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    semgrep-core contributing

    The following explains how to build semgrep-core so you can make and test changes to the OCaml code. Once you have semgrep-core installed, you can refer to semgrep-contributing to see how to build and run the Semgrep application.

    Building semgrep-core


    semgrep-core is written primarily in OCaml. You will need to install OCaml and its package manager OPAM. On macOS, it should consist in doing:

    brew install opam
    opam init
    opam switch create 4.14.0 ocaml-base-compiler.4.14.0
    opam switch 4.14.0
    eval $(opam env)

    Install pkg-config in your environment. On a mac this is

    brew install pkg-config

    Installing for the first time

    The root Makefile contains targets that take care of building the right things. It is commented. Please refer to it and keep it up-to-date.

    Run the below commands at the repository root, inside your pipenv shell.

    To link all necessary dependencies, run

    make dev-setup

    Next, to install semgrep-core, run

    make core

    Finally, test the installation with

    bin/semgrep-core -help

    At this point, you have the semgrep-core binary, so if you would like to finish the Semgrep installation, go to the Python-side instructions.

    Installing after a change

    Unless there is a significant dependency change, you will not need to run make dev-setup again.

    We have provided useful targets to help you build and link the entire semgrep project, including both semgrep-core and semgrep. You may find these helpful.

    To install the latest OCaml binaries and semgrep binary after pulling source code changes from git, run

    make rebuild

    To install after you make a change locally, run

    make build    # or just `make`

    After making either of these targets, semgrep will run with all your local changes, OCaml and Python both.

    NOTE: Because this updates the semgrep binary, if you do not have your Python environment configured properly, you will encounter errors when running these commands. Follow the procedure under Development


    In practice, it is not always convenient to use make build or make rebuild. make rebuild will update everything within the project; make build will compile and install all the binaries. You can do this yourself in a more targeted fashion.

    Below is a flow appropriate for frequent developers of semgrep-core

    After you pull, run

    git submodule update --recursive

    This will update internal dependencies. (We suggest aliasing it to uu)

    After tree-sitter is updated, you may need to reconfigure it. If so, run

    make config

    Developing semgrep-core

    If you are developing semgrep-core, Use Makefile in the repository root for core and core-test targets; the code is primarily in src/.

    The following assumes you are in the repository root.

    After you pull or make a change, compile using


    This will build an executable for semgrep-core in _build/default/src/main/Main.exe (we suggest aliasing this to sc). Try it out by running

    _build/default/src/main/Main.exe -help

    When you are done, test your changes with

    make core-test

    Finally, to update the semgrep-core binary used by semgrep, run

    make copy-core-for-cli

    Testing semgrep-core

    make test in the repository root directory will run tests that check code is correctly parsed and patterns perform as expected. To add a test in an appropriate language subdirectory, tests/patterns/[LANG], create a target file (expected file extension given language) and a .sgrep file with a pattern. The testing suite will check that all places with a comment with ERROR were matches found by the .sgrep file. See existing tests for more clarity.

    Development environment

    You can use Visual Studio Code (vscode) to edit the code of Semgrep. The reason-vscode Marketplace extension adds support for OCaml/Reason.

    The OCaml and Reason IDE extension by @freebroccolo is another valid extension, but it seems not as actively maintained as reason-vscode.

    The source of Semgrep contains also a .vscode/ directory at its root containing a task file to automatically build Semgrep from vscode.

    Note that dune and ocamlmerlin must be in your PATH for vscode to correctly build and provide cross-reference on the code. In case of problems, do:

    cd /path/to/semgrep
    eval $(opam env)
    dune --version # just checking dune is in your PATH
    ocamlmerlin -version # just checking ocamlmerlin is in your PATH
    code .

    Testing performance

    Exploring results from a slow run of Semgrep

    Interpreting the result object

    For full timing information, run Semgrep with --time and --json flags. In addition, you can add time at the beginning of the command to get the true wall time. The --json argument produces a large amount of output, so redirecting the output to a file with -o is recommended.

