Reads exactly buf.count bytes; treats a short read (EOF reached early) as an error rather than a quiet truncation. Use when the caller wants binary fidelity — e.g. reading a fixed-width header.

Errors

Returns .Err(invalidInput()) if EOF is reached before buf.count bytes have been collected.

Examples

var header = Array[UInt8](repeating: 0, count: 16); try readExact(file, into: header.asSlice()); // must read 16 bytes