CODEWITHSUNDEEP
creadline
hertzsprung
hertzsprung

Reputation: 9893

Why does readline() echo stdin to stdout?

I've written a small C program that uses readline("") to read input from stdin. Rather than interacting with it from the terminal, I've written a test harness which executes the C program and gives it input:

test harness --stdout--> | --stdin--> C program
         ^------stdin--- | <--stdout-----/

This test harness is receiving an echo of its output, even though the program itself. Why is readline() echoing its input, and can I disable this? Or should I not be using readline() in a non-interactive environment?

Upvotes: 3

Views: 1968

Answers (2)

Low power
Low power

Reputation: 273

By default Readline echos user input to stdout, however it is configurable.

From readline documentation:

Variable: FILE * rl_outstream

The stdio stream to which Readline performs output.

Just set rl_outstream to stderr in beginning of your program, so it will echoing to stderr instead; For example:

#include <readline/readline.h>
#include <stdio.h>
...
int main() {
    rl_outstream = stderr;
...

In fact bash(1) does this too.

And I also agree that if your program isn't designed to be used interactively, your should use another simpler way to get lines from stdin, such as getline(3), fgets(3) or getchar(3).

Upvotes: 2

user149341
user149341

Reputation:

The readline library is solely intended for interactive use. Your program should probably fall back to using fgets() if isatty(STDIN_FILENO) is false.

Upvotes: 5

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