[中文版本](/posts/c教程/zh-CN/05-输入输出与文件/044-Working with stdin, stdout, and stderr)

Every C program automatically starts with three open streams connected to your environment, the keyboard, the terminal screen, and the error console. They are the standard I/O streams that make your programs flexible and scriptable.

Understanding these three streams is crucial for writing tools that can interact with files, pipes, and other programs, the essence of Unix-style design.

The Three Standard Streams

Stream	Purpose	Typical Device	Example Use
stdin	Standard Input	Keyboard (or file via<)	scanf,fgets
stdout	Standard Output	Screen (or file via>)	printf,puts,fprintf(stdout, …)
stderr	Standard Error	Screen (separate from stdout)	fprintf(stderr, …)

These are all of typeFILE *. You can treat them like normal file pointers, reading, writing, or redirecting them.

Basic Example

#include <stdio.h>
int main(void) {
    char name[50];
    printf("Enter your name: ");
    fgets(name, sizeof(name), stdin);
    fprintf(stdout, "Hello, %s", name);
    fprintf(stderr, "Note: This is an example error message.\n");
    return 0;
}

Output:

Enter your name: Alice
Hello, Alice
Note: This is an example error message.

Redirection in the Shell

You can redirect each stream separately:

./program < input.txt > output.txt 2> errors.log

• < replacesstdin(read input from file)
• > replacesstdout(write normal output to file)
• 2> replacesstderr(write errors to file)

You can also combine them:

./program > all_output.txt 2>&1

This merges both output and error streams into one file.

Reading from stdin

You can build programs that process input dynamically, one line at a time:

#include <stdio.h>
int main(void) {
    char line[100];
    printf("Enter text (Ctrl+D to stop):\n");
    while (fgets(line, sizeof(line), stdin))
        printf("You said: %s", line);
    return 0;
}

Now your program behaves like a Unix filter, it can read from a file, a pipe, or a keyboard input interchangeably.

Example:

echo "hello" | ./program

Output:

You said: hello

Writing to stdout and stderr

stdout is for normal program output, whilestderr is for error messages or logs.

#include <stdio.h>
int main(void) {
    fprintf(stdout, "Everything is fine.\n");
    fprintf(stderr, "Warning: something might be wrong.\n");
    return 0;
}

You can suppress normal output but keep errors:

./program > /dev/null

Output:

Warning: something might be wrong.

Tiny Code: Word Counter Using stdin/stdout

This small program mimics a simplified version of the Unixwc command.

#include <stdio.h>
#include <ctype.h>
int main(void) {
    int ch, words = 0, in_word = 0;
    while ((ch = getchar()) != EOF) {
        if (isspace(ch))
            in_word = 0;
        else if (!in_word) {
            in_word = 1;
            words++;
        }
    }
    printf("Word count: %d\n", words);
    return 0;
}

Try:

echo "C is small but powerful" | ./wordcount

Output:

Word count: 4

Mixing stdout and stderr

Sometimes you need to log progress tostderr while outputting results tostdout. That way, logs don’t pollute the actual data.

#include <stdio.h>
int main(void) {
    for (int i = 0; i < 3; i++) {
        fprintf(stderr, "Processing item %d...\n", i + 1);
        fprintf(stdout, "Item %d processed\n", i + 1);
    }
    return 0;
}

Redirect logs separately:

./program > result.txt 2> log.txt

Flushing Buffers

Output streams are buffered, data isn’t written until the buffer is full or flushed. To ensure output appears immediately:

fflush(stdout);  // flush output buffer

This is useful for interactive programs.

Why It Matters

These three streams give your program flexibility:

• Work interactively (keyboard/screen)
• Work in batch (file input/output)
• Chain with other tools using pipes

They are the foundation of the Unix philosophy: small programs that do one thing well and can be composed together.

Try It Yourself

1. Write a program that reads from stdin and prints only lines containing a given keyword.
2. Print errors to stderr if no keyword is provided.
3. Redirect input and output from files using< and>.
4. Add progress messages to stderr and redirect them to a separate log.
5. Combine everything into a small “filter” tool that processes text from pipelines.

Withstdin,stdout, andstderr, your C programs become tools that fit seamlessly into real workflows, able to interact with files, other programs, and users alike. Next, you’ll explore buffered I/O, understanding how the C library optimizes performance through read and write buffers usingfgets,fputs, and more.