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io.ReadAtLeast() Function in Golang with Examples

  • Last Updated : 05 May, 2020

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 ReadAtLeast() function in Go language is used to read from the stated reader “r” into the stated buffer “buf” until it has read for at least the minimum number of stated bytes. 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 ReadAtLeast(r Reader, buf []byte, min int) (n int, err error)

Here, “r” is the reader stated, “buf” is the buffer stated, and “min” is the minimum number of bytes until which the reader reads into the given buffer.

Return value: It returns the number of bytes that the stated buffer copies and also returns an error if the number of bytes reads are less than the minimum number of bytes. Here, “n” returned will be greater than the “min” bytes if and only if the error is nil. However, the error returned is “EOF” if and only if no bytes are read.

Note: If an EOF takes place after reading fewer bytes than the stated “min” bytes then this method returns ErrUnexpectedEOF error. But, if the stated minimum number of bytes is more than the length of the stated buffer then this method returns ErrShortBuffer error. However, if the stated reader returns an error after reading at least the stated minimum bytes then the error is declined.



Example 1:




// Golang program to illustrate the usage of
// io.ReadAtLeast() 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("Geeks")
  
    // Defining buffer of specified length
    // using make keyword
    buffer := make([]byte, 6)
  
    // Calling ReadAtLeast method with its parameters
    n, err := io.ReadAtLeast(reader, buffer, 3)
  
    // If error is not nil then panics
    if err != nil {
        panic(err)
    }
  
    // Prints output
    fmt.Printf("Number of bytes in the buffer: %d\n", n)
    fmt.Printf("Content in buffer: %s\n", buffer)
}

Output:

Number of bytes in the buffer: 5
Content in buffer: Geeks

Here, the ‘n’ returned i.e, 5 is greater than ‘min’ i.e, 3 as an error is nil.

Example 2:




// Golang program to illustrate the usage of
// io.ReadAtLeast() 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")
  
    // Defining buffer of specified length
    // using make keyword
    buffer := make([]byte, 4)
  
    // Calling ReadAtLeast method with its parameters
    n, err := io.ReadAtLeast(reader, buffer, 5)
  
    // If error is not nil then panics
    if err != nil {
        panic(err)
    }
  
    // Prints output
    fmt.Printf("Number of bytes in the buffer: %d\n", n)
    fmt.Printf("Content in buffer: %s\n", buffer)
}

Output:

panic: short buffer

goroutine 1 [running]:
main.main()
    /tmp/sandbox041442440/prog.go:29 +0x20f

Here, the buffer stated in the above code has length lesser than the “min” bytes stated so an error is thrown.




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