Skip to main content

5. File Input/Output

5.1 Why File I/O?

So far, all our programs lose their data when they terminate. File I/O allows programs to:

  • Save data permanently
  • Read data from external sources
  • Create logs and reports
  • Share data between programs

Python vs C File Operations:

Python C
f = open("file.txt", "r") FILE *f = fopen("file.txt", "r");
f.write("text") fprintf(f, "text");
content = f.read() fscanf(f, "%s", buffer);
f.close() fclose(f);

5.2 File Pointer

In C, files are accessed through file pointers of type FILE*:

FILE *filePointer;

The FILE type is defined in <stdio.h>.

5.3 Opening Files - fopen()

Function Signature:

FILE *fopen(const char *filename, const char *mode);

File Opening Modes:

Mode Description If File Exists If File Doesn't Exist
"r" Read only Opens file Returns NULL
"w" Write only Overwrites content Creates new file
"a" Append Appends to end Creates new file
"r+" Read and write Opens file Returns NULL
"w+" Read and write Overwrites content Creates new file
"a+" Read and append Opens file Creates new file
"rb" Read binary Opens file Returns NULL
"wb" Write binary Overwrites content Creates new file
"ab" Append binary Appends to end Creates new file

Example:

FILE *file;

// Open file for reading
file = fopen("data.txt", "r");

// Always check if file opened successfully
if (file == NULL) {
    printf("Error: Could not open file!\n");
    return 1;
}

// ... file operations ...

fclose(file);

5.4 Closing Files - fclose()

Always close files after use to:

  • Free system resources
  • Ensure all data is written to disk
  • Prevent data corruption
int fclose(FILE *filePointer);

Returns:

  • 0 on success
  • EOF on error

Example:

FILE *file = fopen("data.txt", "r");
if (file != NULL) {
    // ... operations ...
    fclose(file);
}

5.5 Reading from Text Files

5.5.1 fscanf() - Formatted Input

Similar to scanf(), but reads from a file:

int fscanf(FILE *stream, const char *format, ...);

Example:

FILE *file = fopen("numbers.txt", "r");
if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

int num;
while (fscanf(file, "%d", &num) == 1) {
    printf("Read: %d\n", num);
}

fclose(file);

5.5.2 fgets() - Line Input

Reads a line from a file:

char *fgets(char *str, int n, FILE *stream);

Example:

FILE *file = fopen("text.txt", "r");
if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

char line[256];
while (fgets(line, sizeof(line), file) != NULL) {
    printf("%s", line);
}

fclose(file);

5.5.3 fgetc() - Character Input

Reads a single character:

int fgetc(FILE *stream);

Example:

FILE *file = fopen("text.txt", "r");
if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

int ch;
while ((ch = fgetc(file)) != EOF) {
    putchar(ch);
}

fclose(file);

5.6 Writing to Text Files

5.6.1 fprintf() - Formatted Output

Similar to printf(), but writes to a file:

int fprintf(FILE *stream, const char *format, ...);

Example:

FILE *file = fopen("output.txt", "w");
if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

fprintf(file, "Student ID: %d\n", 12345);
fprintf(file, "Name: %s\n", "Alice Johnson");
fprintf(file, "GPA: %.2f\n", 3.75);

fclose(file);

5.6.2 fputs() - String Output

Writes a string to a file:

int fputs(const char *str, FILE *stream);

Example:

FILE *file = fopen("output.txt", "w");
if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

fputs("Hello, World!\n", file);
fputs("This is a test.\n", file);

fclose(file);

5.6.3 fputc() - Character Output

Writes a single character:

int fputc(int char, FILE *stream);

Example:

FILE *file = fopen("output.txt", "w");
if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

for (char ch = 'A'; ch <= 'Z'; ch++) {
    fputc(ch, file);
}

fclose(file);

5.7 File Position Functions

5.7.1 fseek() - Move File Pointer

int fseek(FILE *stream, long offset, int origin);

Origin values:

  • SEEK_SET - Beginning of file
  • SEEK_CUR - Current position
  • SEEK_END - End of file

Example:

FILE *file = fopen("data.txt", "r");

// Move to beginning
fseek(file, 0, SEEK_SET);

// Move 10 bytes from current position
fseek(file, 10, SEEK_CUR);

// Move to end of file
fseek(file, 0, SEEK_END);

5.7.2 ftell() - Get Current Position

long ftell(FILE *stream);

Example:

FILE *file = fopen("data.txt", "r");
long position = ftell(file);
printf("Current position: %ld\n", position);

5.7.3 rewind() - Reset to Beginning

void rewind(FILE *stream);

Example:

FILE *file = fopen("data.txt", "r");

// Read some data...

rewind(file);  // Go back to beginning

5.8 Checking End of File - feof()

int feof(FILE *stream);

Returns non-zero if end of file is reached.