    See the following example for the full command:

    time semgrep --config=auto --time --json -o result.json PATH/TO/SRC

    Substitute the optional placeholder PATH/TO/SRC with the path to your source code.

    Here is an example result object.

          { "results": [], 
    "paths": {},
    "errors": [],
    "time": {
    "max_memory_bytes": 48693248,
    "profiling_times": {
    "config_time": 0.0624239444732666,
    "core_time": 0.11341428756713867,
    "ignores_time": 0.00017690658569335938,
    "total_time": 0.17628788948059082
    "rules": [
    "id": "test-rule"
    "rules_parse_time": 0.0013418197631835938,
    "targets": [
    "match_times": [
    "num_bytes": 340,
    "parse_times": [
    "path": "",
    "run_time": 0.011521100997924805
    "total_bytes": 340

    All the information about timing is contained under time.

    The first section is profiling_times. This contains wall time durations of various relevant steps:

    • Getting the rule config files (config_time)
    • Running the main engine (core_time)
    • Processing the ignores (ignores_time)

    The total_time field represents the sum of these steps.

    The remaining fields report engine performance. Together, rule_parse_time and targets should capture all the time spent running semgrep-core.

    rule_parse_time is straightforward. It records the time spent parsing the rules file.

    targets poses more difficulty. Since files are run in parallel, the amount of time spent parsing (parse_times) and matching (match_times) will inevitably be meaningless compared against total_time or core_time. Therefore, the total run time (run_time) of each target for each rule is taken within the parallel run. This helps contextualize the time spent parsing and matching each target. The sum of the run times thus can (and usually should) be longer than the total time.

    The lists match_times and parse_times are in the same order as rules. That is, the match time of rule rules[0] is match_times[0].

    Note that parse_times is given for each rule, but a file should only be parsed once (the first number). Afterwards, the parse time represents the time spent retrieving the file's AST from the cache.

    Negative values in the metrics

    When a time is not measured, by default it has the value -1. It is common to a have a normal runtime, but -1 for the parse time or match time; this indicates an error in parsing.

    Tips for exploring Semgrep results

    There are several scripts already written to analyze and summarize these timing data. Find them in scripts/processing-output. If you have a timing file, you can run

    python [your_timing_file]

    You may need to adjust the line result_times = results based on whether you have a timing file or the full results (in which case this should be result_times = results["time"])

    Profiling code

    You can pass the -profile command-line argument to semgrep-core to get a short profile of the code. For example, running:

    cd semgrep-core
    ./bin/semgrep-core -profile -e foo tests/python

    will output:

    profiling result
    Main total : 1.975 sec 1 count
    Parse_python.parse : 0.828 sec 1 count

    You can also instead set the environment variable SEMGREP_CORE_PROFILE to 1 to get the same information:

    cd semgrep-core
    ./bin/semgrep-core -e foo tests/python

    will output:

    profiling result
    Main total : 1.975 sec 1 count
    Parse_python.parse : 0.828 sec 1 count

    This is especially useful when you don't call directly semgrep-core, but instead use the python wrapper semgrep.

    Note that since semgrep 0.82, you can pass the --dump-command-for-core (or the shorter -d) to semgrep to get the command the python wrapper will use to call semgrep-core (this is an hidden option, which is why you will not see it in semgrep --help). For example:

    semgrep --dump-command-for-core --config bench/zulip/input/rules/zulip/rules.zulip.semgrep.yml.yaml bench/zulip/input/zulip/

    will output:

    Running 10 rules...
    /home/pad/github/semgrep/cli/src/semgrep/bin/semgrep-core -json -rules semgrep_rules.yaml -j 20 -targets semgrep_targets.txt -timeout 30 -timeout_threshold 0 -max_memory 0 -json_time -fast

    where semgrep_rules.yaml and semgrep_targets.txt are files created by semgrep that respectively contain the list of rules and targets. It is easy then to copy-paste this command and possibly add a -profile or -debug to get more information.

