How to divide by 1000000 without resulting in undefined reference to '__udivdi3'
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Introduction
Dividing numbers in programming by large values like 1,000,000 can sometimes involve specific challenges or obscure linker errors, especially in embedded systems or when dealing with unconventional integer sizes (e.g., 64-bit integers on 32-bit systems). One such issue, encountered primarily in embedded scenarios using certain compilers or language distributions, is the "`undefined reference to '__udivdi3'`" error. In this article, we will explore methods to perform this large division without encountering such obstacles, using technical explanations and examples to guide your understanding.
The Problem: Undefined Reference to `__udivdi3`
The error "`undefined reference to '__udivdi3'`" usually arises when a program attempts to perform division on `uint64_t` (or 64-bit) integers in environments where native support for such operations is not available. This is typical in scenarios where a 32-bit system is tasked with performing 64-bit arithmetic using libraries that do not provide the requisite division function. The `__udivdi3` function is typically used by the GNU Compiler Collection (GCC) in such situations to perform 64-bit unsigned divisions.
Technical Explanation
When the compiler encounters a need to divide a 64-bit number and there is no native CPU instruction to handle it, the C runtime library steps in to provide support functions. The function `__udivdi3` is one of these, part of GCC's runtime support library for handling unsigned division operations on double-word integers (`di` stands for "double integer").
Why the Error Occurs
- Lack of CPU Support: The target hardware may not support 64-bit division natively, necessitating a software-driven division function.
- Library Issues: The linking process cannot find the `__udivdi3` definition, often because the standard GCC runtime library (`libgcc`) isn't correctly linked with your application.
- Compiler/Linker Flags: Incorrect compilation flags can prevent the necessary runtime from being linked, or the flags do not specify linking against `libgcc`.
Solutions
Method 1: Ensure Proper Linking
Ensure that the compilation and linking processes include the essential runtime libraries, particularly `libgcc`. This can typically be achieved by using the following compiler and linking flags:
