Macro calls have the form:

name(arg1,arg2, ..., argn)

The ( must immediately follow the name of the macro. If the name of a defined macro is not followed by a (, it is deemed to be a call of that macro with no arguments. Potential macro names consist of alphanumeric characters and underscore (_), where the first character is not a digit.

Leading unquoted blanks, TABs, and NEWLINEs are ignored while collecting arguments. Left and right single quotes are used to quote strings. The value of a quoted string is the string stripped of the quotes.

When a macro name is recognized, its arguments are collected by searching for a matching right parenthesis. If fewer arguments are supplied than are in the macro definition, the trailing arguments are taken to be NULL. Macro evaluation proceeds normally during the collection of the arguments, and any commas or right parentheses that happen to turn up within the value of a nested call are as effective as those in the original input text. After argument collection, the value of the macro is pushed back onto the input stream and rescanned.

eval

Evaluates its argument as an arithmetic expression, using 32-bit signed-integer arithmetic. The following operators are supported: parentheses, unary -, unary +, !, ~, *, /, %, +, -, relationals, bitwise &, |, &&, and ||. Octal and hex numbers may be specified as in C. The second argument specifies the radix for the result; the default is 10. The third argument may be used to specify the minimum number of digits in the result.

eval

Evaluates its argument as an arithmetic expression, using 32-bit signed-integer arithmetic. The following operators are supported: parentheses, unary -, unary +, !, ~, *, /, %, +, -, <<, >>, relationals, bitwise &, |, &&, and ||. Precedence and associativity are as in C. Octal and hex numbers may also be specified as in C. The second argument specifies the radix for the result; the default is 10. The third argument may be used to specify the minimum number of digits in the result.

ifdef

If the first argument is defined, the value is the second argument, otherwise the third. If there is no third argument, the value is NULL. The word unix is predefined.

ifelse

This macro has three or more arguments. If the first argument is the same string as the second, then the value is the third argument. If not, and if there are more than four arguments, the process is repeated with arguments 4, 5, 6 and 7. Otherwise, the value is either the fourth string, or, if it is not present, NULL.

include

Returns the contents of the file named in the argument.

incr

Returns the value of its argument incremented by 1. The value of the argument is calculated by interpreting an initial digit-string as a decimal number.

index

Returns the position in its first argument where the second argument begins (zero origin), or -1 if the second argument does not occur.

len

Returns the number of characters in its argument.

m4exit

This macro causes immediate exit from m4. Argument 1, if given, is the exit code; the default is 0.

m4wrap

Argument 1 will be pushed back at final EOF; example: m4wrap(`cleanup()')

maketemp

Fills in a string of "X" characters in its argument with the current process ID.

popdef

Removes current definition of its argument(s), exposing the previous one, if any.

pushdef

Like define, but saves any previous definition.

shift

Returns all but its first argument. The other arguments are quoted and pushed back with commas in between. The quoting nullifies the effect of the extra scan that will subsequently be performed.

sinclude

This macro is identical to include, except that it says nothing if the file is inaccessible.

substr

Returns a substring of its first argument. The second argument is a zero origin number selecting the first character; the third argument indicates the length of the substring. A missing third argument is taken to be large enough to extend to the end of the first string.

syscmd

This macro executes the command given in the first argument. No value is returned.

sysval

This macro is the return code from the last call to syscmd.

translit

Transliterates the characters in its first argument from the set given by the second argument to the set given by the third. No abbreviations are permitted.

traceon

This macro with no arguments, turns on tracing for all macros (including built-ins). Otherwise, turns on tracing for named macros.

traceoff

Turns off trace globally and for any macros specified. Macros specifically traced by traceon can be untraced only by specific calls to traceoff.

undefine

Removes the definition of the macro named in its argument.

undivert

This macro causes immediate output of text from diversions named as arguments, or all diversions if no argument. Text may be undiverted into another diversion. Undiverting discards the diverted text.

An example of a single m4 input file capable of generating two output files follows. The file file1.m4 could contain lines such as:

if(VER, 1, do_something) if(VER, 2, do_something)

The makefile for the program might include:

file1.1.c : file1.m4 m4 -D VER=1 file1.m4 > file1.1.c ... file1.2.c : file1.m4 m4 -D VER=2 file1.m4 > file1.2.c ...

The -U option can be used to undefine VER. If file1.m4 contains:

if(VER, 1, do_something) if(VER, 2, do_something) ifndef(VER, do_something)

then the makefile would contain:

file1.0.c : file1.m4 m4 -U VER file1.m4 > file1.0.c ... file1.1.c : file1.m4 m4 -D VER=1 file1.m4 > file1.1.c ... file1.2.c : file1.m4 m4 -D VER=2 file1.m4 > file1.2.c ...