nim-c-bindings

name	nim-c-bindings
description	Bind C libraries to Nim and make them build reliably across Linux, macOS, and Windows, including headers, types, linking, shared-library loading, and CI/release workflows. Use when wrapping a C dependency, fixing Nim FFI build issues, or setting up cross-platform GitHub Actions for a Nim native library project.

Nim C Bindings & CI

Rules

Binding Fundamentals

• Use importc with cdecl for C APIs unless the library explicitly requires a different calling convention (e.g., stdcall).
• Represent opaque C handles as type Name = ptr object types. Use incompleteStruct for partial/opaque structs to avoid size/layout mismatches.
• Use {.bycopy.} on structs that Nim must pass by value to C.
• Declare the C header in the binding when the compiler needs the C definitions for compilation, for example header: "foo.h".
• Use {.push callconv: cdecl, importc.} blocks when many declarations share the same convention.
• Match C storage with Nim's C-compatible types such as cint, cuint, clong, culong, clonglong, csize_t, cfloat, cdouble, cstring, and pointer types.
• Preserve the complete field order for concrete structs passed or returned by value.
• When the requested deliverable is a raw binding, import C symbols directly. Put ergonomic aliases and conversions in a separate wrapper layer.

Linking

• System libraries: link with -l<name> only. Do not hardcode -L paths — the OS toolchain already knows where system libs live.
• Local/third-party libraries: add both -L<dir> and -l<name> (or .lib/.dll.lib paths on Windows MSVC).
• Use repository-relative paths for vendored dependencies, for example third_party/libfoo, to keep builds hermetic.
• Distinguish linker-resolved imports from Nim runtime dynlib imports. header supplies declarations for generated C; a runtime-only dynlib proc should not also use header.

Runtime Library Resolution

• Vendored/local shared libs: colocate the .so/.dylib/.dll next to the executable. Do not rely on environment variables for discovery.
• System-installed libs: do not copy DLLs/shared libs next to the executable. Rely on the platform's normal system loader configuration. Use environment variables only as temporary overrides.
• On Linux, add rpath $ORIGIN only when loading colocated shared libs. From the shell, pass --passL:"-Wl,-rpath,\$ORIGIN". In Nim source, use {.passL: "-Wl,-rpath,\\$ORIGIN".}.
• Do not use build-tree-only rpaths or absolute paths to non-system shared libraries.

Platform Rules

Platform	Toolchain	Deps	Link flags	Runtime
Linux	System GCC/Clang	apt	-l<name> or -L<dir> -l<name>	Colocate local .so; rpath $ORIGIN if needed
macOS	Apple Clang	Homebrew	staticExec("brew --prefix") for -I/-L	Colocate local .dylib
Windows	MSVC (--cc:vcc)	vcpkg only	.lib/.dll.lib full paths	Colocate .dll next to .exe

• On macOS, resolve include/link paths via staticExec("brew --prefix <formula>").
• On Windows, use MSVC via --cc:vcc. Use vcpkg only — avoid Chocolatey and MSYS2. Never mix toolchains.
• On Windows, do not rely on implicit -l<name> for MSVC. Use explicit .lib or .dll.lib paths.
• On Windows CI, export VCPKG_ROOT to the installed triplet root and prepend its bin to PATH.

CI & Release

• CI is the authoritative spec for supported platforms, toolchains, and flags. Local workflows must be compatible with CI.
• Keep test builds simple: compile, then run, with minimal environment mutation.
• Align the local toolchain with CI (e.g., MSVC + vcpkg x64-windows-release with --cc:vcc).
• Use the reference workflows as starting points — references/ci.yml for validation, references/release.yml for tagged releases.

Verification

• Compile against the real header and call the real C implementation.
• Exercise concrete structs passed and returned by value. Compare Nim layout with C sizeof and offsetof probes when layout is in doubt.
• Test bundled shared libraries after copying the executable and library into a clean directory.
• For dynlib, verify runtime loading without also linking that library.

Workflow

1. Identify the C API. Determine calling convention, opaque vs value types, and ownership.
2. Write bindings. Use importc, correct calling convention, incompleteStruct for opaque types, bycopy for value types. Add header pragma.
3. Set up linking. System libs: -l only. Local libs: -L + -l with repo-relative paths. Windows: explicit .lib paths.
4. Handle runtime. Colocate local shared libs. For system-installed libs, rely on the normal loader search path. Add rpath $ORIGIN on Linux only for colocated local libs.
5. Verify the ABI. Run a real C round trip for the types and calls used by the binding.
6. Add CI. Copy references/ci.yml, adapt placeholders (<package>, <src/main.nim>, dependency lists).
7. Add release. Copy references/release.yml, adapt placeholders, configure draft releases.
8. Test locally, then push. Verify the build works locally with the same flags CI uses.

Common Mistakes

Mistake	Why it's wrong
Hardcoding -L paths for system libraries	OS toolchain already knows where they are; breaks portability
Mixing MSVC + MSYS2/MinGW on Windows	Incompatible ABIs, linker errors, runtime crashes
Using implicit -l<name> with MSVC	MSVC doesn't resolve libs the same way GCC does; use explicit .lib paths
Relying on env vars for vendored shared libs	Fragile across machines; colocate instead
Build-tree-only rpaths	Breaks when the binary moves; use $ORIGIN for colocated libs
Guessing Windows dependency paths	Non-deterministic; use vcpkg with known install roots
Compiling C sources into the executable when the task requires an existing shared library	Bypasses shared-library link and relocation behavior
Adding Nim-bodied wrappers to a requested raw binding surface	Hides whether the C API was imported directly and changes the requested layer

References

• references/raw_binding.md — Mapping an opaque handle and by-value C struct.
• references/ci.yml — Building a C-dependent project on Linux, macOS, and Windows with Nim, Atlas, and vcpkg.
• references/release.yml — Publishing tagged cross-platform archives in a draft release.