Understand what happens between source and running program, so build errors, linking failures, and optimisation surprises become tractable. Use when builds fail obscurely or behaviour differs between debug and release.
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---
name: compilers-and-toolchains
description: Understand what happens between source and running program, so build errors, linking failures, and optimisation surprises become tractable. Use when builds fail obscurely or behaviour differs between debug and release.
---
# Compilers and toolchains
Source becomes a running program through several stages, each with its
own failure modes. Most confusing build problems are a stage boundary:
a preprocessor definition, a missing symbol at link time, or an
optimisation that changed behaviour the code relied on.
## Method
1. **Locate the failing stage first.** Preprocessing, compilation,
linking, and loading fail differently, and the error message names
the stage if you know how to read it.
2. **Understand linking as symbol resolution.** Undefined symbol errors
mean a declaration without a definition; duplicate symbols mean two
definitions. Both are about what the linker can see.
3. **Know what optimisation is permitted to do.** Reordering,
inlining, and eliminating code that has no observable effect, which
is why undefined behaviour produces different results between builds.
4. **Keep debug information even in release builds.** Symbols make
production stack traces readable and cost only disk (see
stack-trace-reading).
5. **Understand static versus dynamic linking.** Static builds are
self-contained and larger; dynamic builds share libraries and can
fail at load time on a different machine.
6. **Make builds reproducible.** Pinned toolchain versions and no
embedded timestamps mean the same source produces the same binary,
which matters for both debugging and supply chain (see
supply-chain-security).
7. **Read the intermediate output when stuck.** Preprocessed source,
assembly, and the symbol table answer questions that guessing about
the compiler cannot.
## Boundaries
Details differ enormously between languages and toolchains. Interpreted
and just-in-time compiled languages have different stages with
analogous problems. Aggressive optimisation exposes undefined behaviour
rather than causing it, so the fix is the code (see c-memory-safety).
