File I/O

In C programming, File I/O (Input/Output) refers to reading from and writing to files stored on secondary storage (like hard disks), rather than working only with console (keyboard/screen).

Why is this important:

• Data persists even after the program ends.
• Allows handling large data.
• Used in real-world applications like databases, log systems, and configuration files.

File Handling in C

C uses a file pointer (FILE *) to manage files. All file operations are performed through this pointer.

The header file:

code.c

#include <stdio.h>

is required for file operations.

Key Steps in File I/O:

Open a file (fopen) -> Perform operations (read/write/append) -> Close the file (fclose) -> File Modes in C

When opening a file using fopen(filename, mode), the mode defines what you can do.

Mode	Meaning
"r"	Read (file must exist)
"w"	Write (create new or overwrite existing)
"a"	Append (write at end of file)
"r+"	Read + Write (file must exist)
"w+"	Read + Write (create new or overwrite)
"a+"	Read + Append

• For binary files, add “b” → e.g., “rb”, “wb”, “ab”.

Opening a File

code.c

FILE *fp;
fp = fopen("data.txt", "r"); // open for reading

if(fp == NULL) {
    printf("Error opening file!\n");
    return 1;
}

• Always check if file opened successfully.

Closing a File

code.c

fclose(fp);

• Frees resources and ensures data is written properly.

Writing to a File

Using fprintf()

code.c

FILE *fp = fopen("data.txt", "w");
fprintf(fp, "Hello, World!\n");
fclose(fp);

Using fputs()

code.c

fputs("Hello, File Handling!", fp);

Using fputc()

code.c

fputc('A', fp);

Reading from a File

Using fscanf()

code.c

FILE *fp = fopen("data.txt", "r");
char str[50];
fscanf(fp, "%s", str);

• Problem: Stops at whitespace.

Using fgets()

code.c

fgets(str, sizeof(str), fp);

Using fgetc()

code.c

char ch = fgetc(fp);

Using fread() and fwrite() (Binary Files)

code.c

fread(buffer, size, count, fp);
fwrite(buffer, size, count, fp);

• Used for structures, images, and other non-text data.

File Positioning Functions

Function	Purpose
ftell(fp)	Returns current position in file
fseek(fp, offset, origin)	Move file pointer
rewind(fp)	Reset pointer to beginning

Example:

code.c

fseek(fp, 0, SEEK_END);
long size = ftell(fp);
rewind(fp);

Detecting End of File (EOF)

code.c

while(!feof(fp)) {
    char ch = fgetc(fp);
    if(ch != EOF)
            putchar(ch);
}

• Better approach: Check return value of fgetc() instead of feof().

Error Handling in File I/O

• Use perror() or strerror(errno) for descriptive errors.

code.c

if(fp == NULL) {
    perror("File opening failed");
}