    You can also use the SEMGREP_CORE_DEBUG environment variable to add debugging information, for example:

    pipenv run semgrep -f ../semgrep-core/tests/PERF/ajin.yaml ../semgrep-core/tests/PERF/three.js

    will output:

    Debug mode On
    Executed as: semgrep-core -lang javascript -rules_file /tmp/tmpy5pzp3p_ -j 8 ../semgrep-core/tests/PERF/three.js
    Profile mode On
    disabling -j when in profiling mode
    PARSING: ../semgrep-core/tests/PERF/three.js
    saving rules file for debugging in: /tmp/semgrep_core_rule-97ae74.yaml
    profiling result
    Main total : 1.975 sec 1 count
    Parse_js.parse : 0.828 sec 1 count
    Semgrep.check : 0.791 sec 1 count
    Semgrep.match_sts_sts : 0.559 sec 185064 count

    Benchmarking code

    We have two sets of benchmarks, one on a suite of real repos against real rulesets (real benchmarks), another that highlights specific slow (rule, file) pairs (micro benchmarks).

    To run the micro benchmarks, go to perf/perf-matching/, and run ./run-perf-suite.

    To run the real benchmarks, go to perf, and run ./run-benchmarks. See the perf readme for more details on how these are set up.

    There are a number of flags (./run-benchmarks --help to see them) which may be helpful if you are using the benchmarks for local development. For example, ./run-benchmarks --plot_benchmarks will output a graph of the benchmark results at the end.

    If you are concerned about performance, the recommended way to test is to hide your change behind a flag and add that flag to run-benchmarks. Add a flag in src/configuring/ These are ref cells, so you can check whether the flag is enabled or not via !Flag_semgrep.your_flag. In src/core_cli/, go to options, and add a flag that sets the appropriate Flag_semgrep. Then, in perf/run-benchmarks, go to the SemgrepVariants list, and add your variant.

    You can also test the impact of your change by running ./run_benchmarks --std_only in perf, which will only run the default version of semgrep.

    In these next sections we will give an overview of semgrep-core and then some tips for making common changes to semgrep-core. These are only tips; without seeing an error, we cannot know its cause and proper resolution, but hopefully it gives useful direction.


    The following assume you are in the root of the repository.


    • To compile: make
    • To run the test suite: make test
    • To install the semgrep-core binary: make install
    • The semgrep-core executable produced by make: _build/default/src/main/Main.exe (alias sc)

    Running (examples in Python):

    • To match a rule file against a target: sc -rules [your-rule].yaml [your-target].py -lang python
    • To match a pattern against a target: sc -f [your-pattern].sgrep [your-target].py -lang python
    • To dump a pattern AST: sc -dump_pattern [your-pattern].sgrep -lang python
    • To dump a target AST: sc -dump_ast [your-target].py -lang python
    • To dump a pattern Python AST: pf -dump_python [your_pattern].sgrep -sgrep_mode -lang python
    • To dump a target Python AST: pf -dump_python [your_pattern].sgrep -lang python


    • To get the semgrep-core command the python wrapper will use: semgrep --dump-command-for-core --config [your-config] [your-target-directory]

    Try it out: sc -f tests/python/dots_stmts.sgrep tests/python/ -lang python

    semgrep-core overview

    Entry point

    The entry point to semgrep-core is, in src/core_cli/. This is where you add command-line arguments. It calls functions depending on the mode in which semgrep-core was invoked (-config for a yaml file, -f for a single pattern, etc.)

    When invoked by semgrep, semgrep-core is called by default with -config. This corresponds to the function semgrep_with_rules_file, which in turn calls semgrep_with_rules. These functions will parse and then match the rule and targets.


    semgrep-core uses external modules to parse code into augmented language-specific abstract syntax trees (ASTs). Though we call these ASTs, they additionally contain token information such as parentheses that are traditionally only present in concrete syntax trees (CSTs) so that we can output results in the correct range.

