The problem with binding age

Oct 13, 2025 by wbrum.org 144 views

Introduction

In Julia, the ccall function is used to call C functions from Julia code. However, when using ccall with certain types of arguments, it can sometimes fail to statically evaluate the arguments due to binding ages. This can lead to unexpected errors and behavior. In this article, we will explore this issue and provide examples of how to avoid it.

What is binding age?

Binding age refers to the age of a binding, which is the time at which a binding is created. In Julia, bindings are created when a variable is assigned a value. The binding age of a variable is the time at which it was last assigned a value.

The problem with binding age

When using ccall with certain types of arguments, Julia needs to statically evaluate the arguments before calling the C function. However, if the binding age of an argument is too high, Julia may not be able to statically evaluate it, leading to an error.

Example

Let's consider the following example:

using Zstd_jll
const prefix = "Zstd version: "
const sym = :ZSTD_versionString
get_zstd_version() = prefix * unsafe_string(ccall((sym, libzstd), Cstring, ()))

In this example, we define a constant prefix and a constant sym. We then define a function get_zstd_version that uses ccall to call the ZSTD_versionString function from the libzstd library. However, when we run this code, we get an error:

julia> get_zstd_version()
ERROR: TypeError: in ccall: first argument not a pointer or valid constant expression, expected Ptr, got a value of type Tuple{Symbol, String}

The error message indicates that the first argument to ccall is not a pointer or a valid constant expression.

Why does this happen?

The reason for this error is that the binding age of the prefix constant is too high. When we define prefix as a constant, Julia creates a binding for it. However, when we define sym as a constant after prefix, Julia creates another binding for sym. The binding age of prefix is now higher than the binding age of sym.

When we use ccall with sym as the first argument, Julia needs to statically evaluate the first argument. However, since the binding age of prefix is too high, Julia cannot statically evaluate it, leading to an error.

Solution

To avoid this error, we can simply swap the order of the constant definitions:

using Zstd_jll
const sym = :ZSTD_versionString
const prefix = "Zstd version: "
get_zstd_version() = prefix * unsafe_string(ccall((sym, libzstd), Cstring, ()))

By swapping the order of the constant definitions, we ensure that the binding age of sym is lower than the binding age of prefix. This allows Julia to statically evaluate the first argument to ccall, and the code runs without errors.

Conclusion

In conclusion, the ccall function can sometimes fail to statically evaluate arguments due to binding ages. This can lead to unexpected errors and behavior. By understanding the concept of binding age and how it affects ccall, we can avoid this error and write more robust code.

Best Practices

To avoid this error, follow these best practices:

Define constants in the order of their usage.
Avoid defining constants with high binding ages before defining constants with lower binding ages.
Use const to define constants that are used in ccall functions.

Common Pitfalls

Defining constants with high binding ages before defining constants with lower binding ages.
Using ccall with arguments that have high binding ages.
Not understanding the concept of binding age and how it affects ccall.

Example Use Cases

Using ccall to call C functions from Julia code.
Defining constants with low binding ages to avoid errors with ccall.
Understanding the concept of binding age and how it affects ccall.

Code Snippets

Example code that demonstrates the error
Example code that demonstrates the solution

API Documentation

ccall
Binding age
Constant expressions
ccall sometimes fails to statically evaluate arguments due to binding ages ===========================================================

Q&A

Q: What is binding age?

A: Binding age refers to the age of a binding, which is the time at which a binding is created. In Julia, bindings are created when a variable is assigned a value. The binding age of a variable is the time at which it was last assigned a value.

Q: Why does binding age affect ccall?

A: When using ccall with certain types of arguments, Julia needs to statically evaluate the arguments before calling the C function. However, if the binding age of an argument is too high, Julia may not be able to statically evaluate it, leading to an error.

Q: What is the solution to this problem?

A: To avoid this error, you can simply swap the order of the constant definitions. This ensures that the binding age of the argument is lower than the binding age of the constant.

Q: What are some best practices to avoid this error?

A: Here are some best practices to avoid this error:

Define constants in the order of their usage.
Avoid defining constants with high binding ages before defining constants with lower binding ages.
Use const to define constants that are used in ccall functions.

Q: What are some common pitfalls to avoid?

A: Here are some common pitfalls to avoid:

Defining constants with high binding ages before defining constants with lower binding ages.
Using ccall with arguments that have high binding ages.
Not understanding the concept of binding age and how it affects ccall.

Q: What are some related topics to this issue?

A: Here are some related topics to this issue:

Q: What are some example use cases for this issue?

A: Here are some example use cases for this issue:

Using ccall to call C functions from Julia code.
Defining constants with low binding ages to avoid errors with ccall.
Understanding the concept of binding age and how it affects ccall.

Q: What are some code snippets that demonstrate this issue?

A: Here are some code snippets that demonstrate this issue:

Example code that demonstrates the error
Example code that demonstrates the solution

Q: What are some API documentation resources for this issue?

A: Here are some API documentation resources for this issue:

Conclusion

In conclusion, the ccall function can sometimes fail to statically evaluate arguments due to binding ages. By understanding the concept of binding age and how it affects ccall, you can avoid this error and write more robust code.