C Standard Library stdio.h

What You’ll Learn

<stdio.h> (Standard Input/Output) is the first header most beginners meet. It powers printf and scanf for console I/O, plus fopen, fprintf, and fgets for files. Every C program that talks to the user or reads data from disk relies on it.

Definition and Usage

stdio.h declares functions and types for formatted and unformatted I/O. A FILE * represents an open stream—a file on disk, or the predefined streams stdin (input), stdout (output), and stderr (errors).

Console programs use printf/scanf with stdout and stdin. File programs call fopen to get a FILE *, read or write with fgets, fprintf, fread, etc., then fclose to release resources.

📝 Syntax

Include the header:

#include <stdio.h>

Common console functions

• printf(format, ...) — formatted output to stdout
• scanf(format, ...) — formatted input from stdin
• getchar() / putchar(c) — single character I/O
• puts(s) / fgets(buf, n, stream) — line-oriented strings
• snprintf(buf, size, format, ...) — safe formatted string (C99)

Common file functions

• fopen(path, mode) — open file; modes: "r", "w", "a", "rb", etc.
• fclose(stream) — close an open stream
• fprintf(stream, format, ...) — formatted write to any stream
• fscanf(stream, format, ...) — formatted read from a stream
• fgets(buf, n, stream) / fputs(s, stream) — line I/O
• fread / fwrite — binary block I/O
• fflush(stream) — push buffered output out

Error and status helpers

• perror(msg) — print message + system error (uses errno)
• feof(stream) / ferror(stream) / clearerr(stream)

Important macros and streams

FILE *fp;
stdin, stdout, stderr   /* predefined streams */
EOF                     /* end-of-input indicator */
NULL                    /* null pointer (also in stddef.h) */
FOPEN_MAX, FILENAME_MAX /* implementation limits */

Headers and linking

• #include <stdio.h> — part of the C standard library; linked automatically.
• perror works with errno from <errno.h> (often included indirectly).
• Compile: gcc program.c -std=c11 -o program.

Examples Gallery

Compile with gcc file.c -std=c11 -o out. File examples create or read example.txt in the current directory.

#include <stdio.h>

int main(void) {
    int num;

    printf("Enter an integer: ");
    fflush(stdout);
    if (scanf("%d", &num) != 1) {
        fprintf(stderr, "Invalid input.\n");
        return 1;
    }
    printf("You entered: %d\n", num);

    return 0;
}

Enter an integer: 36
You entered: 36

#include <stdio.h>

int main(void) {
    FILE *file = fopen("example.txt", "w");

    if (file == NULL) {
        perror("Error opening file for write");
        return 1;
    }

    fprintf(file, "Hello, World!\n");
    fprintf(file, "Line two from stdio.h demo.\n");

    if (fclose(file) != 0) {
        perror("Error closing file");
        return 1;
    }

    printf("Wrote example.txt successfully.\n");
    return 0;
}

Wrote example.txt successfully.

#include <stdio.h>

int main(void) {
    FILE *file = fopen("example.txt", "r");
    char buffer[100];

    if (file == NULL) {
        perror("Error opening file for read");
        return 1;
    }

    printf("Contents of example.txt:\n");
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        printf("%s", buffer);
    }

    if (ferror(file)) {
        perror("Error while reading");
        fclose(file);
        return 1;
    }

    fclose(file);
    return 0;
}

Contents of example.txt:
Hello, World!
Line two from stdio.h demo.

#include <stdio.h>

int main(void) {
    FILE *file = fopen("nonexistent.txt", "r");

    if (file == NULL) {
        perror("Error opening file");
        return 1;
    }

    fclose(file);
    return 0;
}

Error opening file: No such file or directory

#include <stdio.h>

int main(void) {
    char name[32];
    char greeting[64];
    int n;

    printf("Your name: ");
    fflush(stdout);

    if (fgets(name, sizeof(name), stdin) == NULL) {
        return 1;
    }

    n = snprintf(greeting, sizeof(greeting),
                 "Hello, %s", name);

    if (n < 0 || (size_t)n >= sizeof(greeting)) {
        fprintf(stderr, "Greeting truncated.\n");
    }

    fputs(greeting, stdout);
    return 0;
}

Your name: Ada
Hello, Ada

🚀 Common Use Cases

• Console programs — menus, quizzes, calculators with printf/scanf.
• Log files — fprintf to append timestamps and messages.
• Config files — read key/value lines with fgets and parse.
• Data import/export — CSV or text reports written with fprintf.
• Debugging — quick traces to stderr without mixing into stdout.
• Binary files — fread/fwrite for images, structs, or archives.

📝 Notes

• scanf("%s") has no length limit—use scanf("%31s", buf) or fgets instead.
• Always match printf format specifiers to argument types (%d for int, %zu for size_t, etc.).
• fopen mode "w" destroys existing file contents; use "a" to append.
• Text mode ("r"/"w") may translate newlines on Windows; "rb"/"wb" for exact bytes.
• EOF is not a character stored in files—it is a return value from read functions.
• Close every fopen with fclose; leaking FILE * handles wastes resources.

⚡ Optimization

stdio buffering reduces system calls—avoid calling fflush more than needed. For large binary transfers, fread/fwrite with a reasonably sized buffer (e.g. 4096 or 8192 bytes) beats byte-by-byte fgetc. For heavy formatted logging, consider writing plain text with fputs or batching fprintf calls.

💡 Best Practices