pcrestack
PCRESTACK(3) PCRESTACK(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE DISCUSSION OF STACK USAGE
When you call pcre_exec(), it makes use of an internal function called
match(). This calls itself recursively at branch points in the pat-
tern, in order to remember the state of the match so that it can back
up and try a different alternative if the first one fails. As matching
proceeds deeper and deeper into the tree of possibilities, the recur-
sion depth increases.
Not all calls of match() increase the recursion depth; for an item
such as a* it may be called several times at the same level, after
matching different numbers of a’s. Furthermore, in a number of cases
where the result of the recursive call would immediately be passed
back as the result of the current call (a "tail recursion"), the func-
tion is just restarted instead.
The pcre_dfa_exec() function operates in an entirely different way,
and hardly uses recursion at all. The limit on its complexity is the
amount of workspace it is given. The comments that follow do NOT apply
to pcre_dfa_exec(); they are relevant only for pcre_exec().
You can set limits on the number of times that match() is called, both
in total and recursively. If the limit is exceeded, an error occurs.
For details, see the section on extra data for pcre_exec() in the
pcreapi documentation.
Each time that match() is actually called recursively, it uses memory
from the process stack. For certain kinds of pattern and data, very
large amounts of stack may be needed, despite the recognition of "tail
recursion". You can often reduce the amount of recursion, and there-
fore the amount of stack used, by modifying the pattern that is being
matched. Consider, for example, this pattern:
([^<]|<(?!inet))+
It matches from wherever it starts until it encounters "<inet" or the
end of the data, and is the kind of pattern that might be used when
processing an XML file. Each iteration of the outer parentheses
matches either one character that is not "<" or a "<" that is not fol-
lowed by "inet". However, each time a parenthesis is processed, a
recursion occurs, so this formulation uses a stack frame for each
matched character. For a long string, a lot of stack is required. Con-
sider now this rewritten pattern, which matches exactly the same
strings:
([^<]++|<(?!inet))+
This uses very much less stack, because runs of characters that do not
contain "<" are "swallowed" in one item inside the parentheses. Recur-
sion happens only when a "<" character that is not followed by "inet"
is encountered (and we assume this is relatively rare). A possessive
quantifier is used to stop any backtracking into the runs of non-"<"
characters, but that is not related to stack usage.
This example shows that one way of avoiding stack problems when match-
ing long subject strings is to write repeated parenthesized subpat-
terns to match more than one character whenever possible.
In environments where stack memory is constrained, you might want to
compile PCRE to use heap memory instead of stack for remembering back-
up points. This makes it run a lot more slowly, however. Details of
how to do this are given in the pcrebuild documentation. When built in
this way, instead of using the stack, PCRE obtains and frees memory by
calling the functions that are pointed to by the pcre_stack_malloc and
pcre_stack_free variables. By default, these point to malloc() and
free(), but you can replace the pointers to cause PCRE to use your own
functions. Since the block sizes are always the same, and are always
freed in reverse order, it may be possible to implement customized
memory handlers that are more efficient than the standard functions.
In Unix-like environments, there is not often a problem with the stack
unless very long strings are involved, though the default limit on
stack size varies from system to system. Values from 8Mb to 64Mb are
common. You can find your default limit by running the command:
ulimit -s
Unfortunately, the effect of running out of stack is often SIGSEGV,
though sometimes a more explicit error message is given. You can nor-
mally increase the limit on stack size by code such as this:
struct rlimit rlim;
getrlimit(RLIMIT_STACK, &rlim);
rlim.rlim_cur = 100*1024*1024;
setrlimit(RLIMIT_STACK, &rlim);
This reads the current limits (soft and hard) using getrlimit(), then
attempts to increase the soft limit to 100Mb using setrlimit(). You
must do this before calling pcre_exec().
PCRE has an internal counter that can be used to limit the depth of
recursion, and thus cause pcre_exec() to give an error code before it
runs out of stack. By default, the limit is very large, and unlikely
ever to operate. It can be changed when PCRE is built, and it can also
be set when pcre_exec() is called. For details of these interfaces,
see the pcrebuild and pcreapi documentation.
As a very rough rule of thumb, you should reckon on about 500 bytes
per recursion. Thus, if you want to limit your stack usage to 8Mb, you
should set the limit at 16000 recursions. A 64Mb stack, on the other
hand, can support around 128000 recursions. The pcretest test program
has a command line option (-S) that can be used to increase the size
of its stack.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 05 June 2007
Copyright (c) 1997-2007 University of Cambridge.
PCRESTACK(3)
