Reducing pcre2_match()'s stack usage¶

You can often reduce the amount of recursion, and therefore 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 followed 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. Consider 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. Recursion 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 matching long subject strings is to write repeated parenthesized subpatterns to match more than one character whenever possible.

Compiling PCRE2 to use heap instead of stack for pcre2_match()¶

In environments where stack memory is constrained, you might want to compile PCRE2 to use heap memory instead of stack for remembering back-up points when pcre2_match() is running. This makes it run more slowly, however. Details of how to do this are given in the pcre2build documentation. When built in this way, instead of using the stack, PCRE2 gets memory for remembering backup points from the heap. By default, the memory is obtained by calling the system malloc() function, but you can arrange to supply your own memory management function. For details, see the section entitled "The match context" in the pcre2api documentation. Since the block sizes are always the same, it may be possible to implement a customized memory handler that is more efficient than the standard function. The memory blocks obtained for this purpose are retained and re-used if possible while pcre2_match() is running. They are all freed just before it exits.

Limiting pcre2_match()'s stack usage¶

You can set limits on the number of times the internal match() function is called, both in total and recursively. If a limit is exceeded, pcre2_match() returns an error code. Setting suitable limits should prevent it from running out of stack. The default values of the limits are very large, and unlikely ever to operate. They can be changed when PCRE2 is built, and they can also be set when pcre2_match() is called. For details of these interfaces, see the pcre2build documentation and the section entitled "The match context" in the pcre2api 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 pcre2test test program has a modifier called "find_limits" which, if applied to a subject line, causes it to find the smallest limits that allow a a pattern to match. This is done by calling pcre2_match() repeatedly with different limits.

Limiting pcre2_dfa_match()'s stack usage¶

The recursion limit, as described above for pcre2_match(), also applies to pcre2_dfa_match(), whose use of recursive function calls for recursions in the pattern can lead to runaway stack usage. The non-recursive match limit is not relevant for DFA matching, and is ignored.

Changing stack size in Unix-like systems¶

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 normally 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 pcre2_match().

Changing stack size in Mac OS X¶

Using setrlimit(), as described above, should also work on Mac OS X. It is also possible to set a stack size when linking a program. There is a discussion about stack sizes in Mac OS X at this web site: http://developer.apple.com/qa/qa2005/qa1419.html.