How to read arbitrary amounts of data directly from a file in Go?-CodePudding

Without reading the contents of a file into memory, how can I read "x" bytes from the file so that I can specify what x is for every separate read operation?

I see that the Read method of various Readers takes a byte slice of a certain length and I can read from a file into that slice. But in that case the size of the slice is fixed, whereas what I would like to do, ideally, is something like:

func main() {
    f, err := os.Open("./file.txt")
    if err != nil {
        panic(err)
    }

    someBytes := f.Read(2)
    someMoreBytes := f.Read(4)
}

bytes.Buffer has a Next method which behaves very closely to what I would want, but it requires an existing buffer to work, whereas I'm hoping to read an arbitrary amount of bytes from a file without needing to read the whole thing into memory.

What is the best way to accomplish this?

Thank you for your time.

CodePudding user response：

you can do something like this:

f, err := os.Open("/tmp/dat")
check(err)
b1 := make([]byte, 5)
n1, err := f.Read(b1)
check(err)
fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))

for more reading please check site.

CodePudding user response：

Use this function:

// readN reads and returns n bytes from the reader.
// On error, readN returns the partial bytes read and
// a non-nil error.
func readN(r io.Reader, n int) ([]byte, error) {
    b := make([]byte, n)        // Allocate buffer for result
    n, err := io.ReadFull(r, b) // ReadFull ensures buffer is filled or error is returned.
    return b[:n], err
}

Call like this:

someBytes, err := readN(f, 2)
if err != nil  { /* handle error here */
someMoreBytes := readN(f, 4)
if err != nil  { /* handle error here */