TLDR:
my42bytes, err := ioutil.ReadAll(io.LimitReader(myReader, 42))
Full answer:
@monicuta mentioned io.ReadFull which works great. Here I provide another method. It works by chaining ioutil.ReadAll and io.LimitReader together. Let's read the doc first:
$ go doc ioutil.ReadAll
func ReadAll(r io.Reader) ([]byte, error)
ReadAll reads from r until an error or EOF and returns the data it read. A
successful call returns err == nil, not err == EOF. Because ReadAll is
defined to read from src until EOF, it does not treat an EOF from Read as an
error to be reported.
$ go doc io.LimitReader
func LimitReader(r Reader, n int64) Reader
LimitReader returns a Reader that reads from r but stops with EOF after n
bytes. The underlying implementation is a *LimitedReader.
So if you want to get 42 bytes from myReader, you do this
import (
"io"
"io/ioutil"
)
func main() {
// myReader := ...
my42bytes, err := ioutil.ReadAll(io.LimitReader(myReader, 42))
if err != nil {
panic(err)
}
//...
}
Here is the equivalent code with io.ReadFull
$ go doc io.ReadFull
func ReadFull(r Reader, buf []byte) (n int, err error)
ReadFull reads exactly len(buf) bytes from r into buf. It returns the number
of bytes copied and an error if fewer bytes were read. The error is EOF only
if no bytes were read. If an EOF happens after reading some but not all the
bytes, ReadFull returns ErrUnexpectedEOF. On return, n == len(buf) if and
only if err == nil. If r returns an error having read at least len(buf)
bytes, the error is dropped.
import (
"io"
)
func main() {
// myReader := ...
buf := make([]byte, 42)
_, err := io.ReadFull(myReader, buf)
if err != nil {
panic(err)
}
//...
}
Compared to io.ReadFull, an advantage is that you don't need to manually make a buf, where len(buf) is the number of bytes you want to read, then pass buf as an argument when you Read
Instead you simply tell io.LimitReader you want at most 42 bytes from myReader, and call ioutil.ReadAll to read them all, returning the result as a slice of bytes. If successful, the returned slice is guaranteed to be of length 42.
