Integer expressions are evaluated with
eval(expression, opt radix, opt width)
which expands to the value of expression.
Expressions can contain the following operators, listed in order of decreasing precedence.
* / %
== != > >= < <=
All operators, except exponentiation, are left associative.
Note that many
m4 implementations use `^' as an alternate
operator for the exponentiation, while many others use `^' for the
bitwise exclusive-or. GNU
m4 changed its behavior: it used to
exponentiate for `^', it now computes the bitwise exclusive-or.
Numbers without special prefix are given decimal. A simple `0' prefix introduces an octal number. `0x' introduces an hexadecimal number. `0b' introduces a binary number. `0r' introduces a number expressed in any radix between 1 and 36: the prefix should be immediately followed by the decimal expression of the radix, a colon, then the digits making the number. For any radix, the digits are `0', `1', `2', .... Beyond `9', the digits are `a', `b' ... up to `z'. Lower and upper case letters can be used interchangeably in numbers prefixes and as number digits.
Parentheses may be used to group subexpressions whenever needed. For the
relational operators, a true relation returns
1, and a false
Here are a few examples of use of
eval(-3 * 5) =>-15 eval(index(`Hello world', `llo') >= 0) =>1 define(`square', `eval(($1)**2)') => square(9) =>81 square(square(5)+1) =>676 define(`foo', `666') => eval(`foo'/6) error-->51.eval:14: m4: Bad expression in eval: foo/6 => eval(foo/6) =>111
As the second to last example shows,
eval does not handle macro
names, even if they expand to a valid expression (or part of a valid
expression). Therefore all macros must be expanded before they are
If radix is specified, it specifies the radix to be used in the
expansion. The default radix is 10. The result of
always taken to be signed. The width argument specifies a minimum
output width. The result is zero-padded to extend the expansion to the
eval(666, 10) =>666 eval(666, 11) =>556 eval(666, 6) =>3030 eval(666, 6, 10) =>0000003030 eval(-666, 6, 10) =>-000003030
Take note that radix cannot be larger than 36.
The builtin macro
eval is recognized only when given arguments.
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