data to both scanf() and read()?

Edit: Sorry, I should have clarified: I'm looking for a way to do this without modifying the source. Thanks to the people who answered and commented anyway.

Answer 1

#include <stdio.h>
#include <unistd.h>

int main()
{
    int n;
    char str[16];
    setbuf(stdin, NULL); // Ensure that there's no buffering for stdin
    scanf("%d", &n);
    printf("n: %d\n", n);

    int count = read(STDIN_FILENO, str, 16);
    printf("str: %s\n", str);
    printf("read %d bytes\n", count);
}

As the previous answers said, scanf is causing your problem because it uses a buffer. So you can make sure it doesn't by callingsetbuf(stdin,NULL).

Answer 2

It is not recommended at all to use both file descriptor functions (read) and stream functions (scanf) for the same file descriptor. Functions using FILE * (i.e. fread/fprintf/scanf/...) are buffering data while functions using file descriptors (i.e. read/write/...) do not use those buffers. In your case, the easiest way to fix the program is to use fread instead of read. The program may look like:

#include <stdio.h>
#include <unistd.h>

int main() {
  int n;
  char str[16];
  scanf("%d", &n);
  printf("n: %d\n", n);

  int count = fread(str, 16, 1, stdin);
  printf("str: %s\n", str);
  printf("read %d bytes\n", count);
}

In your original example, scanf has read the input ahead and stored it in its buffer. Because the input was so short and was available immediately it was entirely read to the buffer and your invocation of read had nothing more to read.

This does not happen when you enter the input directly from a terminal, because scanf does not buffer data beyond one line when reading from a terminal device. This would also not happen if you create a time pause in your piped input for example by a command:

(echo -en '45\n'; sleep 1; echo -en 'text\n') | ./program