io.PipeWriter.CloseWithError() Function in Golang with Examples

In Go language, io packages supply fundamental interfaces to the I/O primitives. And its principal job is to enclose the ongoing implementations of such king of primitives. The PipeWriter.CloseWithError() function in Go language is used to close the writer. However, successive reads from the PipeReader i.e, read half of the pipe will not return any bytes and the error err or EOF error is returned if the error is nil. Moreover, this function is defined under the io package. Here, you need to import the “io” package in order to use these functions.

Syntax:

func (w *PipeWriter) CloseWithError(err error) error

Here, “w” is a pointer to the PipeWriter. Where PipeWriter is the write half of a pipe and “err” is the error as stated by us in the code which is printed if an error occurs.

Return value: It returns an error if any as stated by us else it returns an EOF if the error is nil.

Note: This method never rewrites the former error if it resides and every time returns “nil”.

Example 1:

// Golang program to illustrate the usage of
// io.PipeWriter.CloseWithError() function

// Including main package
package main

// Importing fmt and io
import (
    "fmt"
    "io"
)

// Calling main
func main() {

    // Calling Pipe method
    pipeReader, pipeWriter := io.Pipe()

    // Defining data parameter of Read method
    data := make([]byte, 20)

    // Reading data into the buffer stated
    go func() {
        pipeReader.Read(data)

        // Calling CloseWithError() method with
        // its parameter
        pipeReader.CloseWithError(fmt.Errorf("There is an "+
                              "error in the code written!"))
    }()

    // Using for loop
    for i := 0; i < 1; i++ {

        // Calling pipeWriter.Write() method
        n, err := pipeWriter.Write([]byte("GeeksforGeeks!"))

        // If error is not nil panic
        if err != nil {
            panic(err)
        }

        // Prints the content written
        fmt.Printf("%v\n", string(data))

        // Prints the number of bytes
        fmt.Printf("%v\n", n)
    }
}

Output:

GeeksforGeeks!
14

Here, no error is returned as CloseWithError() method was not called here because the loop stops after one read operation only.

Example 2:

// Golang program to illustrate the usage of
// io.PipeWriter.CloseWithError() function

// Including main package
package main

// Importing fmt and io
import (
    "fmt"
    "io"
)

// Calling main
func main() {

    // Calling Pipe method
    pipeReader, pipeWriter := io.Pipe()

    // Defining data parameter of Read method
    data := make([]byte, 20)

    // Reading data into the buffer stated
    go func() {
        pipeReader.Read(data)

        // Calling CloseWithError() method with
        // its parameter
        pipeReader.CloseWithError(fmt.Errorf("There is"+
                      " an error in the code written!"))
    }()

    // Using for loop
    for i := 0; i < 2; i++ {

        // Calling pipeWriter.Write() method
        n, err := pipeWriter.Write([]byte("GeeksforGeeks!"))

        // If error is not nil panic
        if err != nil {
            panic(err)
        }

        // Prints the content written
        fmt.Printf("%v\n", string(data))

        // Prints the number of bytes
        fmt.Printf("%v\n", n)
    }
}

Output:

GeeksforGeeks!
14
panic: There is an error in the code written!

goroutine 1 [running]:
main.main()
    /tmp/sandbox246824679/prog.go:39 +0x3c2

Here, an error is thrown as CloseWithError() method is called because “for” loop stops after the second iteration. And the error returned here is as stated by us.