    When semgrep-core receives a rule or a target, it will first need to parse it. The functions that do this are located in src/parsing/.

    • If it reads a rule, it will go through, which uses to parse the code-like portions of the rule
    • If it reads a target, it will go through

    Depending on the language, and will invoke parsers to parse the code. For example, if we have Java code, it will first be parsed into a Java-specific AST.

    Converting to the generic AST

    semgrep-core does not match based on the Java AST. It has a generic AST, defined in (in libs/ast_generic/), which all language-specific ASTs are converted to.

    The functions for this conversion are in either languages/[LANG]/generic/. They are named with the appropriate language in a consistent convention.


    The matching functions are contained in src/engine/ (e.g., and src/matching/ (e.g. There are several possible matchers to invoke

    • spacegrep (for generic mode)
    • regexp (to match by regexp instead of semgrep patterns)
    • comby (an experimental mode for languages we don't yet support)
    • pattern (the main mode)

    We will only talk about the last for now. In most cases, will invoke the check function in This will visit the target AST and try to match the pattern to it at each point. If the pattern and the target node correspond, it will call the relevant function in

    The core of the matching is done by The logic for whether two expressions, statements, etc. match is contained within this file.

    Reporting results

    The results of the match will be returned to the calling function in (for example, semgrep_with_rules). From there, the results are formatted and outputted.

    There are two modes for outputting: JSON and text. JSON output is processed by functions in in semgrep-core/src/reporting/

    Fixing a parse error

    Before you start fixing a parse error, you need to know what parser was used. This bears some explanation.

    Guide to parsers

    The parsers used by semgrep fall into these categories:

    • legacy parsers (pfff): implemented directly in OCaml via a parser generator
    • tree-sitter parsers: third-party parsers implemented as tree-sitter grammars
    • generic parser (spacegrep): fallback for unsupported languages, comes with its own matching engine

    For each language, we need a parser for target files and a parser for semgrep patterns. For a given language, ideally both would use the same parser. For historical reasons, some languages use a legacy parser for patterns and a tree-sitter parser for target code. Here's the breakdown by language as of February 2021:

    • legacy parser for both pattern and target:
      • OCaml
      • PHP
      • Python
    • legacy parser for pattern, tree-sitter parser for target:
      • C
      • Go
      • Java
      • JavaScript, JSX, JSON
      • Ruby
      • TypeScript, TSX
    • tree-sitter parser for both pattern and target:
      • C#
      • Kotlin
      • Lua
      • R
      • Rust

    Fixing a pfff parse error

    Parsing With pfff

    pfff is an OCaml project that we plug into semgrep-core as a git submodule. It uses menhir to generate parsers from a defined grammar.

    Consider a Python pattern (or target). To parse it into a generic AST form, we transform the code as follows:

    Text -- (via Lexer_python.mll) --> Tokens -- (via Parser_python.mly) --> Ast_python -- (via --> AST_generic

    These files live in different places. Specifically,

    • Lexer_python.mll is in semgrep-core/src/pfff/lang_python/parsing
    • Parser_python.mly is in semgrep-core/src/pfff/lang_python/parsing
    • is in semgrep-core/src/pfff/lang_python/parsing
    • is in semgrep-core/src/parsing/pfff
    • is in semgrep-core/src/core/ast

    You will notice that the first three, Lexer_python.mll, Parser_python.mly, and are in semgrep-core/src/pfff/, which is a submodule. This means that when you modify them, you modify the submodule rather than semgrep-core. You can develop as usual---pfff is compiled when you run make in semgrep-core/---but will need to go through an extra step to make a pull request (explained later).

    When a language is particularly complicated, it can be convenient to first parse into a CST, then convert to the AST. Currently, we only do this for PHP. In this case, there is an extra step:

    Tokens -- (via Parser_php.mly) --> Cst_php -- (via --> Ast_php

    The lexers and parsers apply for both patterns and targets of a given language. To avoid parsing invalid targets, we have a function Flag_semgrep.sgrep_guard which fails when parsing constructs that only appear in patterns if a target is being parsed.

