fread() / fwrite()

// Reads binary data from a file.
// Return value: the number of blocks read (check with feof() or ferror()).
size_t fread(void *ptr, size_t size, size_t count, FILE *stream);

// Writes binary data to a file.
// Return value: the number of blocks written (if different from count, an error occurred).
size_t fwrite(const void *ptr, size_t size, size_t count, FILE *stream);

Parameter	Description
ptr	The buffer to store data into (for reading), or the starting address of the data to write (for writing).
size	The size of one block in bytes. For a struct, pass sizeof(struct X).
count	The number of blocks to read or write.
stream	A FILE pointer opened with "rb" or "wb".

sample_fread_fwrite.c

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

typedef struct {
    int id;
    double score;
} Record;

int main(void) {
    // Writes struct data to a binary file.
    Record rec = {42, 98.5};
    FILE *fp = fopen("data.bin", "wb"); // Opens the file in binary write mode.
    if (fp == NULL) {
        perror("fopen");
        return EXIT_FAILURE;
    }
    size_t written = fwrite(&rec, sizeof(Record), 1, fp);
    if (written != 1) {
        fprintf(stderr, "Write failed.\n");
    }
    fclose(fp);

    // Reads struct data from the binary file.
    Record rec2 = {0};
    fp = fopen("data.bin", "rb"); // Opens the file in binary read mode.
    if (fp == NULL) {
        perror("fopen");
        return EXIT_FAILURE;
    }
    size_t read_count = fread(&rec2, sizeof(Record), 1, fp);
    fclose(fp);

    if (read_count == 1) {
        printf("id = %d\n", rec2.id);       // Outputs: id = 42
        printf("score = %.1f\n", rec2.score); // Outputs: score = 98.5
    }
    return 0;
}

gcc fread_fwrite.c -o fread_fwrite
./fread_fwrite
id = 42
score = 98.5

fread_fwrite2.c — Writing and reading an array of structs in one call

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

#define MAX_NAME 32

typedef struct {
    int   id;
    char  name[MAX_NAME];
    float power;
} Fighter;

int main(void) {
    Fighter fighters[] = {
        {1, "Son Goku",  9000.0f},
        {2, "Vegeta",    8000.0f},
        {3, "Piccolo",   3500.0f},
    };
    int count = (int)(sizeof(fighters) / sizeof(fighters[0]));

    // Write the entire array at once.
    FILE *fp = fopen("fighters.bin", "wb");
    if (fp == NULL) { perror("fopen"); return EXIT_FAILURE; }
    fwrite(fighters, sizeof(Fighter), count, fp);
    fclose(fp);

    // Read the entire array at once.
    Fighter loaded[3];
    fp = fopen("fighters.bin", "rb");
    if (fp == NULL) { perror("fopen"); return EXIT_FAILURE; }
    size_t n = fread(loaded, sizeof(Fighter), count, fp);
    fclose(fp);

    for (size_t i = 0; i < n; i++) {
        printf("id=%d name=%-10s power=%.1f\n",
               loaded[i].id, loaded[i].name, loaded[i].power);
    }
    remove("fighters.bin");
    return 0;
}

gcc fread_fwrite2.c -o fread_fwrite2
./fread_fwrite2
id=1 name=Son Goku   power=9000.0
id=2 name=Vegeta     power=8000.0
id=3 name=Piccolo    power=3500.0

fread_fwrite3.c — Appending to a binary file and reading all entries

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

typedef struct {
    int   id;
    float score;
} Entry;

int main(void) {
    // Initial write
    Entry e1 = {1, 85.5f};
    FILE *fp = fopen("scores.bin", "wb");
    if (fp == NULL) { perror("fopen"); return EXIT_FAILURE; }
    fwrite(&e1, sizeof(Entry), 1, fp);
    fclose(fp);

    // Append additional entries.
    Entry e2 = {2, 92.0f};
    Entry e3 = {3, 78.3f};
    fp = fopen("scores.bin", "ab"); // "ab" = binary append mode
    if (fp == NULL) { perror("fopen"); return EXIT_FAILURE; }
    fwrite(&e2, sizeof(Entry), 1, fp);
    fwrite(&e3, sizeof(Entry), 1, fp);
    fclose(fp);

    // Read all entries.
    fp = fopen("scores.bin", "rb");
    if (fp == NULL) { perror("fopen"); return EXIT_FAILURE; }
    Entry buf;
    while (fread(&buf, sizeof(Entry), 1, fp) == 1) {
        printf("id=%d score=%.1f\n", buf.id, buf.score);
    }
    fclose(fp);
    remove("scores.bin");
    return 0;
}

gcc fread_fwrite3.c -o fread_fwrite3
./fread_fwrite3
id=1 score=85.5
id=2 score=92.0
id=3 score=78.3

Always specify binary mode ("rb", "wb", "ab") when opening a file. Opening in text mode causes automatic newline conversion ('\n' ↔ "\r\n") on Windows, which will corrupt binary data.

If the return value of fread() is less than the requested count, it means either the end of file (EOF) was reached or an error occurred. Use feof() / ferror() to determine the cause.

Reading or writing multiple blocks at once reduces the number of system calls and improves performance. You can control the file position with fseek().