Input text stream

_images/input-text-stream.svg

Function prototypes for handling input text stream are declared in main/read.h. The file exists in Exuberant Ctags, too. However, the names functions are changed when overhauling --line-directive option. (In addition macros were converted to functions for making data structures for the input text stream opaque.)

Ctags has 3 groups of functions for handling input: input, bypass, and raw. Parser developers should use input group. The rest of two are for ctags main part.

inputFile type and the functions of input group

Note

The original version of this section was written before inputFile type and File variable are made private.

inputFile is the type for representing the input file and stream for a parser. It was declared in main/read.h but now it is defined in main/read.c.

Ctags uses a file static variable File having type inputFile for maintaining the input file and stream. File is also defined in main/read.c as inputFile is.

fp and line are the essential fields of File. fp having type well known MIO declared in main/mio.h. By calling functions of input group (getcFromInputFile and readLineFromInputFile), a parser gets input text from fp.

The functions of input group updates fields input and source of File variable. These two fields has type inputFileInfo. These two fields are for mainly tracking the name of file and the current line number. Usually ctags uses only input field. source field is used only when #line directive is found in the current input text stream.

A case when a tool generates the input file from another file, a tool can record the original source file to the generated file with using the #line directive. source field is used for tracking/recording the information appeared on #line directives.

Regex pattern matching are also done behind calling the functions of this group.

The functions of bypass group

The functions of bypass group (readLineFromBypass and readLineFromBypassSlow) are used for reading text from fp field of File static variable without updating input and source fields of File variable.

Parsers may not need the functions of this group. The functions are used in ctags main part. The functions are used to make pattern fields of tags file, for example.

The functions of raw group

The functions of this group (readLineRaw and readLineRawWithNoSeek) take a parameter having type MIO; and don’t touch File static variable.

Parsers may not need the functions of this group. The functions are used in ctags main part. The functions are used to load option files, for example.

Output tag stream

_images/output-tag-stream.svg

Ctags provides makeTagEntry to parsers as an entry point for writing tag information to MIO. makeTagEntry calls writeTagEntry if the parser does not set useCork field. writeTagEntry calls writerWriteTag. writerWriteTag just calls writeEntry of writer backends. writerTable variable holds the four backends: ctagsWriter, etagsWriter, xrefWriter, and jsonWriter. One of them is chosen depending on the arguments passed to ctags.

If useCork is set, the tag information goes to a queue on memory. The queue is flushed when useCork in unset. See “cork API” for more details.

cork API

Background and Idea

cork API is introduced for recording scope information easier.

Before introducing cork API, a scope information must be recorded as strings. It is flexible but memory management is required. Following code is taken from clojure.c (with some modifications).

if (vStringLength (parent) > 0)
{
        current.extensionFields.scope[0] = ClojureKinds[K_NAMESPACE].name;
        current.extensionFields.scope[1] = vStringValue (parent);
}

makeTagEntry (&current);

parent, scope [0] and scope [1] are vStrings. The parser must manage their life cycles; the parser cannot free them till the tag referring them via its scope fields are emitted, and must free them after emitting.

cork API provides more solid way to hold scope information. cork API expects parent, which represents scope of a tag(current) currently parser dealing, is recorded to a tags file before recording the current tag via makeTagEntry function.

For passing the information about parent to makeTagEntry, tagEntryInfo object was created. It was used just for recording; and freed after recording. In cork API, it is not freed after recording; a parser can reused it as scope information.

How to use

See a commit titled with “clojure: use cork”. I applied cork API to the clojure parser.

Cork API can be enabled and disabled per parser, and is disabled by default. So there is no impact till you enables it in your parser.

useCork field is introduced in parserDefinition type:

typedef struct {
...
                unsigned int useCork;
...
} parserDefinition;

Set CORK_QUEUE to useCork like:

extern parserDefinition *ClojureParser (void)
{
        ...
        parserDefinition *def = parserNew ("Clojure");
        ...
        def->useCork = CORK_QUEUE;
        return def;
}

When ctags running a parser with useCork being CORK_QUEUE, all output requested via makeTagEntry function calling is stored to an internal queue, not to tags file. When parsing an input file is done, the tag information stored automatically to the queue are flushed to tags file in batch.