    Identifying the error

    The source of the error can be anywhere along the Text --> AST_generic path, so you will want to identify which file is causing it.

    First, create a minimum failing case. If you are debugging a rule, isolate this to an individual pattern if possible, saved in a .sgrep file.

    For simplicity, we will use Python in the examples, but you can substitute Python for any language parsed with pfff.

    If the problem is in Lexer_python.mll, you will probably get a helpful error message which should tell you what you need to change.

    If the problem is in Parser_python.mly, you will probably not get a helpful error message, because the error will be reported in the generated parser, not the grammar. To identify which production within the grammar is problematic, you will want to see what AST the parser is trying to produce. Modify the failing case minimally until it parses successfully.

    Now, you need to see what generic AST is produced by this similar code. You can actually do this in the playground, by going to Tools -> Dump AST. On the command line, you can run

    • For a pattern:
      sc -dump_pattern -f [your_pattern].sgrep -lang python
    • For a target:
      sc -dump_ast [your_target].py -lang python

    where sc is an alias for semgrep-core/_build/default/src/cli/Main.exe, the executable produced by running make in semgrep-core/. If you have installed semgrep-core, you can instead use semgrep-core here, but each time you make a change you will need to compile (make) and then install (make install).

    By default, tokens are not shown in full in the dumped AST. Their presence is indicated by ().

    You may also find it useful to see the Python AST representation of the pattern. Just as make produces an executable for semgrep-core in semgrep-core/_build/default/src/cli/Main.exe, it also produces one for pfff in semgrep-core/_build/default/src/pfff/cli/Main.exe (alias to pf for these docs).

    To dump the Python AST, run

    • For a pattern:
      pf -dump_python [your_pattern].sgrep -sgrep_mode -lang python
    • For a target:
      pf -dump_python [your_target].py -lang python

    (Note that -sgrep_mode does not always work with incomplete programs. You may need to wrap your pattern so that it is a valid program for that language, except for semgrep constructs such as ...)

    Fixing the error

    At this point, the relevant change you need to make will vary depending on your goal. It may be as simple as adding ... as a possible case. It may require you to introduce a new construct and add it to AST_generic and Ast_python. As a rule of thumb, prefer to avoid changing AST_generic if possible. This will also make your life easier!

    If you add a pattern-specific feature, remember to use Flag_semgrep.sgrep_guard so that an invalid target does not parse successfully.

    When you change the grammar, it is important that you do not introduce conflicts. Check the conflicts before you start by forcing dune to compile the grammar. (You can either use make clean and read through the output or make a change in Parser_python.mly, run make, then remove the change and run make again.) Then, after you change the grammar, see if there are any more conflicts than there were before your change.

    It can sometimes be okay to introduce a shift/reduce conflict, though avoid doing this if possible. It is never okay to introduce a reduce/reduce conflict. To understand why, read about LR(1) parsers.

    If you do introduce a conflict, you can figure out how to resolve it by running

    menhir --explain Parser_python.mly

    This will produce the file Parser_python.conflicts in the same folder as Parser_python.mly, which will show the two possible interpretations Menhir is considering for each conflict.

    Unfortunately, it will also produce and Parser_python.mli, which will confuse dune when it tries to build. Remove these files before you run make again.

    Committing the fix

    Once you have made your desired pattern or target parse, you need to make sure it doesn't break anything else. In semgrep-core/, run make test. If at the end it says Ok, you can commit your fix!

    First, if you have any changes in pfff, go into the semgrep-core/src/pfff/ directory, checkout develop, pull, and then make a pull request as usual with your changes. This will make a PR to pfff.

    When you change files in pfff, semgrep-core will realize that pfff is different (though not which file within pfff). If you go back up to semgrep-core/ and run git status, you will see modified: src/pfff (modified content). To pin your latest pfff changes to semgrep-core, add src/pfff.

