C — File Handling

Opening and closing files

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    fprintf(fp, "Hello, file!\n");
    fclose(fp);

    printf("File written successfully\n");
    return 0;
}

File modes

Mode	Description
"r"	Read (file must exist)
"w"	Write (creates or truncates)
"a"	Append (creates if needed)
"r+"	Read + write (file must exist)
"w+"	Read + write (creates or truncates)
"a+"	Read + append (creates if needed)
"rb"	Read binary
"wb"	Write binary

Writing to files

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "w");
    if (!fp) {
        perror("Error");
        return 1;
    }

    // fprintf: formatted output
    fprintf(fp, "Name: %s\n", "Alice");
    fprintf(fp, "Score: %d\n", 95);
    fprintf(fp, "Average: %.2f\n", 92.5);

    // fputs: write a string
    fputs("This is a line\n", fp);

    // fputc: write a single character
    fputc('A', fp);
    fputc('\n', fp);

    fclose(fp);
    return 0;
}

Reading from files

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Write test data first
    FILE *fp = fopen("data.txt", "w");
    fprintf(fp, "Alice 95\n");
    fprintf(fp, "Bob 87\n");
    fprintf(fp, "Charlie 92\n");
    fclose(fp);

    // Read it back
    fp = fopen("data.txt", "r");
    if (!fp) {
        perror("Error");
        return 1;
    }

    char name[50];
    int score;

    // fscanf: formatted input
    while (fscanf(fp, "%s %d", name, &score) == 2) {
        printf("Name: %s, Score: %d\n", name, score);
    }

    fclose(fp);
    return 0;
}

Reading line by line

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

int main() {
    FILE *fp = fopen("lines.txt", "w");
    fprintf(fp, "First line\n");
    fprintf(fp, "Second line\n");
    fprintf(fp, "Third line\n");
    fclose(fp);

    fp = fopen("lines.txt", "r");
    char buffer[256];
    int lineNum = 0;

    // fgets: read one line at a time (recommended)
    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
        lineNum++;
        // Remove trailing newline
        buffer[strcspn(buffer, "\n")] = '\0';
        printf("Line %d: %s\n", lineNum, buffer);
    }

    fclose(fp);
    return 0;
}

Reading entire file

#include <stdio.h>
#include <stdlib.h>

char *readFile(const char *filename) {
    FILE *fp = fopen(filename, "r");
    if (!fp) return NULL;

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

    char *buffer = malloc(size + 1);
    if (!buffer) {
        fclose(fp);
        return NULL;
    }

    fread(buffer, 1, size, fp);
    buffer[size] = '\0';

    fclose(fp);
    return buffer;
}

int main() {
    char *content = readFile("data.txt");
    if (content) {
        printf("File contents:\n%s\n", content);
        free(content);
    }
    return 0;
}

Binary file I/O

#include <stdio.h>

typedef struct {
    char name[50];
    int age;
    float score;
} Record;

int main() {
    Record records[] = {
        {"Alice", 30, 95.5f},
        {"Bob", 25, 87.0f},
        {"Charlie", 35, 92.3f}
    };
    int count = sizeof(records) / sizeof(records[0]);

    // Write binary
    FILE *fp = fopen("records.bin", "wb");
    fwrite(records, sizeof(Record), count, fp);
    fclose(fp);

    // Read binary
    Record readRecords[3];
    fp = fopen("records.bin", "rb");
    fread(readRecords, sizeof(Record), count, fp);
    fclose(fp);

    for (int i = 0; i < count; i++) {
        printf("%s: %d, %.1f\n",
               readRecords[i].name,
               readRecords[i].age,
               readRecords[i].score);
    }

    return 0;
}

File position

#include <stdio.h>

int main() {
    FILE *fp = fopen("pos.txt", "w+");
    fprintf(fp, "ABCDEFGHIJ");

    // ftell: current position
    printf("Position: %ld\n", ftell(fp)); // 10

    // fseek: move to position
    fseek(fp, 0, SEEK_SET); // Beginning
    printf("Position: %ld\n", ftell(fp)); // 0

    fseek(fp, 5, SEEK_SET); // 5 bytes from start
    char c = fgetc(fp);
    printf("Char at 5: %c\n", c); // F

    fseek(fp, -3, SEEK_END); // 3 bytes before end
    c = fgetc(fp);
    printf("Char at end-3: %c\n", c); // H

    fclose(fp);
    return 0;
}

Error checking

#include <stdio.h>

int main() {
    FILE *fp = fopen("nonexistent.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        // Or: fprintf(stderr, "Error: %s\n", strerror(errno));
        return 1;
    }

    // Check for read errors
    int c;
    while ((c = fgetc(fp)) != EOF) {
        putchar(c);
    }

    if (ferror(fp)) {
        fprintf(stderr, "Error reading file\n");
    }

    fclose(fp);
    return 0;
}

Mini Practice

Write C code that:

1. Writes a list of names and scores to a file
2. Reads the file back and calculates the average score
3. Copies a file line by line to another file
4. Writes and reads a binary file containing structs

Up Next

In the next lesson, you'll learn about Memory Management — malloc, calloc, realloc, and free.