Trimming: Dynamic Dispatch In `ccall` Not Caught By Verifier

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Introduction

In the world of programming, dynamic dispatch is a powerful technique used to resolve function calls at runtime. However, when it comes to the Julia programming language, dynamic dispatch can sometimes lead to unexpected behavior. In this article, we will explore a specific issue related to dynamic dispatch in Julia, specifically when using the ccall function. We will examine a code snippet that demonstrates this issue and provide a detailed explanation of what is happening.

The Code Snippet

Let's take a look at the code snippet that demonstrates this issue:

module MyApp

using Zstd_jll: get_libzstd_path

Base.@ccallable function main()::Cint
    println(Core.stdout, unsafe_string(ccall((:ZSTD_versionString, get_libzstd_path()), Cstring, ())))
    return 0
end

end

This code defines a module MyApp that uses the Zstd_jll package to get the path to the libzstd library. It then defines a function main that uses the ccall function to call the ZSTD_versionString function from the libzstd library. The ccall function is used to call a C function from Julia, and the Base.@ccallable macro is used to specify the calling convention.

The Issue

When we run this code, we get a segmentation fault error:

$ ./jll_crash
[75746] signal 11 (1): Segmentation fault
in expression starting at none:0
jl_lookup_generic_ at /home/topolarity/repos/julia2/src/gf.c:3623 [inlined]
ijl_apply_generic at /home/topolarity/repos/julia2/src/gf.c:3696
main at /home/topolarity/repos/julia2/jll_crash.jl:6
main at ./jll_crash (unknown line)
unknown function (ip: 0x7f48d5de3d8f) at /lib/x86_64-linux-gnu/libc.so.6
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
_start at ./jll_crash (unknown line)
Allocations: 1 (Pool: 1; Big: 0); GC: 0

The issue here is that the dynamic dispatch in the ccall function is not caught by the verifier. The verifier is a mechanism in Julia that checks the type of a function call at compile-time to ensure that it is safe. However, in this case, the dynamic dispatch in the ccall function is not caught by the verifier, which leads to a segmentation fault error.

What's Happening

So, what's happening here? When we use the ccall function to call a C function from Julia, Julia needs to resolve the function call at runtime. However, the ccall function is not a Julia function, but rather a C function that is called from Julia. This means that the dynamic dispatch in the ccall function is not caught by the verifier, which is a mechanism that checks the type of a function call at compile-time.

In this case, the ccall function is calling the ZSTD_versionString function from the libzstd library. However, the ZSTD_versionString function is not a Julia function, but rather a C function that is called from Julia. This means that the dynamic dispatch in the ccall function is not caught by the verifier, which leads to a segmentation fault error.

Conclusion

In conclusion, the use of get_libzstd_path() instead of libzstd causes a dynamic dispatch that is not caught by the verifier. This leads to a segmentation fault error when we run the code. This issue is related to the way that Julia handles dynamic dispatch in the ccall function, and it highlights the importance of understanding the type system in Julia.

Best Practices

To avoid this issue, it's best to use the libzstd library directly instead of using the get_libzstd_path() function. This will ensure that the dynamic dispatch in the ccall function is caught by the verifier, and it will prevent the segmentation fault error.

Additionally, it's always a good idea to use the @ccall macro to specify the calling convention when calling a C function from Julia. This will ensure that the function call is resolved correctly at runtime.

Future Work

In the future, it would be great to see improvements in the way that Julia handles dynamic dispatch in the ccall function. This could involve adding more checks to the verifier to ensure that dynamic dispatch is caught correctly, or it could involve adding more features to the @ccall macro to make it easier to specify the calling convention.

References

Q: What is dynamic dispatch in Julia?

A: Dynamic dispatch in Julia is a mechanism that allows the type of a function call to be resolved at runtime. This means that the type of the function being called is not known until the function is actually called.

Q: What is the ccall function in Julia?

A: The ccall function in Julia is a function that allows you to call a C function from Julia. It is used to call functions that are not written in Julia, but are instead written in C.

Q: What is the @ccall macro in Julia?

A: The @ccall macro in Julia is a macro that allows you to specify the calling convention when calling a C function from Julia. It is used to ensure that the function call is resolved correctly at runtime.

Q: What is the verifier in Julia?

A: The verifier in Julia is a mechanism that checks the type of a function call at compile-time to ensure that it is safe. It is used to prevent type errors at runtime.

Q: Why is dynamic dispatch in ccall not caught by the verifier?

A: Dynamic dispatch in ccall is not caught by the verifier because the ccall function is not a Julia function, but rather a C function that is called from Julia. This means that the type of the function being called is not known until the function is actually called, which is at runtime.

Q: What is the issue with using get_libzstd_path() instead of libzstd?

A: The issue with using get_libzstd_path() instead of libzstd is that it causes a dynamic dispatch that is not caught by the verifier. This leads to a segmentation fault error when the code is run.

Q: How can I avoid this issue?

A: To avoid this issue, you can use the libzstd library directly instead of using the get_libzstd_path() function. This will ensure that the dynamic dispatch in the ccall function is caught by the verifier, and it will prevent the segmentation fault error.

Q: What are some best practices for using ccall in Julia?

A: Some best practices for using ccall in Julia include:

  • Using the @ccall macro to specify the calling convention when calling a C function from Julia.
  • Ensuring that the type of the function being called is known at compile-time.
  • Avoiding dynamic dispatch in ccall by using the libzstd library directly instead of using the get_libzstd_path() function.

Q: What are some future improvements that could be made to Julia's ccall function?

A: Some future improvements that could be made to Julia's ccall function include:

  • Adding more checks to the verifier to ensure that dynamic dispatch is caught correctly.
  • Adding more features to the @ccall macro to make it easier to specify the calling convention.
  • Improving the performance of the ccall function by reducing the overhead of dynamic dispatch.

Q: Where can I find more information about Julia's ccall function?

A: You can find more information about Julia's ccall function in the Julia documentation, which can be found at https://docs.julialang.org/en/v1/.