How to pass a file pointer to a different function

If I have created a file in my main() function:

output, err := os.Create("D:\\output.txt")

And I want everything that another function in the program prints, to be put in that file using:

output.WriteString(str)

How could I pass a pointer to that file so that function could write to it?
Also, is there any other way I should use to write a string to a file, or WriteString is succicient?

Have your function take a pointer as a parameter using the * type modifier, and just pass your file object as-is since os.Create already returns a pointer:

func WriteStringToFile(f *os.File) {
    n, err := f.WriteString("foobar")
}

// ..

output, err := os.Create("D:\\output.txt")
WriteStringToFile(output)

Also, please note that it is good practice not to ignore errors.

To write strings into a file can be done in a few different ways, especially if you want to avoid using the os.File object directly, and only use the io.Writer interface. For example:

fmt.Fprint(output, "foo bar")

Simply define a function that can take *File pointer as argument:

func Write(output *os.File) {
 (...)
}

Write(&output) //call function.

}

Also you may want to ensure that file is closed in the end using:

defer output.Close()

Using an interface such as io.Writer is the way to go. Many types in Go fulfill the io.Writer just by having a Write method. os.File is one of those types.