Example:

FILE *file = fopen("data.txt", "r");
char ch;

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

fclose(file);

5.9 Practical Example: Student Records Manager

#include <stdio.h>
#include <string.h>

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

// Save student to file
void saveStudent(const char *filename, Student s) {
    FILE *file = fopen(filename, "a");  // Append mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return;
    }
    
    fprintf(file, "%d,%s,%.2f,%d\n", s.id, s.name, s.gpa, s.age);
    fclose(file);
    
    printf("Student record saved successfully!\n");
}

// Load all students from file
void loadStudents(const char *filename) {
    FILE *file = fopen(filename, "r");
    if (file == NULL) {
        printf("No records found!\n");
        return;
    }
    
    Student s;
    printf("\n=== Student Records ===\n");
    
    while (fscanf(file, "%d,%49[^,],%f,%d\n", 
                  &s.id, s.name, &s.gpa, &s.age) == 4) {
        printf("ID: %d | Name: %s | GPA: %.2f | Age: %d\n",
               s.id, s.name, s.gpa, s.age);
    }
    
    fclose(file);
}

// Search for student by ID
int searchStudent(const char *filename, int searchID) {
    FILE *file = fopen(filename, "r");
    if (file == NULL) {
        printf("Error opening file!\n");
        return 0;
    }
    
    Student s;
    while (fscanf(file, "%d,%49[^,],%f,%d\n", 
                  &s.id, s.name, &s.gpa, &s.age) == 4) {
        if (s.id == searchID) {
            printf("\n--- Student Found ---\n");
            printf("ID: %d\n", s.id);
            printf("Name: %s\n", s.name);
            printf("GPA: %.2f\n", s.gpa);
            printf("Age: %d\n", s.age);
            fclose(file);
            return 1;
        }
    }
    
    fclose(file);
    printf("Student not found!\n");
    return 0;
}

int main() {
    int choice;
    Student s;
    
    do {
        printf("\n=== Student Management System ===\n");
        printf("1. Add Student\n");
        printf("2. View All Students\n");
        printf("3. Search Student by ID\n");
        printf("4. Exit\n");
        printf("Enter choice: ");
        scanf("%d", &choice);
        
        switch (choice) {
            case 1:
                printf("\nEnter Student ID: ");
                scanf("%d", &s.id);
                
                printf("Enter Student Name: ");
                scanf(" %[^\n]", s.name);
                
                printf("Enter Student GPA: ");
                scanf("%f", &s.gpa);
                
                printf("Enter Student Age: ");
                scanf("%d", &s.age);
                
                saveStudent("students.txt", s);
                break;
                
            case 2:
                loadStudents("students.txt");
                break;
                
            case 3: {
                int searchID;
                printf("\nEnter Student ID to search: ");
                scanf("%d", &searchID);
                searchStudent("students.txt", searchID);
                break;
            }
                
            case 4:
                printf("Exiting program...\n");
                break;
                
            default:
                printf("Invalid choice!\n");
        }
    } while (choice != 4);
    
    return 0;
}

5.10 Binary File Operations

Binary files store data in raw binary format, which is more efficient for storing structures.

5.10.1 Writing Binary Data - fwrite()

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

Parameters:

  • ptr - Pointer to data to write
  • size - Size of each element
  • nmemb - Number of elements
  • stream - File pointer

Example:

typedef struct {
    int id;
    char name[50];
    float gpa;
} Student;

Student s = {12345, "Alice Johnson", 3.75};

FILE *file = fopen("students.dat", "wb");
if (file != NULL) {
    fwrite(&s, sizeof(Student), 1, file);
    fclose(file);
}

5.10.2 Reading Binary Data - fread()

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

Parameters:

  • ptr - Pointer to memory where data will be stored
  • size - Size of each element
  • nmemb - Number of elements
  • stream - File pointer

Example:

Student s;

FILE *file = fopen("students.dat", "rb");
if (file != NULL) {
    while (fread(&s, sizeof(Student), 1, file) == 1) {
        printf("ID: %d, Name: %s, GPA: %.2f\n", 
               s.id, s.name, s.gpa);
    }
    fclose(file);
}

5.10.3 Binary vs Text Files

Aspect Text Files Binary Files
Human Readable Yes No
Size Larger Smaller
Speed Slower Faster
Portability More portable Less portable
Precision May lose precision Full precision
Best for Configuration files, logs Large data, structures