When calling makeTagEntry with a tagEntryInfo object (parent), it returns an integer. The integer can be used as handle for referring the object after calling.

static int parent = CORK_NIL;
...
parent = makeTagEntry (&e);

The handle can be used by setting to a scopeIndex field of current tag, which is in the scope of parent.

current.extensionFields.scopeIndex = parent;

When passing current to makeTagEntry, the scopeIndex is referred for emitting the scope information of current.

scopeIndex must be set to CORK_NIL if a tag is not in any scope. When using scopeIndex of current, NULL must be assigned to both current.extensionFields.scope[0] and current.extensionFields.scope[1]. initTagEntry function does this initialization internally, so you generally you don’t have to write the initialization explicitly.

Automatic full qualified tag generation

If a parser uses the cork API for recording and emitting scope information, ctags can reuse it for generating full qualified (FQ) tags. Set requestAutomaticFQTag field of parserDefinition to TRUE then the main part of ctags emits FQ tags on behalf of the parser if --extras=+q is given.

An example can be found in DTS parser:

extern parserDefinition* DTSParser (void)
{
        static const char *const extensions [] = { "dts", "dtsi", NULL };
        parserDefinition* const def = parserNew ("DTS");
        ...
        def->requestAutomaticFQTag = TRUE;
        return def;
}

Setting requestAutomaticFQTag to TRUE implies setting useCork to CORK_QUEUE.

tokenInfo API

In Exuberant Ctags, a developer can write a parser anyway; only input stream and tagEntryInfo data structure is given.

However, while maintaining Universal Ctags I (Masatake YAMATO) think we should have a framework for writing parser. Of course the framework is optional; you can still write a parser without the framework.

To design a framework, I have studied how @b4n (Colomban Wendling) writes parsers. tokenInfo API is the first fruit of my study.

TBW

Multiple parsers

Guest parser (promise API)

See “Guest parser: Applying a parser to specified areas of input file” about the concept of guest parsers.

Background and Idea

More than one programming languages can be used in one input text stream. promise API allows a host parser running a guest parser in the specified area of input text stream.

e.g. Code written in c language (C code) is embedded in code written in Yacc language (Yacc code). Let’s think about this input stream.

/* foo.y */
 %token
         END_OF_FILE 0
         ERROR               255
         BELL                1

 %{
 /* C language */
 int counter;
 %}
 %right      EQUALS
 %left       PLUS MINUS
 ...
 %%
 CfgFile             :       CfgEntryList
                         { InterpretConfigs($1); }
                 ;

 ...
 %%
 int
 yyerror(char *s)
 {
     (void)fprintf(stderr,"%s: line %d of %s\n",s,lineNum,
                                         (scanFile?scanFile:"(unknown)"));
     if (scanStr)
         (void)fprintf(stderr,"last scanned symbol is: %s\n",scanStr);
     return 1;
 }

In the input the area started from %{ to %} and the area started from the second %% to the end of file are written in C. Yacc can be called host language, and C can be called guest language.

Ctags may choose the Yacc parser for the input. However, the parser doesn’t know about C syntax. Implementing C parser in the Yacc parser is one of approach. However, ctags has already C parser. The Yacc parser should utilize the existing C parser. The promise API allows this.

See also “Guest parser: Applying a parser to specified areas of input file” about more concept and examples of the guest parser.

Usage

See a commit titled with “Yacc: run C parser in the areas where code is written in C”. I applied promise API to the Yacc parser.

The parser for host language must track and record the start and the end of a guest language. Pairs of line number and byte offset represents the start and end. When the start and end are fixed, call makePromise with (1) the guest parser name, (2) start, and (3) end. (This description is a bit simplified the real usage.)

Let’s see the actual code from “parsers/yacc.c”.

struct cStart {
        unsigned long input;
        unsigned long source;
};

Both fields are for recording start. input field is for recording the value returned from getInputLineNumber. source is for getSourceLineNumber. See “inputFile” for the difference of the two.

enter_c_prologue shown in the next is a function called when %{ is found in the current input text stream. Remember, in yacc syntax, %{ is a marker of C code area.

static void enter_c_prologue (const char *line CTAGS_ATTR_UNUSED,
                             const regexMatch *matches CTAGS_ATTR_UNUSED,
                             unsigned int count CTAGS_ATTR_UNUSED,
                             void *data)
{
       struct cStart *cstart = data;


       readLineFromInputFile ();
       cstart->input  = getInputLineNumber ();
       cstart->source = getSourceLineNumber ();
}

The function just records the start line. It calls readLineFromInputFile because the C code may start the next line of the line where the marker is.

leave_c_prologue shown in the next is a function called when %}, the end marker of C code area, is found in the current input text stream.

static void leave_c_prologue (const char *line CTAGS_ATTR_UNUSED,
                             const regexMatch *matches CTAGS_ATTR_UNUSED,
                             unsigned int count CTAGS_ATTR_UNUSED,
                             void *data)
{
       struct cStart *cstart = data;
       unsigned long c_end;

       c_end = getInputLineNumber ();
       makePromise ("C", cstart->input, 0, c_end, 0, cstart->source);
}

After recording the line number of the end of the C code area, leave_c_prologue calls makePromise.

