11.2 Evaluating integer expressions

Integer expressions are evaluated with eval:

Expressions can contain the following operators, listed in order of decreasing precedence.

+ -

Unary plus and minus

**

Exponentiation

* / %

Multiplication, division and modulo

+ -

Addition and subtraction

<< >>

Shift left or right

== != > >= < <=

Relational operators

!

Logical negation

~

Bitwise negation

&

Bitwise and

^

Bitwise exclusive-or

|

Bitwise or

&&

Logical and

||

Logical or

All operators, except exponentiation, are left associative.

Note that some older m4 implementations use `^' as an alternate operator for exponentiation, although POSIX requires the C behavior of bitwise exclusive-or. On the other hand, the precedence of `~' and `!' are different in GNU m4 than they are in C, matching the precedence in traditional m4 implementations. This behavior is likely to change in a future version to match POSIX, so use parentheses to force the desired precedence.

Within expression, (but not radix or width), numbers without a special prefix are decimal. A simple `0' prefix introduces an octal number. `0x' introduces a 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 radix 1, leading zeros are ignored and all remaining digits must be `1'; for all other radices, 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 relation return 0.

Here are a few examples of use of eval.

     eval(`-3 * 5')
     =>-15
     eval(index(`Hello world', `llo') >= 0)
     =>1
     eval(`0r1:0111 + 0b100 + 0r3:12')
     =>12
     define(`square', `eval(`('$1`)**2')')
     =>
     square(`9')
     =>81
     square(square(`5')`+1')
     =>676
     define(`foo', `666')
     =>
     eval(`foo/6')
     error-->m4:stdin:8: bad expression in eval: foo/6
     =>
     eval(foo/6)
     =>111

As the last two lines show, 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 passed to eval.

All evaluation is done with 32-bit signed integers, assuming 2's-complement with wrap-around. The shift operators are defined in GNU m4 by doing an implicit bit-wise and of the right-hand operand with 0x1f, and sign-extension with right shift.

     eval(0x80000000 / -1)
     =>-2147483648
     eval(0x80000000 % -1)
     =>0
     eval(0x7fffffff)
     =>2147483647
     incr(eval(0x7fffffff))
     =>-2147483648
     eval(-4 >> 33)
     =>-2

If radix is specified, it specifies the radix to be used in the expansion. The default radix is 10; this is also the case if radix is the empty string. It is an error if the radix is outside the range of 1 through 36, inclusive. The result of eval is always taken to be signed. No radix prefix is output, and for radices greater than 10, the digits are lower case. The width argument specifies the minimum output width, excluding any negative sign. The result is zero-padded to extend the expansion to the requested width. It is an error if the width is negative. On error, the expansion of eval is empty.

     eval(`666', `10')
     =>666
     eval(`666', `11')
     =>556
     eval(`666', `6')
     =>3030
     eval(`666', `6', `10')
     =>0000003030
     eval(`-666', `6', `10')
     =>-0000003030
     eval(`10', `', `0')
     =>10
     `0r1:'eval(`10', `1', `11')
     =>0r1:01111111111
     eval(`10', `16')
     =>a