(m4.info)Builtin

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5.8 Indirect call of builtins
=============================

Builtin macros can be called indirectly with 'builtin':

 -- Builtin: builtin (NAME, [ARGS...])
     Results in a call to the builtin NAME, which is passed the rest of
     the arguments ARGS.  If NAME does not name a builtin, an error
     message is printed, and the expansion is void.

     The macro 'builtin' is recognized only with parameters.

   This can be used even if NAME has been given another definition that
has covered the original, or been undefined so that no macro maps to the
builtin.

     pushdef(`define', `hidden')
     =>
     undefine(`undefine')
     =>
     define(`foo', `bar')
     =>hidden
     foo
     =>foo
     builtin(`define', `foo', defn(`divnum'))
     =>
     foo
     =>0
     builtin(`define', `foo', `BAR')
     =>
     foo
     =>BAR
     undefine(`foo')
     =>undefine(foo)
     foo
     =>BAR
     builtin(`undefine', `foo')
     =>
     foo
     =>foo

   The NAME argument only matches the original name of the builtin, even
when the '--prefix-builtins' option (or '-P', *note Invoking m4:
Operation modes.) is in effect.  This is different from 'indir', which
only tracks current macro names.

     $ m4 -P
     m4_builtin(`divnum')
     =>0
     m4_builtin(`m4_divnum')
     error->m4:stdin:2: undefined builtin `m4_divnum'
     =>
     m4_indir(`divnum')
     error->m4:stdin:3: undefined macro `divnum'
     =>
     m4_indir(`m4_divnum')
     =>0

   Note that 'indir' and 'builtin' can be used to invoke builtins
without arguments, even when they normally require parameters to be
recognized; but it will provoke a warning, and result in a void
expansion.

     builtin
     =>builtin
     builtin()
     error->m4:stdin:2: undefined builtin `'
     =>
     builtin(`builtin')
     error->m4:stdin:3: Warning: too few arguments to builtin `builtin'
     =>
     builtin(`builtin',)
     error->m4:stdin:4: undefined builtin `'
     =>
     builtin(`builtin', ``'
     ')
     error->m4:stdin:5: undefined builtin ``'
     error->'
     =>
     indir(`index')
     error->m4:stdin:7: Warning: too few arguments to builtin `index'
     =>