Of course "C" stands for C language, the name of guest parser. Available parser names can be listed by running ctags with --list-languages option. In this example two 0 characters are provided as the 3rd and 5th argument. They are byte offsets of the start and the end of the C language area from the beginning of the line which is 0 in this case. In general, the guest language’s section does not have to start at the beginning of the line in which case the two offsets have to be provided. Compilers reading the input character by character can obtain the current offset by calling getInputLineOffset().

Internal design

_images/promise.svg

A host parser cannot run a guest parser directly. What the host parser can do is just asking the ctags main part scheduling of running the guest parser for specified area which defined with the start and end. These scheduling requests are called promises.

After running the host parser, before closing the input stream, the ctags main part checks the existence of promise(s). If there is, the main part makes a sub input stream and run the guest parser specified in the promise. The sub input stream is made from the original input stream by narrowing as requested in the promise. The main part iterates the above process till there is no promise.

Theoretically a guest parser can be nested; it can make a promise. The level 2 guest is also just scheduled. (However, I have never tested such a nested guest parser).

Why not running the guest parser directly from the context of the host parser? Remember many parsers have their own file static variables. If a parser is called from the parser, the variables may be crashed.

API for subparser

See “Subparser: Tagging definitions of higher (upper) level language” about the concept of subparser.

Note

Consider using optlib when implementing a subparser. It is much more easy and simple. See “Defining a subparser” for details.

Outline

You have to work on both sides: a base parser and subparsers.

A base parser must define a data structure type (baseMethodTable) for its subparsers by extending struct subparser defined in main/subparser.h. A subparser defines a variable (subparser var) having type baseMethodTable by filling its fields and registers subparser var to the base parser using dependency API.

The base parser calls functions pointed by baseMethodTable of subparsers during parsing. A function for probing a higher level language may be included in baseMethodTable. What kind of fields should be included in baseMethodTable is up to the design of a base parser and the requirements of its subparsers. A method for probing is one of them.

Registering a subparser var to a base parser is enough for the bottom up choice. For handling the top down choice (e.g. specifying --language-force=<subparser> in a command line), more code is needed.

In the top down choice, the subparser must call scheduleRunningBasepaser, declared in main/subparser.h, in its parser method. Here, parser method means a function assigned to the parser member of the parserDefinition of the subparser. scheduleRunningBaseparser takes an integer argument that specifies the dependency used for registering the subparser var.

By extending struct subparser you can define a type for your subparser. Then make a variable for the type and declare a dependency on the base parser.

Fields of subparser type

Here the source code of Autoconf/m4 parsers is referred as an example.

main/types.h:

struct sSubparser;
typedef struct sSubparser subparser;

main/subparser.h:

typedef enum eSubparserRunDirection {
        SUBPARSER_BASE_RUNS_SUB = 1 << 0,
        SUBPARSER_SUB_RUNS_BASE = 1 << 1,
        SUBPARSER_BI_DIRECTION  = SUBPARSER_BASE_RUNS_SUB|SUBPARSER_SUB_RUNS_BASE,
} subparserRunDirection;

struct sSubparser {
        ...

        /* public to the parser */
        subparserRunDirection direction;

        void (* inputStart) (subparser *s);
        void (* inputEnd) (subparser *s);
        void (* exclusiveSubparserChosenNotify) (subparser *s, void *data);
};

A subparser must fill the fields of subparser.

direction field specifies how the subparser is called. See “Direction flags” in “Running multiple parsers on an input file” about direction flags, and see “Direction flags” in “Extending ctags with Regex parser (optlib)” for examples of using the direction flags.

direction field Direction Flag
SUBPARSER_BASE_RUNS_SUB shared (default)
SUBPARSER_SUB_RUNS_BASE dedicated
SUBPARSER_BI_DIRECTION bidirectional

If a subparser runs exclusively and is chosen in top down way, set SUBPARSER_SUB_RUNS_BASE flag. If a subparser runs coexisting way and is chosen in bottom up way, set SUBPARSER_BASE_RUNS_SUB. Use SUBPARSER_BI_DIRECTION if both cases can be considered.

SystemdUnit parser runs as a subparser of iniconf base parser. SystemdUnit parser specifies SUBPARSER_SUB_RUNS_BASE because unit files of systemd have very specific file extensions though they are written in iniconf syntax. Therefore we expect SystemdUnit parser is chosen in top down way. The same logic is applicable to YumRepo parser.

