Make C parser reentrant

[wangjia184]

The *.y template file generated by BNFC using global varilables to store the parsed AST and internal status.

/* Global variables holding parse results for entrypoints. */
Proc YY_RESULT_Proc_ = 0;
ListProc YY_RESULT_ListProc_ = 0;

This means the parser is not thread-safe.

Please consider enabling Thread Local Storage for them.
Here is a thread_local macro suggested here.

#ifndef thread_local
# if __STDC_VERSION__ >= 201112 && !defined __STDC_NO_THREADS__
#  define thread_local _Thread_local
# elif defined _WIN32 && ( \
       defined _MSC_VER || \
       defined __ICL || \
       defined __DMC__ || \
       defined __BORLANDC__ )
#  define thread_local __declspec(thread) 
/* note that ICC (linux) and Clang are covered by __GNUC__ */
# elif defined __GNUC__ || \
       defined __SUNPRO_C || \
       defined __xlC__
#  define thread_local __thread
# else
#  error "Cannot define thread_local"
# endif
#endif

[andreasabel]

I am not very proficient in C. Could you contribute a pull request?

Or, if you do not want to touch the Haskell code, could you describe precisely (e.g. by diffs) how the output of the C Backend should change to implement your feature?

[wangjia184]

Hi @andreasabel

• if I run a single test case (means the test case call the C parser only once), everything works fine;
• If I run my test cases, even used mutex to synchronize the calls to the C parser. It raises occasional errors. And the error changes, sometime it says syntax error in different location; sometimes it says memory error...

It seems BNFC C parser is not ready to be used in this way. So I give up this approach, and wrap the C parser into a child process. And everytime I need to parse a source file, launch the child process and exit after parsing the file.

The following changes are what I have attempted in *.y file.

First I added a macro in the top part.

#ifndef thread_local
# if __STDC_VERSION__ >= 201112 && !defined __STDC_NO_THREADS__
#  define thread_local _Thread_local
# elif defined _WIN32 && ( \
       defined _MSC_VER || \
       defined __ICL || \
       defined __DMC__ || \
       defined __BORLANDC__ )
#  define thread_local __declspec(thread) 
/* note that ICC (linux) and Clang are covered by __GNUC__ */
# elif defined __GNUC__ || \
       defined __SUNPRO_C || \
       defined __xlC__
#  define thread_local __thread
# else
#  error "Cannot define thread_local"
# endif
#endif

Then added thread_local before all global variables under the following line

/* Global variables holding parse results for entrypoints. */
thread_local Proc YY_RESULT_Proc_ = 0;
thread_local ListProc YY_RESULT_ListProc_ = 0;
...

Reset all global variables in the beginning of the parser method.

Proc pProc(FILE *inp)
{
  YY_RESULT_Proc_ = 0;
  YY_RESULT_ListProc_ = 0;
  //...

//....
}

It just does not work and fail with occasional and different errors..

[andreasabel]

With bison you have to do some extra work to get a reentrant parser. https://www.gnu.org/software/bison/manual/html_node/Pure-Decl.html

Maybe we could add change BNFC to generate reentrant parsers by default.