Write/Save data in CSV using GO language

The basic problem is that you are calling os.Create. The documentation for os.Create says

Create creates the named file with mode 0666 (before umask), truncating it if it already exists. If successful, methods on the returned File can be used for I/O; the associated file descriptor has mode O_RDWR. If there is an error, it will be of type *PathError.

So each call to os.Create will remove all content from the file you passed. Instead what you want is probably os.OpenFile with the os.O_CREATE, os.O_WRONLY and os.O_APPEND flags. This will make sure that the file will be created, if it doesn't exists, but won't truncate it.

But there is another problem in your code. You are calling defer csvfile.Close() inside the loop. A deferred function will only be executed once the function returns and not after the loop iteration. This can lead to problems, especially since you are opening the same file over and over again.

Instead you should open the file once before the loop so that you only need to close it once. Like this:

package main

import (
    "encoding/csv"
    "fmt"
    "net/http"
    "os"
)

func PageRequest(w http.ResponseWriter, r *http.Request) {
    // Default page number is 1
    if len(r.URL.Path) <= 1 {
        r.URL.Path = "/1"
    }

    // Page number is not negative or 0
    page.Abs(page)
    if page.Cmp(one) == -1 {
        page.SetInt64(1)
    }

    // Page header
    fmt.Fprintf(w, PageHeader, pages, previous, next)

    // Save in csv
    csvfile, err := os.OpenFile("marks.csv", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    defer csvfile.Close()

    writer := csv.NewWriter(csvfile)
    defer writer.Flush()

    // Marks for UID
    UID, length := compute(start)
    for i := 0; i < length; i++ {
        key := UID[i]

        fmt.Fprintf(w, key.fname, key.marks, key.lname, key.remarks)

        records := [][]string{{key.fname, key.marks, key.lname, key.remarks}}

        for _, record := range records {
            err := writer.Write(record)
            if err != nil {
                fmt.Println("Error:", err)
                return
            }
        }
    }

    // Page Footer
    fmt.Fprintf(w, PageFooter, previous, next)
}