fread and fwrite
- C course
- Intermediate
- Lesson 51/116
- 5 min
Learn binary read/write with fread and fwrite.
fread and fwrite
fread and fwrite are the basic way to read and write binary data to/from a file. They're used for structured data, not text; for example arrays and structs.
Example: int arr[3] = {1, 2, 3}; fwrite(arr, sizeof(int), 3, f);. This writes 3 integers to the file. fread(arr2, sizeof(int), 3, f); reads them back. Parameters: data pointer, element size, element count, file. Binary modes "wb" and "rb" are used; this matters for cross-platform consistency.
fwrite parameters
fwrite has four parameters: data pointer, element size, element count, and file. This structure lets you write multiple elements efficiently in one call.
Example: int x = 42; fwrite(&x, sizeof(int), 1, f);. Writes a single integer. Or int arr[10]; fwrite(arr, sizeof(int), 10, f); writes ten integers. sizeof(int) is 4 on most platforms; but using sizeof rather than hardcoding preserves portability. fread has the same parameter structure; it returns the number of elements read.
Practice Write a binary array and read back
Declare int arr[3] = {10, 20, 30};. Open with FILE *f = fopen("nums.bin", "wb"); (check NULL). Write with fwrite(arr, sizeof(int), 3, f);, fclose(f);. Declare int arr2[3];; reopen with f = fopen("nums.bin", "rb"); (check NULL). Read with fread(arr2, sizeof(int), 3, f);, fclose(f);. Print with printf("%d %d %d\n", arr2[0], arr2[1], arr2[2]);.
Starter code
#include <stdio.h>
// Open nums.bin with fopen (binary write mode)
// Declare int arr[3] = {10, 20, 30}, write with fwrite
// fclose, then reopen in read mode
// Read with fread, print with printf
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
int main(void) {
int arr[3] = {10, 20, 30};
FILE *f = fopen("nums.bin", "wb");
if (f == NULL) return 1;
fwrite(arr, sizeof(int), 3, f);
fclose(f);
int arr2[3];
f = fopen("nums.bin", "rb");
if (f == NULL) return 1;
fread(arr2, sizeof(int), 3, f);
fclose(f);
printf("%d %d %d\n", arr2[0], arr2[1], arr2[2]);
return 0;
}Expected output
10 20 30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match fread/fwrite concepts
| Term | Meaning |
|---|---|
fwrite |
Binary write |
fread |
Binary read |
sizeof(int) |
Element size |
3 |
Element count |
2 Complete the fwrite call
int x = 42;
fwrite(&x, sizeof(int), ____, f);int x = 42;
fwrite(&x, sizeof(int), 1, f);Explanation: Writing one element means count is 1.
3 What does fwrite's second parameter specify?
- Size of each element Correct answer
- Total bytes
- Buffer pointer
- File name
Explanation: The second parameter is the size of each element (like sizeof(int)).