Why Can\'t I Read Data I Just Wrote to a Temporary File in Go?

How to Read Data Just Written to a Temp File

Problem Outline

When attempting to write data to a temporary file and then read it in Go, users may encounter difficulties. Despite successfully writing the data to the file, retrieving it subsequently proves elusive.

Solution

The crux of the issue lies in how ioutil.TempFile operates. This function creates a temporary file and opens it for both reading and writing. Consequently, the pointer within the file is positioned at the end of the data after the write operation.

To address this challenge, it is necessary to seek the beginning of the file using *os.File.Seek before attempting to read from it. This operation resets the pointer to the start, enabling the subsequent read operation to access the written data.

Implementation

The following code sample demonstrates the corrected implementation:

package main

import (
    "bufio"
    "fmt"
    "io/ioutil"
    "log"
    "os"
    "path/filepath"
)

func main() {
    tmpFile, err := ioutil.TempFile("", fmt.Sprintf("%s-", filepath.Base(os.Args[0])))
    if err != nil {
        log.Fatal("Could not create temporary file", err)
    }
    defer tmpFile.Close()

    fmt.Println("Created temp file: ", tmpFile.Name())

    fmt.Println("Writing some data to the temp file")
    if _, err = tmpFile.WriteString("test data"); err != nil {
        log.Fatal("Unable to write to temporary file", err)
    } else {
        fmt.Println("Data should have been written")
    }

    fmt.Println("Trying to read the temp file now")

    // Seek the pointer to the beginning
    tmpFile.Seek(0, 0)
    s := bufio.NewScanner(tmpFile)
    for s.Scan() {
        fmt.Println(s.Text())
    }
    if err = s.Err(); err != nil {
        log.Fatal("error reading temp file", err)
    }
}

Optimizing for Performance

For optimal performance in the above solution, consider using a bytes.Buffer instead of a temporary file. This buffer can be passed to a bufio.Reader for convenient reading of the data.

Additionally, the s.Scan() loop can be replaced with ioutil.ReadAll() for efficient reading of all data into a byte slice.

The above is the detailed content of Why Can\'t I Read Data I Just Wrote to a Temporary File in Go?. For more information, please follow other related articles on the PHP Chinese website!