5.11 Complete Binary File Example

#include <stdio.h>
#include <string.h>

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

// Save student to binary file
void saveStudentBinary(const char *filename, Student s) {
    FILE *file = fopen(filename, "ab");  // Append binary
    if (file == NULL) {
        printf("Error opening file!\n");
        return;
    }
    
    fwrite(&s, sizeof(Student), 1, file);
    fclose(file);
    
    printf("Student saved to binary file!\n");
}

// Load all students from binary file
void loadStudentsBinary(const char *filename) {
    FILE *file = fopen(filename, "rb");  // Read binary
    if (file == NULL) {
        printf("No records found!\n");
        return;
    }
    
    Student s;
    printf("\n=== Student Records (Binary) ===\n");
    
    while (fread(&s, sizeof(Student), 1, file) == 1) {
        printf("ID: %d | Name: %s | GPA: %.2f | Age: %d\n",
               s.id, s.name, s.gpa, s.age);
    }
    
    fclose(file);
}

// Count number of records in binary file
int countRecords(const char *filename) {
    FILE *file = fopen(filename, "rb");
    if (file == NULL) {
        return 0;
    }
    
    // Seek to end of file
    fseek(file, 0, SEEK_END);
    
    // Get file size
    long fileSize = ftell(file);
    
    fclose(file);
    
    // Calculate number of records
    return fileSize / sizeof(Student);
}

// Update student record by ID
int updateStudent(const char *filename, int id, Student newData) {
    FILE *file = fopen(filename, "rb+");  // Read and write binary
    if (file == NULL) {
        printf("Error opening file!\n");
        return 0;
    }
    
    Student s;
    int found = 0;
    long position = 0;
    
    // Search for student
    while (fread(&s, sizeof(Student), 1, file) == 1) {
        if (s.id == id) {
            // Move back to the position of this record
            fseek(file, position, SEEK_SET);
            
            // Write updated data
            fwrite(&newData, sizeof(Student), 1, file);
            
            found = 1;
            printf("Student record updated!\n");
            break;
        }
        position = ftell(file);
    }
    
    fclose(file);
    
    if (!found) {
        printf("Student ID %d not found!\n", id);
    }
    
    return found;
}

// Delete student record by ID
int deleteStudent(const char *filename, int id) {
    FILE *file = fopen(filename, "rb");
    FILE *temp = fopen("temp.dat", "wb");
    
    if (file == NULL || temp == NULL) {
        printf("Error opening files!\n");
        return 0;
    }
    
    Student s;
    int found = 0;
    
    // Copy all records except the one to delete
    while (fread(&s, sizeof(Student), 1, file) == 1) {
        if (s.id == id) {
            found = 1;
            continue;  // Skip this record
        }
        fwrite(&s, sizeof(Student), 1, temp);
    }
    
    fclose(file);
    fclose(temp);
    
    // Replace original file with temp file
    remove(filename);
    rename("temp.dat", filename);
    
    if (found) {
        printf("Student record deleted!\n");
    } else {
        printf("Student ID %d not found!\n", id);
    }
    
    return found;
}

int main() {
    int choice;
    Student s;
    
    do {
        printf("\n=== Student Management System (Binary Files) ===\n");
        printf("1. Add Student\n");
        printf("2. View All Students\n");
        printf("3. Count Records\n");
        printf("4. Update Student\n");
        printf("5. Delete Student\n");
        printf("6. Exit\n");
        printf("Enter choice: ");
        scanf("%d", &choice);
        
        switch (choice) {
            case 1:
                printf("\nEnter Student ID: ");
                scanf("%d", &s.id);
                
                printf("Enter Student Name: ");
                scanf(" %[^\n]", s.name);
                
                printf("Enter Student GPA: ");
                scanf("%f", &s.gpa);
                
                printf("Enter Student Age: ");
                scanf("%d", &s.age);
                
                saveStudentBinary("students.dat", s);
                break;
                
            case 2:
                loadStudentsBinary("students.dat");
                break;
                
            case 3: {
                int count = countRecords("students.dat");
                printf("\nTotal records: %d\n", count);
                break;
            }
                
            case 4: {
                int updateID;
                printf("\nEnter Student ID to update: ");
                scanf("%d", &updateID);
                
                printf("Enter new Student Name: ");
                scanf(" %[^\n]", s.name);
                
                printf("Enter new Student GPA: ");
                scanf("%f", &s.gpa);
                
                printf("Enter new Student Age: ");
                scanf("%d", &s.age);
                
                s.id = updateID;
                updateStudent("students.dat", updateID, s);
                break;
            }
                
            case 5: {
                int deleteID;
                printf("\nEnter Student ID to delete: ");
                scanf("%d", &deleteID);
                deleteStudent("students.dat", deleteID);
                break;
            }
                
            case 6:
                printf("Exiting program...\n");
                break;
                
            default:
                printf("Invalid choice!\n");
        }
    } while (choice != 6);
    
    return 0;
}

No comments to display

Back to top