Input/Output Operations. Files.

FILE *f = fopen("data.txt", "r");       // 1. opening

if (f == NULL) {                        // 2. mandatory CHECK
    printf("Cannot open the file!\n");
    return 1;
}

/* ... reading or writing ... */        // 3. processing

fclose(f);                              // 4. closing

size_t fread (void *p, size_t size, size_t nr, FILE *f);
size_t fwrite(const void *p, size_t size, size_t nr, FILE *f);

// p    - the address of the memory zone
// size - the size of one record, in bytes
// nr   - how many records are transferred
// f    - the stream
// returns the number of records actually transferred

#include <stdio.h>

int main(void)
{
    FILE *f = fopen("data.txt", "w");     // WARNING: erases existing contents
    if (f == NULL) {
        printf("Cannot create the file!\n");
        return 1;
    }

    fprintf(f, "John Smith;20;9.15\n");
    fprintf(f, "Mary Johnson;21;8.70\n");
    fprintf(f, "Radu Georgescu;20;9.60\n");

    fclose(f);
    printf("File created.\n");

    // appending at the end, without erasing the contents
    f = fopen("data.txt", "a");
    if (f) {
        fprintf(f, "Ana Dumitrescu;22;9.85\n");
        fclose(f);
        printf("Record added.\n");
    }
    return 0;
}

#include <stdio.h>

int main(void)
{
    FILE *f = fopen("data.txt", "r");
    if (f == NULL) {
        printf("The file does not exist!\n");
        return 1;
    }

    char line[200];
    int nrLines = 0;

    // correct version: the condition is precisely the success of the read
    while (fgets(line, sizeof(line), f) != NULL) {
        nrLines++;
        printf("%3d: %s", nrLines, line);
    }

    printf("\nTotal %d lines\n", nrLines);
    fclose(f);

    // formatted reading, field by field
    f = fopen("data.txt", "r");
    char name[50];
    int age;
    float average;

    printf("\nStructured reading:\n");
    while (fscanf(f, "%49[^;];%d;%f\n", name, &age, &average) == 3) {
        printf("  %-20s %3d years  average %.2f\n", name, age, average);
    }
    fclose(f);

    return 0;
}

#include <stdio.h>

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

int main(void)
{
    Student group[3] = {
        {"John Smith",    20, 9.15f},
        {"Mary Johnson",  21, 8.70f},
        {"Radu Georgescu",20, 9.60f}
    };

    // binary write: the whole array, in a single call
    FILE *f = fopen("students.dat", "wb");
    if (!f) return 1;
    fwrite(group, sizeof(Student), 3, f);
    fclose(f);

    // reading record number 1 (the second one), WITHOUT reading the first
    f = fopen("students.dat", "rb");
    if (!f) return 1;

    Student one;
    fseek(f, 1 * sizeof(Student), SEEK_SET);   // direct positioning
    fread(&one, sizeof(Student), 1, f);

    printf("Record 1: %s, %d years, average %.2f\n",
           one.name, one.age, one.average);

    // the size of the file
    fseek(f, 0, SEEK_END);
    long size = ftell(f);
    printf("File size: %ld bytes = %ld records\n",
           size, size / (long)sizeof(Student));

    fclose(f);
    return 0;
}

#include <stdio.h>

int main(void)
{
    FILE *source = fopen("data.txt", "rb");
    if (!source) { printf("The source is missing\n"); return 1; }

    FILE *dest = fopen("copy.txt", "wb");
    if (!dest) { fclose(source); return 1; }

    char buffer[1024];
    size_t bytesRead;
    long total = 0;

    // copying in blocks, much faster than byte by byte
    while ((bytesRead = fread(buffer, 1, sizeof(buffer), source)) > 0) {
        fwrite(buffer, 1, bytesRead, dest);
        total += bytesRead;
    }

    printf("Copied %ld bytes\n", total);

    fclose(source);
    fclose(dest);
    return 0;
}