Autoconf parser specifies SUBPARSER_BI_DIRECTION. For input file having name configure.ac, by pattern matching, Autoconf parser is chosen in top down way. In other hand, for file name foo.m4, Autoconf parser can be chosen in bottom up way.

inputStart is called before the base parser starting parsing a new input file. inputEnd is called after the base parser finishing parsing the input file. Universal Ctags main part calls these methods. Therefore, a base parser doesn’t have to call them.

exclusiveSubparserChosenNotify is called when a parser is chosen as an exclusive parser. Calling this method is a job of a base parser.

Extending subparser type

The m4 parser extends subparser type like following:

parsers/m4.h:

typedef struct sM4Subparser m4Subparser;
struct sM4Subparser {
        subparser subparser;

        bool (* probeLanguage) (m4Subparser *m4, const char* token);

        /* return value: Cork index */
        int  (* newMacroNotify) (m4Subparser *m4, const char* token);

        bool (* doesLineCommentStart)   (m4Subparser *m4, int c, const char *token);
        bool (* doesStringLiteralStart) (m4Subparser *m4, int c);
};

Put subparser as the first member of the extended struct (here sM4Subparser). In addition the first field, 4 methods are defined in the extended struct.

Till choosing a subparser for the current input file, the m4 parser calls probeLanguage method of its subparsers each time when find a token in the input file. A subparser returns true if it recognizes the input file is for the itself by analyzing tokens passed from the base parser.

parsers/autoconf.c:

extern parserDefinition* AutoconfParser (void)
{
        static const char *const patterns [] = { "configure.in", NULL };
        static const char *const extensions [] = { "ac", NULL };
        parserDefinition* const def = parserNew("Autoconf");

        static m4Subparser autoconfSubparser = {
                .subparser = {
                        .direction = SUBPARSER_BI_DIRECTION,
                        .exclusiveSubparserChosenNotify = exclusiveSubparserChosenCallback,
                },
                .probeLanguage  = probeLanguage,
                .newMacroNotify = newMacroCallback,
                .doesLineCommentStart = doesLineCommentStart,
                .doesStringLiteralStart = doesStringLiteralStart,
        };

probeLanguage function defined in autoconf.c is connected to the probeLanguage member of autoconfSubparser. The probeLanguage function of Autoconf is very simple:

parsers/autoconf.c:

static bool probeLanguage (m4Subparser *m4, const char* token)
{
        return strncmp (token, "m4_", 3) == 0
                || strncmp (token, "AC_", 3) == 0
                || strncmp (token, "AM_", 3) == 0
                || strncmp (token, "AS_", 3) == 0
                || strncmp (token, "AH_", 3) == 0
                ;
}

This function checks the prefix of passed tokens. If known prefix is found, Autoconf assumes this is an Autoconf input and returns true.

parsers/m4.c:

if (m4tmp->probeLanguage
        && m4tmp->probeLanguage (m4tmp, token))
{
        chooseExclusiveSubparser ((m4Subparser *)tmp, NULL);
        m4found = m4tmp;
}

The m4 parsers calls probeLanguage function of a subparser. If true is returned chooseExclusiveSubparser function which is defined in the main part. chooseExclusiveSubparser calls exclusiveSubparserChosenNotify method of the chosen subparser.

The method is implemented in Autoconf subparser like following:

parsers/autoconf.c:

static void exclusiveSubparserChosenCallback (subparser *s, void *data)
{
        setM4Quotes ('[', ']');
}

It changes quote characters of the m4 parser.

Making a tag in a subparser

Via calling callback functions defined in subparsers, their base parser gives chance to them making tag entries.

The m4 parser calls newMacroNotify method when it finds an m4 macro is used. The Autoconf parser connects newMacroCallback function defined in parser/autoconf.c.

parsers/autoconf.c:

static int newMacroCallback (m4Subparser *m4, const char* token)
{
        int keyword;
        int index = CORK_NIL;

        keyword = lookupKeyword (token, getInputLanguage ());

        /* TODO:
           AH_VERBATIM
         */
        switch (keyword)
        {
        case KEYWORD_NONE:
                break;
        case KEYWORD_init:
                index = makeAutoconfTag (PACKAGE_KIND);
                break;

...

extern parserDefinition* AutoconfParser (void)
{
        ...
        static m4Subparser autoconfSubparser = {
                .subparser = {
                        .direction = SUBPARSER_BI_DIRECTION,
                        .exclusiveSubparserChosenNotify = exclusiveSubparserChosenCallback,
                },
                .probeLanguage  = probeLanguage,
                .newMacroNotify = newMacroCallback,

In newMacroCallback function, the Autoconf parser receives the name of macro found by the base parser and analysis whether the macro is interesting in the context of Autoconf language or not. If it is interesting name, the Autoconf parser makes a tag for it.

