Several options allow
m4 to behave more like a preprocessor.
Macro definitions and deletions can be made on the command line, the
search path can be altered, and the output file can track where the
input came from. These features occur with the following options:
m4search directory for included files that are not found in the current working directory. See Search Path, for more details. This option may be given more than once.
m4is used as a front end to a compiler. Source file name and line number information is conveyed by directives of the form ‘#line linenum "file"’, which are inserted as needed into the middle of the output. Such directives mean that the following line originated or was expanded from the contents of input file file at line linenum. The ‘"file"’ part is often omitted when the file name did not change from the previous directive.
Synchronization directives are always given on complete lines by themselves. When a synchronization discrepancy occurs in the middle of an output line, the associated synchronization directive is delayed until the next newline that does not occur in the middle of a quoted string or comment.
define(`twoline', `1 2') ⇒#line 2 "stdin" ⇒ changecom(`/*', `*/') ⇒ define(`comment', `/*1 2*/') ⇒#line 5 ⇒ dnl no line hello ⇒#line 7 ⇒hello twoline ⇒1 ⇒#line 8 ⇒2 comment ⇒/*1 ⇒2*/ one comment `two three' ⇒#line 10 ⇒one /*1 ⇒2*/ two ⇒three goodbye ⇒#line 12 ⇒goodbye