M4 comes with a number of predefined macros, or you can write your own macros by calling the define() function. (If you’re new to m4, this repeated scanning may surprise you, but it’s one key to m4 s power.) Quoting text, like ` text‘, prevents expansion. If so, the token is replaced by the macro’s value, and then that text is pushed back onto the input to be rescanned. M4 copies its input (from files or standard input) to standard output. It checks each token (a name, a quoted string, or any single character that’s not a part of either a name or a string) to see if it’s the name of a macro. m4manipulates files, performs arithmetic, has functions for handling strings, and can do much more. Unlike sed and other languages, m4 is designed specifically for macro processing. ABSDIFF() saves typing - and the chance for error. V1 replaces a everywhere, and v2 replace b. sed can also test and branch, for some rudimentary decision-making.Īs another example, here’s a C program with a cpp macro named ABSDIFF() that accepts two arguments, a and b. So, let’s dig in.Ī simple way to do macro substitution is with tools like sed and cpp. For instance, the command sed’s/XPRESIDENTX/President Bush/’ reads lines of text, changing every occurrence of XPRESIDENTX to President Bush. While you won’t become an m4 wizard in three pages (or in six, as the discussion of m4continues next month), but you can master the basics. (You’ll see some of them.) As of this writing, the latest GNU version was 1.4.2, released in August 2004. The GNU version of m4 has some extensions from the original V7 version. It’s worth knowing because you can do things with m4 that are hard to do any other way. m4 is a powerful macro processor that’s been part of Unix for some 30 years, but it’s almost unknown - except for special purposes, such as generating the sendmail.cf file. If you’re a C programmer, you know cpp, the C preprocessor, a simple macro processor. For instance, a macro processor can convert one language into another. A macro processor scans input text for defined symbols - the macros - and replaces that text by other text, or possibly by other symbols.

M4 common licensing