Input and Output in C Programming Language

Input and output (I/O) operations are fundamental in any programming language, as they allow programs to interact with users and other systems. In C, the standard library provides functions for handling input and output through various streams. This blog will introduce you to the basics of I/O in C, including standard input, output, and error handling, with practical examples.

Standard Input and Output
In C, the standard input (stdin), standard output (stdout), and standard error (stderr) are predefined file streams. The stdio.h library provides functions to read from and write to these streams.
 
Output Functions
1. printf
The printf function is used to print formatted output to the standard output (usually the console). The function takes a format string followed by a list of arguments.
#include <stdio.h>
 
int main() {
    int age = 25;
    float height = 5.9;
    char grade = 'A';
 
    printf("Age: %d\n", age);
    printf("Height: %.1f\n", height);
    printf("Grade: %c\n", grade);
 
    return 0;
}

In this example:

• %d is a format specifier for integers.

• %.1f is a format specifier for floating-point numbers with one decimal place.

• %c is a format specifier for characters.

 
2. puts
The puts function writes a string to the standard output followed by a newline character.
#include <stdio.h>
 
int main() {
    puts("Hello, World!");
    return 0;
}
 
Input Functions
1. scanf
The scanf function reads formatted input from the standard input (usually the keyboard). The function takes a format string followed by pointers to variables where the input will be stored.
#include <stdio.h>
 
int main() {
    int age;
    float height;
    char grade;
 
    printf("Enter your age: ");
    scanf("%d", &age);
 
    printf("Enter your height: ");
    scanf("%f", &height);
 
    printf("Enter your grade: ");
    scanf(" %c", &grade); // Note the space before %c to consume any leftover newline character
 
    printf("Age: %d\n", age);
    printf("Height: %.1f\n", height);
    printf("Grade: %c\n", grade);
 
    return 0;
}

 
2. gets
The gets function reads a string from the standard input until a newline character is encountered. However, gets is considered unsafe due to the potential for buffer overflow. It is recommended to use fgets instead.
#include <stdio.h>
 
int main() {
    char name[50];
 
    printf("Enter your name: ");
    fgets(name, sizeof(name), stdin); // safer alternative to gets
 
    printf("Your name is: %s", name);
 
    return 0;
}

 
Handling Standard Error
The standard error stream (stderr) is used to output error messages. You can use fprintf to write to stderr.
#include <stdio.h>
 
int main() {
    int age;
 
    printf("Enter your age: ");
    if (scanf("%d", &age) != 1) {
        fprintf(stderr, "Invalid input! Please enter a number.\n");
        return 1;
    }
 
    printf("Age: %d\n", age);
    return 0;
}

 
File I/O
In addition to standard I/O, C provides functions for file input and output. The fopen, fclose, fread, fwrite, fprintf, and fscanf functions are commonly used for file operations.

Writing to a File
#include <stdio.h>
 
int main() {
    FILE *file = fopen("output.txt", "w");
    if (file == NULL) {
        fprintf(stderr, "Error opening file!\n");
        return 1;
    }
 
    fprintf(file, "Hello, File!\n");
    fclose(file);
 
    return 0;
}

 
Reading from a File
#include <stdio.h>
 
int main() {
    FILE *file = fopen("output.txt", "r");
    if (file == NULL) {
        fprintf(stderr, "Error opening file!\n");
        return 1;
    }
 
    char buffer[100];
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        printf("%s", buffer);
    }
 
    fclose(file);
    return 0;
}

 
Final Remarks
Understanding input and output functions in C is crucial for interacting with users and external systems. The stdio.h library provides a comprehensive set of functions for handling standard I/O and file operations. By mastering these functions, you can create robust and user-friendly C programs. Practice using these I/O functions to gain confidence and enhance your programming skills.