Binary reads and writes in C

fread and fwrite move records, and both return how many they handled.

#include <stdio.h>

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

size_t save_records(const char *path, const Record *rows, size_t n) {
    FILE *f = fopen(path, "wb");
    if (f == NULL) {
        return 0;
    }
    size_t written = fwrite(rows, sizeof(Record), n, f);
    fclose(f);
    return written;
}

size_t load_records(const char *path, Record *rows, size_t max) {
    FILE *f = fopen(path, "rb");
    if (f == NULL) {
        return 0;
    }
    size_t read = fread(rows, sizeof(Record), max, f);
    fclose(f);
    return read;
}

How it works

1. The element size and count are separate arguments.
2. A short return value means end of file or an error.
3. Struct layout differs between machines, so this is not a portable format.

Keywords and builtins used here

• FILE
• NULL
• char
• const
• double
• if
• int
• load_records
• return
• save_records
• size_t
• sizeof
• struct
• typedef

The run, in numbers

Lines

26

Characters to type

481

Tokens

150

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 90 seconds.

Type this snippet

Step 2 of 4 in Writing & binary, step 6 of 12 in Files & I/O.

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