# linkers-lto

A skill for picking a linker, fixing link-order errors and enabling LTO in GCC and Clang builds.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 608 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/binaries/linkers-lto/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/linkers-lto

## What it does

The skill walks an agent through nine areas: linker selection, essential flags, link order, LTO with GCC and Clang, dead-code stripping, symbol visibility, common linker errors and map files. It carries a comparison of GNU ld, gold and lld with invocation flags such as `-fuse-ld=lld`, plus a flags reference in `references/flags.md`.

Link order gets its own section. GNU ld reads archives left to right, so a library has to come after the objects that need it. When two archives depend on each other, the skill points to `-Wl,--start-group` and `-Wl,--end-group` to repeat the search until symbols resolve.

With LTO, the catch is that `-flto` must be passed again at link time, and archives have to be built with `gcc-ar` and `gcc-ranlib` rather than plain `ar`. Build a static library with plain `ar` and the LTO link breaks. ThinLTO in Clang caches its work, and the cache directory is set with `-Wl,--thinlto-cache-dir=/tmp/thinlto-cache`.

The skill triggers on queries about `-flto`, thin LTO, LTCG, `--gc-sections`, link-order errors, weak symbols and linker scripts.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill linkers-lto -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/linkers-lto/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/linkers-lto/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill linkers-lto -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/linkers-lto/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill linkers-lto -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/linkers-lto/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills linkers-lto
```

2. **Manually: put the files in place**
- File: `.github/skills/linkers-lto/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill linkers-lto -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/binaries/linkers-lto
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/linkers-lto/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill linkers-lto -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/binaries/linkers-lto/SKILL.md

## When to use it
- choose between ld, gold and lld for a given build
- fix `undefined reference` caused by wrong library order
- turn on LTO in a GCC or Clang project without breaking archives
- shrink a binary with `--gc-sections`
- sort out duplicate and weak symbol conflicts

## Example prompts
- I'm building with GCC and want to switch to lld — which flags do I add?
- I keep getting `undefined reference to 'foo'` at link time, help me fix the library order
- Enable ThinLTO for my Clang build and tell me what has to change for the archives

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 3213dde2c6a40b1d.

BEGIN THIRD-PARTY FILE 3213dde2c6a40b1d
````text
---
name: linkers-lto
description: Linker and Link-Time Optimisation (LTO) skill. Use when configuring GNU ld, gold, or lld linker flags, diagnosing link-order issues or undefined symbols at link time, enabling LTO safely in real projects, or understanding inter-module optimisation trade-offs. Activates on queries about linker flags, -flto, thin LTO, LTCG, --gc-sections, link order errors, weak symbols, or linker scripts.
---

# Linkers and LTO

## Purpose

Guide agents through linker selection, common linker flags, link-order issues, LTO setup, and symbol-visibility management.

## Triggers

- "I'm getting `undefined reference` at link time"
- "How do I enable LTO for a real project?"
- "Which linker should I use: ld, gold, or lld?"
- "How do I reduce binary size with `--gc-sections`?"
- "How do I write or understand a linker script?"
- "I have duplicate symbol or weak symbol issues"

## Workflow

### 1. Linker selection

| Linker | Invocation | Strengths |
|--------|-----------|-----------|
| GNU ld (BFD) | default on Linux | Universal, stable |
| gold | `-fuse-ld=gold` | Faster than ld for C++; supports LTO plugins |
| lld (LLVM) | `-fuse-ld=lld` | Fastest, parallel, required for Clang LTO |

```bash
# Use lld with GCC or Clang
gcc -fuse-ld=lld -o prog ...
clang -fuse-ld=lld -o prog ...

# Check which linker is used
gcc -v -o prog main.c 2>&1 | grep 'Invoking'
```

### 2. Essential linker flags

```bash
# Pass linker flags via compiler driver:
# -Wl,flag1,flag2   (comma-separated, no spaces)
# -Wl,flag1 -Wl,flag2  (separate -Wl options)

gcc main.c -o prog \
  -Wl,-rpath,/opt/mylibs/lib \     # runtime library search path
  -Wl,--as-needed \                 # only link libraries that are actually used
  -Wl,--gc-sections \               # remove unused sections (requires -ffunction-sections -fdata-sections)
  -Wl,-z,relro \                    # mark relocations read-only after startup
  -Wl,-z,now \                      # resolve all symbols at startup (full RELRO)
  -L/opt/mylibs/lib -lfoo
```

### 3. Link order matters (GNU ld)

GNU ld processes archives left-to-right. A library must come after the objects that need it.

```bash
# Wrong: libfoo provides symbols needed by main.o
gcc main.o -lfoo libdep.a -o prog   # can fail if libdep.a needs libfoo

# Correct: dependencies after dependents
gcc main.o -lfoo -ldep -o prog

# If there are circular deps between archives:
gcc main.o -Wl,--start-group -lfoo -lbar -Wl,--end-group -o prog
# --start-group/--end-group: repeat search until no new symbols resolved
```

### 4. LTO with GCC
````
END THIRD-PARTY FILE 3213dde2c6a40b1d

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