Error in Bitcoin function ConstevalHexDigit
As a developer working with the Bitcoin project, you are probably not new to its intricacies. I recently ran into an error related to a particular CMake build process. The problem occurs when building bitcoin code using the --build option with multiple threads enabled.
Problem: ConstevalHexDigit function
The problem is with the util::ConstevalHexDigit function, which seems to be trying to call a non-constant expression. In C++11 and later, the consteval function is required for certain operations that must always return constant results.
CMake build options
Let’s see what happens when you run cmake --build build -j$(sysctl -n hw.ncpu). Here’s a breakdown of the possible options:
--build: Specifies the build process to use.
-j$(sysctl -n hw.ncpu): enable simultaneous compilation with several threads. The variable$is used for dynamic substitution of system calls, which allows us to dynamically set the number of cores.
Error
When you compile your code with cmake –build build -j$(sysctl -n hw.ncpu), the compiler detects an attempt to call a nonconstant expression inutil::ConstevalHexDigit`. This is because, according to the C++ standard, a constant expression must be evaluated as an integer constant (with an explicit type), a pointer to an integer constant, or an arithmetic expression that does not contain operators.
Solution
To solve this problem, we can use the constexpr function in our code. These functions can be declared as consteval, which meets the requirements of the C++ standard for calling non-constant expressions.
Here’s how you can rewrite your code using constexpr:
#include
#include "util/consteval_hex_digit.hpp"
static constexpr uint256_t ConstevalHexDigit(const std::string& input) {
// The implementation here remains the same...
}
By declaring ConstevalHexDigit as constexpr, we ensured that it met all the requirements of a nonconstant expression, fixing the bug.