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diff --git a/doc/gawk.texi b/doc/gawk.texi index ba5ffa30..fb17b716 100644 --- a/doc/gawk.texi +++ b/doc/gawk.texi @@ -7171,6 +7171,34 @@ trying to accomplish. It is worth noting that those variants which do not use redirection can cause @code{FILENAME} to be updated if they cause @command{awk} to start reading a new input file. + +@item +If the variable being assigned is an expression with side effects, +different versions of @command{awk} behave differently upon encountering +end-of-file. Some versions don't evaluate the expression; many versions +(including @command{gawk}) do. Here is an example, due to Duncan Moore: + +@ignore +Date: Sun, 01 Apr 2012 11:49:33 +0100 +From: Duncan Moore <duncan.moore@@gmx.com> +@end ignore + +@example +BEGIN @{ + system("echo 1 > f") + while ((getline a[++c] < "f") > 0) @{ @} + print c +@} +@end example + +@noindent +Here, the side effect is the @samp{++c}. Is @code{c} incremented if +end of file is encountered, before the element in @code{a} is assigned? + +@command{gawk} treats @code{getline} like a function call, and evaluates +the expression @samp{a[++c]} before attempting to read from @file{f}. +Other versions of @command{awk} only evaluate the expression once they +know that there is a string value to be assigned. Caveat Emptor. @end itemize @node Getline Summary |