Topic 4- File Handling

Tags: #◇CCPROG2 #◇C Programming #◇Programming #◇Computer Science #◇Files #◇Data Analysis #◇Searching #◇Structured Data #◇Structured Programming #◇Memory Management

◇Overview

File handling in C allows programs to read from and write to files on disk using the <stdio.h> library and the FILE pointer type.

◇Key Concepts

FILE * pointer represents an open file stream fopen() opens a file; fclose() closes it Modes: "r" (read), "w" (write), "a" (append), "rb"/"wb" (binary) Text I/O: fprintf, fscanf, fgets, fputs Binary I/O: fread, fwrite

◇Details / Theory

Opening and Closing Files

FILE *fp = fopen("data.txt", "r");  // Open for reading
if (fp == NULL) {
    printf("Error: Cannot open file.\n");
    return 1;
}

// ... use the file ...

fclose(fp);  // Always close when done

File Modes

Writing to a Text File

FILE *fp = fopen("output.txt", "w");
if (fp != NULL) {
    fprintf(fp, "Name: %s\n", "Alice");
    fprintf(fp, "Score: %d\n", 95);
    fputs("End of record.\n", fp);
    fclose(fp);
}

Reading from a Text File

FILE *fp = fopen("data.txt", "r");
char line[256];

// Read line by line
while (fgets(line, sizeof(line), fp) != NULL) {
    printf("%s", line);
}
fclose(fp);

// Structured reading with fscanf
FILE *fp = fopen("students.txt", "r");
char name[50];
int age;
float gpa;

while (fscanf(fp, "%s %d %f", name, &age, &gpa) == 3) {
    printf("%s is %d with GPA %.2f\n", name, age, gpa);
}
fclose(fp);

Binary File I/O

typedef struct {
    char name[50];
    int age;
} Student;

// Write binary
FILE *fp = fopen("students.bin", "wb");
Student s = {"Alice", 20};
fwrite(&s, sizeof(Student), 1, fp);
fclose(fp);

// Read binary
fp = fopen("students.bin", "rb");
Student s2;
fread(&s2, sizeof(Student), 1, fp);
printf("%s, %d\n", s2.name, s2.age);
fclose(fp);

File Position Functions

Origins for fseek: SEEK_SET (beginning), SEEK_CUR (current), SEEK_END (end).

Checking for End of File

while (!feof(fp)) {
    // Read data...
}

Note: Prefer checking the return value of fread/fgets/fscanf rather than using feof() in loop conditions.

◇Common Pitfalls

Not checking if fopen() returns NULL — leads to segfault. Forgetting to fclose() — causes resource leaks and data loss. Using "w" mode when intending to append — overwrites the file. Using feof() as a loop condition — can read one extra iteration. Mixing text and binary modes on Windows (newline translation issues).

◇Summary

TL;DR — File handling uses FILE * with fopen/fclose. Use fprintf/fscanf/fgets for text and fread/fwrite for binary. Always check for NULL on open and close files when done.

◇Links

Related: ◇Strings — ◇Structs — ◇Recursion — ◇What is an Array Source / Reference:

◇References & Navigation

• Previous Topic: ◇Structs
• Next Topic: ◇Recursion
• Related Notes: ◇Files I-O (via Streams), ◇Computer Hardware - IO Devices