The name of an external program that will be used to format the lines selected with the |gq| operator. The program must take the input on stdin and produce the output on stdout. The Unix program "fmt" is such a program. If the 'formatexpr' option is not empty it will be used instead. Otherwise, if 'formatprg' option is an empty string, the internal format function will be used |C-indenting|. Environment variables are expanded |:set_env|. See |option-backslash| about including spaces and backslashes. This option cannot be set from a |modeline| or in the |sandbox|, for security reasons.