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Diversions are a way of temporarily saving output. The output of
m4
can at any time be diverted to a temporary file, and be
reinserted into the output stream, undiverted, again at a later
time.
Numbered diversions are counted from 0 upwards, diversion number 0
being the normal output stream. The number of simultaneous diversions
is limited mainly by the memory used to describe them, because GNU
m4
tries to keep diversions in memory. However, there is a
limit to the overall memory usable by all diversions taken altogether
(512K, currently). When this maximum is about to be exceeded,
a temporary file is opened to receive the contents of the biggest
diversion still in memory, freeing this memory for other diversions.
When creating the temporary file, m4
honors the value of the
environment variable TMPDIR, and falls back to /tmp.
So, it is theoretically possible that the number and aggregate size of
diversions is limited only by available disk space.
Diversions make it possible to generate output in a different order than the input was read. It is possible to implement topological sorting dependencies. For example, GNU Autoconf makes use of diversions under the hood to ensure that the expansion of a prerequisite macro appears in the output prior to the expansion of a dependent macro, regardless of which order the two macros were invoked in the user's input file.