I cannot write to stdin with /proc/{pid}/fd/0

Question

I have this program:

#include <stdio.h>

int main() {
  char buf[10];
  puts("gimme input:");
  fread(buf, 1, 10, stdin);
  printf("got %s", buf);
}

When I run this and open another terminal I try to write to stdin:

echo "ASDFASDFASDF" > /proc/{pid}/0

ASDFSADFSADF gets printed on the terminal that is running my C program, but fread still doesn't return until I type in the actual terminal. It also does not print any of the text that I wrote to /proc/{pid}/0

Is there something else I have to do to programatically input text to stdin?

Answer 1

If stdin is a terminal, then writing something to stdin will write to the terminal. Reading from the terminal will read whatever is typed into the terminal, not what's written to the terminal. This is just how terminals work.

If you want a program to read from something other than a terminal, you have to direct that to happen. Or, if you want to use a virtual terminal that you can put information into it and have it be read out, you have to direct that to happen.

Probably the simplest solution is to create a pipe with mkpipe and have the program read from the pipe rather than a terminal.

Answer 2

When you execute the echo command output-ing to the File Descriptor 0 you're just sending text. If you check the file descriptor using ls -l probably it is pointing to an device TTY or PTY/PTS. If you check the FD type using lsof it will be tty. It means you need to interact with this FD such as TTY.

Basically you need to simulate the input to get the expected behavior.

You can do this by calling the kernel tool ioctl.tiocsti(). I added a python code into the following similar question: Writing to File descriptor 0 (STDIN) only affects terminal. Program doesn't read