Calling methods of subparsers from a base parser

A base parser can use foreachSubparser macro for accessing its subparsers. A base should call enterSubparser before calling a method of a subparser, and call leaveSubparser after calling the method. The macro and functions are declare in main/subparser.h .

parsers/m4.c:

static m4Subparser * maySwitchLanguage (const char* token)
{
        subparser *tmp;
        m4Subparser *m4found = NULL;

        foreachSubparser (tmp, false)
        {
                m4Subparser *m4tmp = (m4Subparser *)tmp;

                enterSubparser(tmp);
                if (m4tmp->probeLanguage
                        && m4tmp->probeLanguage (m4tmp, token))
                {
                        chooseExclusiveSubparser (tmp, NULL);
                        m4found = m4tmp;
                }
                leaveSubparser();

                if (m4found)
                        break;
        }

        return m4found;
}

foreachSubparser takes a variable having type subparser. For each iteration, the value for the variable is updated.

enterSubparser takes a variable having type subparser. With the calling enterSubparser, the current language (the value returned from getInputLanguage) can be temporary switched to the language specified with the variable. One of the effect of switching is that language field of tags made in the callback function called between enterSubparser and leaveSubparser is adjusted.

Registering a subparser to its base parser

Use DEPTYPE_SUBPARSER dependency in a subparser for registration.

parsers/autoconf.c:

extern parserDefinition* AutoconfParser (void)
{
        parserDefinition* const def = parserNew("Autoconf");

        static m4Subparser autoconfSubparser = {
                .subparser = {
                        .direction = SUBPARSER_BI_DIRECTION,
                        .exclusiveSubparserChosenNotify = exclusiveSubparserChosenCallback,
                },
                .probeLanguage  = probeLanguage,
                .newMacroNotify = newMacroCallback,
                .doesLineCommentStart = doesLineCommentStart,
                .doesStringLiteralStart = doesStringLiteralStart,
        };
        static parserDependency dependencies [] = {
                [0] = { DEPTYPE_SUBPARSER, "M4", &autoconfSubparser },
        };

        def->dependencies = dependencies;
        def->dependencyCount = ARRAY_SIZE (dependencies);

DEPTYPE_SUBPARSER is specified in the 0th element of dependencies function static variable. In the next a literal string “M4” is specified and autoconfSubparser follows. The intent of the code is registering autoconfSubparser subparser definition to a base parser named “M4”.

dependencies function static variable must be assigned to dependencies fields of a variable of parserDefinition. The main part of Universal Ctags refers the field when initializing parsers.

[0] emphasizes this is “the 0th element”. The subparser may refer the index of the array when the subparser calls scheduleRunningBaseparser.

Scheduling running the base parser

For the case that a subparser is chosen in top down, the subparser must call scheduleRunningBaseparser in the main parser method.

parsers/autoconf.c:

static void findAutoconfTags(void)
{
        scheduleRunningBaseparser (0);
}

extern parserDefinition* AutoconfParser (void)
{
        ...
        parserDefinition* const def = parserNew("Autoconf");
        ...
        static parserDependency dependencies [] = {
                [0] = { DEPTYPE_SUBPARSER, "M4", &autoconfSubparser },
        };

        def->dependencies = dependencies;
        ...
        def->parser = findAutoconfTags;
        ...
        return def;
}

A subparser can do nothing actively. A base parser makes its subparser work by calling methods of the subparser. Therefore a subparser must run its base parser when the subparser is chosen in a top down way, The main part prepares scheduleRunningBaseparser function for the purpose.

A subparser should call the function from parser method of parserDefinition of the subparser. scheduleRunningBaseparser takes an integer. It specifies an index of the dependency which is used for registering the subparser.

PackCC compiler-compiler

PackCC is a compiler-compiler; it translates .peg grammar file to .c file. PackCC was originally written by Arihiro Yoshida. Its source repository is at https://github.com/arithy/packcc.

The source tree of PackCC is grafted at misc/packcc directory. Building PackCC and ctags are integrated in the build-scripts of Universal Ctags.

Refer peg/valink.peg as a sample of a parser using PackCC.

Automatic parser guessing (TBW)

Managing regular expression parsers (TBW)

Ghost kind in regex parser (TBW)