src\os\file.go
// WriteString is like Write, but writes the contents of string s rather than
// a slice of bytes.
func (f *File) WriteString(s string) (n int, err error) {
var b []byte
hdr := (*unsafeheader.Slice)(unsafe.Pointer(&b))
hdr.Data = (*unsafeheader.String)(unsafe.Pointer(&s)).Data
hdr.Cap = len(s)
hdr.Len = len(s)
return f.Write(b)
}
// Write writes len(b) bytes from b to the File.
// It returns the number of bytes written and an error, if any.
// Write returns a non-nil error when n != len(b).
func (f *File) Write(b []byte) (n int, err error) {
if err := f.checkValid("write"); err != nil {
return 0, err
}
n, e := f.write(b)
if n < 0 {
n = 0
}
if n != len(b) {
err = io.ErrShortWrite
}
epipecheck(f, e)
if e != nil {
err = f.wrapErr("write", e)
}
return n, err
}