Binary record processor

Implement copy_matching_records.

typedef struct {
    int id;
    unsigned flags;
    double value;
} record_t;

size_t copy_matching_records(
    FILE *input,
    FILE *output,
    int (*matches)(const record_t *)
);

Read record_t values using fread, test each complete record using matches, write matching records using fwrite, and return the number successfully written. Correctly handle end-of-file, partial reads, and write failures. Explain how the caller can distinguish a clean EOF from an input error if your function’s return type cannot report both directly.