io.SectionReader.ReadAt() 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 SectionReader.ReadAt() function in Go language is used to return the number of bytes as read by NewSectionReader method. This method holds a buffer and offsets as its parameters. 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 (s *SectionReader) ReadAt(p []byte, off int64) (n int, err error)

Here, “s” is a pointer to the SectionReader which is returned by the NewSectionReader method, “p” is a buffer of stated byte length, and “off” is the offset of type int64 from where the reading starts.

Return value: It returns the number of bytes of the content returned from the stated buffer of specified length as well as offset and also returns an error if any but if no error occurred then “nil” is returned.

Below examples illustrates the use of above method:

Example 1:

// Golang program to illustrate the usage of
// io.SectionReader.ReadAt() function

// Including main package
package main

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

// Calling main
func main() {

    // Defining reader using NewReader method
    reader := strings.NewReader("GeeksforGeeks\n")

    // Calling NewSectionReader method with its parameters
    r := io.NewSectionReader(reader, 6, 12)

    // Defining buffer using make keyword
    buf := make([]byte, 4)

    // Calling ReadAt method with its parameters
    n, err := r.ReadAt(buf, 2)

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

    // Prints output
    fmt.Printf("Content in buffer: %s\n", buf)
    fmt.Printf("n: %v\n", n)
}

Output:

Content in buffer: Geek
n: 4

In the above example first, content is returned from the NewSectionReader() then that content is read from the given offset in the ReadAt() method till the number of bytes stated in the buffer. Then the number of bytes of the resultant content is returned as output here. Moreover, the content of the buffer above has only 4 bytes so, “4” is returned and there is no error thrown while reading the stated content so the error is “nil”.

Example 2:

// Golang program to illustrate the usage of
// io.SectionReader.ReadAt() function

// Including main package
package main

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

// Calling main
func main() {

    // Defining reader using NewReader method
    reader := strings.NewReader("Computer Science!")

    // Calling NewSectionReader method with its parameters
    r := io.NewSectionReader(reader, 6, 12)

    // Defining buffer using make keyword
    buf := make([]byte, 10)

    // Calling ReadAt method with its parameters
    n, err := r.ReadAt(buf, 2)

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

    // Prints output
    fmt.Printf("Content in buffer: %s\n", buf)
    fmt.Printf("n: %v\n", n)
}

Output:

panic: EOF

goroutine 1 [running]:
main.main()
    /tmp/sandbox798260752/prog.go:31 +0x263

Here, the content in the buffer used in the above code has fewer bytes than stated so, EOF error is thrown.