Source: readfile.go in package io/fs


package fs

import "io"

ReadFileFS is the interface implemented by a file system that provides an optimized implementation of ReadFile.

type ReadFileFS interface {
	FS

ReadFile reads the named file and returns its contents. A successful call returns a nil error, not io.EOF. (Because ReadFile reads the whole file, the expected EOF from the final Read is not treated as an error to be reported.)

	ReadFile(name string) ([]byte, error)
}

ReadFile reads the named file from the file system fs and returns its contents. A successful call returns a nil error, not io.EOF. (Because ReadFile reads the whole file, the expected EOF from the final Read is not treated as an error to be reported.) If fs implements ReadFileFS, ReadFile calls fs.ReadFile. Otherwise ReadFile calls fs.Open and uses Read and Close on the returned file.

func ReadFile(fsys FS, name string) ([]byte, error) {
	if fsys, ok := fsys.(ReadFileFS); ok {
		return fsys.ReadFile(name)
	}

	file, err := fsys.Open(name)
	if err != nil {
		return nil, err
	}
	defer file.Close()

	var size int
	if info, err := file.Stat(); err == nil {
		size64 := info.Size()
		if int64(int(size64)) == size64 {
			size = int(size64)
		}
	}

	data := make([]byte, 0, size+1)
	for {
		if len(data) >= cap(data) {
			d := append(data[:cap(data)], 0)
			data = d[:len(data)]
		}
		n, err := file.Read(data[len(data):cap(data)])
		data = data[:len(data)+n]
		if err != nil {
			if err == io.EOF {
				err = nil
			}
			return data, err
		}
	}