C:
Writing to standard error

How to:

In C, the stderr stream is used to write error messages. Unlike writing to standard output with printf, writing to stderr can be done using fprintf or fputs. Here’s how you can do it:

#include <stdio.h>

int main() {
    fprintf(stderr, "This is an error message.\n");

    fputs("This is another error message.\n", stderr);
    
    return 0;
}

Sample output (to stderr):

This is an error message.
This is another error message.

It’s important to note that while the output appears similar to stdout in the console, when redirection is used in the terminal, the distinction becomes clear:

$ ./your_program > output.txt

This command redirects only the standard output to output.txt, while error messages will still appear on the screen.

Deep Dive

The distinction between stdout and stderr in Unix-based systems dates back to the early days of C and Unix. This separation allows for more robust error handling and logging, as it enables programmers to redirect error messages independent of standard program output. While stderr is unbuffered by default to ensure immediate output of error messages, which helps in debugging crashes and other critical issues, stdout is typically buffered, meaning its output might be delayed until the buffer is flushed (e.g., program completion or manual flushing).

In modern applications, writing to stderr is still relevant, especially for command-line tools and server applications where distinguishing between regular log messages and errors is crucial. However, for more complex error handling, especially in GUI applications or where more sophisticated logging mechanisms are needed, programmers might use dedicated logging libraries that provide more control over message formatting, destinations (e.g., files, network), and severity levels (info, warning, error, etc.).

While stderr provides a fundamental mechanism for error reporting in C, the evolution of programming practices and the availability of advanced logging frameworks mean it is often just the starting point for modern error handling strategies.