XmTextPosition man page
XmTextPosition — Data type for a character position within a text string
XmTextPosition is an integer data type that holds a character's position within a text string for Text and TextField.
An XmTextPosition value conceptually points to the gap between two characters. For example, consider a text string consisting of N characters. A value of 0 refers to the position immediately prior to the first character. A value of 1 refers to the position in between the first and second characters. A value of N refers to the position immediately following the last character. Therefore, the text string of N characters actually contains N + 1 positions.