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13.2 Printing current location
==============================
To make it possible to specify the location of an error, three utility
builtins exist:
-- Builtin: __file__
-- Builtin: __line__
-- Builtin: __program__
Expand to the quoted name of the current input file, the current
input line number in that file, and the quoted name of the current
invocation of `m4'.
errprint(__program__:__file__:__line__: `input error
')
error-->m4:stdin:1: input error
=>
Line numbers start at 1 for each file. If the file was found due to
the `-I' option or `M4PATH' environment variable, that is reflected in
the file name. The syncline option (`-s', Invoking m4), and the
`f' and `l' flags of `debugmode' ( Debug Levels), also use this
notion of current file and line. Redefining the three location macros
has no effect on syncline, debug, or warning message output. Assume
this example is run in the `checks' directory of the GNU M4 package,
using `--include=../examples' in the command line to find the file
`incl.m4' mentioned earlier:
define(`foo', ``$0' called at __file__:__line__')
=>
foo
=>foo called at stdin:2
include(`incl.m4')
=>Include file start
=>foo called at ../examples/incl.m4:2
=>Include file end
=>
Currently, all text wrapped with `m4wrap' ( M4wrap) behaves
as though it came from line 0 of the file "". It is hoped that a
future release of `m4' can overcome this limitation and remember which
file invoked the call to `m4wrap'.
The `__program__' macro behaves like `$0' in shell terminology. If
you invoke `m4' through an absolute path or a link with a different
spelling, rather than by relying on a `PATH' search for plain `m4', it
will affect how `__program__' expands. The intent is that you can use
it to produce error messages with the same formatting that `m4'
produces internally. It can also be used within `syscmd' (
Syscmd) to pick the same version of `m4' that is currently running,
rather than whatever version of `m4' happens to be first in `PATH'.
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