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5.1 Testing if a macro is defined

There are two different builtin conditionals in m4. The first is ifdef:

— Builtin: ifdef (name, string-1, [string-2])

If name is defined as a macro, ifdef expands to string-1, otherwise to string-2. If string-2 is omitted, it is taken to be the empty string (according to the normal rules).

The macro ifdef is recognized only with parameters. 

     ifdef(`foo', ``foo' is defined', ``foo' is not defined')
     =>foo is not defined
     define(`foo', `')
     =>
     ifdef(`foo', ``foo' is defined', ``foo' is not defined')
     =>foo is defined
     ifdef(`no_such_macro', `yes', `no', `extra argument')
     error-->m4:stdin:4: Warning: excess arguments to builtin `ifdef' ignored
     =>no