    Now, make the rest of your pull request for semgrep-core as usual.

    If you haven't changed pfff, don't worry about this. Just make a pull request with your changes.

    Remember to add test cases so that future changes don't break your example! See Testing semgrep-core

    Fixing a Tree-sitter parse error

    There is more information in Adding Support for a Language on tree-sitter which will be helpful. Also, see semgrep-core/src/parsing/tree-sitter/.

    Fixing a match error

    The first thing you will need to do is understand what you expected and why you aren't getting that. If possible, reduce your rule to a single pattern that doesn't match. You may need to experiment with the clauses in your rule. For example, if you are getting too many matches, it may be because the pattern in pattern-not doesn't match what you expect.

    If you are unable to do so, you may need to investigate

    Otherwise, produce a minimal failing pattern/target pair. You will need to compare the ASTs to see which portion is not matching as you expect. Run

    sc -dump_ast [your_target].py -lang py

    and then

    sc -dump_pattern [your_pattern].sgrep -lang py

    It can be hard to figure out where in the AST you are looking. You can make it easier by using a distinctive variable name in the section you're interested in.

    Once you've isolated the parts that aren't matching, try to figure out where they're different, taking into account special features like metavariables and ellipses. It is unlikely (though not impossible) that the problem would ever be that two identical code segments aren't matching or that there is some AST element that ellipses refuse to match. You might find it helpful to write out the AST parts you want to match on a whiteboard, indicating which part is matched by a special feature. Pare down the code as much as possible and try changing the bit you're interested in.

    When you are sure you know what ought to have happened, make it happen. If two pieces of code should match but don't, change to tell it that pattern should match the target.

    Oftentimes, a matching error is actually a parsing error. You may want to change how Parser_python.mly reduces the construct or how it gets converted in Refer to Fixing a Parse Error for advice.

    At the end, confirm the match with

    sc -f [your_pattern].sgrep [your_target].py -lang py

    Fixing an autofix error

    Autofix runs through both semgrep-core and semgrep, but the most common autofix error you can encounter is a kind of incorrect range. This happens because semgrep-core determines the range of a match based on the locations of the tokens stored in the AST. When the range is incorrect, that usually means a token is missing. You can see token location information with

    sc -full_token_info -dump_ast [your_target].py -lang py

    See Fixing a Parse Error for more on parsing

    Debugging resources

    In the process of debugging, you will probably want to print things. We provide a function pr2 in (in semgrep-core/src/pfff/commons/) to print strings. You can also use the Printf module.

    If you would like to print an AST element, you can use a show function. For example, to print a node of type any in AST_generic, you can use

    pr2 (show_any your_node)

    Any type that includes [@@deriving show] in its definition can be converted to a string in this way.

    We also provide some flags that are useful. If you run with -debug, you can see the steps semgrep-core is taking. You can see more information (and change what you want to see) using -log_config_file, which takes a file. You can use one of semgrep-core/log_config.json.ex1 or semgrep-core/log_config.json.ex2 to start.

    Additionally, the OCaml debugger is a great resource.

    Adding support for a language

    There are some cases where we have chosen to implement a new parser in pfff, but in general new languages should use tree-sitter.

    Tree-Sitter parsers

    Tree-sitter parsers exist as individual public projects. They are shared with other users of tree-sitter outside of semgrep. Our ocaml-tree-sitter project adds the necessary extensions for supporting semgrep patterns (ellipsis ... and such). It also contains the machinery for turning a tree-sitter grammar into a usable, typed concrete syntax tree (CST).

    For example, for the Kotlin language we have:

    Assuming the tree-sitter grammar works well enough, most of the work consists in mapping the CST to the generic abstract syntax tree (AST) shared by all languages in semgrep.

    These guides go over the integration work in more details:

    Not finding what you need in this doc? Ask questions in our Community Slack group, or see Support for other ways to get help.