source: code/trunk/vendor/github.com/prometheus/procfs/internal/util/sysreadfile.go@ 822

// Copyright 2018 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//go:build (linux || darwin) && !appengine
// +build linux darwin
// +build !appengine

package util

import (
        "bytes"
        "os"
        "syscall"
)

// SysReadFile is a simplified os.ReadFile that invokes syscall.Read directly.
// https://github.com/prometheus/node_exporter/pull/728/files
//
// Note that this function will not read files larger than 128 bytes.
func SysReadFile(file string) (string, error) {
        f, err := os.Open(file)
        if err != nil {
                return "", err
        }
        defer f.Close()

        // On some machines, hwmon drivers are broken and return EAGAIN.  This causes
        // Go's os.ReadFile implementation to poll forever.
        //
        // Since we either want to read data or bail immediately, do the simplest
        // possible read using syscall directly.
        const sysFileBufferSize = 128
        b := make([]byte, sysFileBufferSize)
        n, err := syscall.Read(int(f.Fd()), b)
        if err != nil {
                return "", err
        }

        return string(bytes.TrimSpace(b[:n])), nil
}