4.2. String Handling in Macros

It’s often useful to be able to handle strings in macros. NASM supports two simple string handling macro operators from which more complex operations can be constructed.

4.2.1. String Length: %strlen

The %strlen macro is like %assign macro in that it creates (or redefines) a numeric value to a macro. The difference is that with %strlen, the numeric value is the length of a string. An example of the use of this would be:

%strlen charcnt 'my string'

In this example, charcnt would receive the value 8, just as if an %assign had been used. In this example, 'my string' was a literal string but it could also have been a single-line macro that expands to a string, as in the following example:

%define sometext 'my string'
%strlen charcnt sometext

As in the first case, this would result in charcnt being assigned the value of 8.

4.2.2. Sub-strings: %substr

Individual letters in strings can be extracted using %substr. An example of its use is probably more useful than the description:

%substr mychar  'xyz' 1         ; equivalent to %define mychar 'x'
%substr mychar  'xyz' 2         ; equivalent to %define mychar 'y'
%substr mychar  'xyz' 3         ; equivalent to %define mychar 'z'

In this example, mychar gets the value of 'y'. As with %strlen (see Section 4.2.1), the first parameter is the single-line macro to be created and the second is the string. The third parameter specifies which character is to be selected. Note that the first index is 1, not 0 and the last index is equal to the value that %strlen would assign given the same string. Index values out of range result in an empty string.