How to use
- Copy the installation command and review the source package before installing it in your own agent environment.
- Check the source document for required tools, credentials and supported hosts.
- Describe your input and desired deliverable; review the output against the source material.
npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill linkers-ltoBefore you start
- Installation does not include a model subscription, API allowance or permission to access your files.
- Repository stars and installation counts describe activity, not an independent quality or security test.
Original skill document
Rendered from the source repository. Headings, examples, code, tables, links, and referenced images are preserved.
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 referenceat 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 -lfoo3. 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 resolved4. LTO with GCC
bash
# Compile
gcc -O2 -flto -ffunction-sections -fdata-sections -c foo.c -o foo.o
gcc -O2 -flto -ffunction-sections -fdata-sections -c bar.c -o bar.o
# Link (must pass -flto again)
gcc -O2 -flto -Wl,--gc-sections foo.o bar.o -o prog
# Archives: must use gcc-ar / gcc-ranlib, not plain ar
gcc-ar rcs libfoo.a foo.o
gcc-ranlib libfoo.aParallel LTO:
bash
gcc -O2 -flto=auto foo.o bar.o -o prog # uses jobserver
gcc -O2 -flto=4 foo.o bar.o -o prog # 4 parallel jobs5. LTO with Clang / lld
bash
# Full LTO
clang -O2 -flto -fuse-ld=lld foo.c bar.c -o prog
# ThinLTO (faster, nearly same quality)
clang -O2 -flto=thin -fuse-ld=lld foo.c bar.c -o prog
# LTO with cmake: set globally
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON) # enables -fltoThinLTO caches work: subsequent builds that reuse unchanged modules are faster. Cache location: specify with -Wl,--thinlto-cache-dir=/tmp/thinlto-cache.
6. Dead-code stripping
bash
# Compile with per-function/per-data sections
gcc -O2 -ffunction-sections -fdata-sections -c foo.c -o foo.o
# Link with garbage collection
gcc -Wl,--gc-sections foo.o -o prog
# Verify what was removed
gcc -Wl,--gc-sections -Wl,--print-gc-sections foo.o -o prog 2>&1 | head -20On macOS, the linker strips dead code by default (-dead_strip).
7. Symbol visibility
Controlling visibility reduces DSO size and enables better LTO:
bash
# Hide all symbols by default, export explicitly
gcc -fvisibility=hidden -O2 -shared -fPIC foo.c -o libfoo.so
# In source, mark exports:
__attribute__((visibility("default"))) int my_public_function(void);Or use a version script:
text
# foo.ver
{
global: my_public_function; my_other_public;
local: *;
};bash
gcc -Wl,--version-script=foo.ver -shared -fPIC -o libfoo.so foo.o8. Common linker errors
| Error | Cause | Fix |
|---|---|---|
undefined reference to 'foo' | Missing library or wrong order | Add -lfoo; move after the object that needs it |
multiple definition of 'foo' | Symbol defined in two TUs | Remove duplicate; use static or extern |
cannot find -lfoo | Library not in search path | Add -L/path/to/lib; install dev package |
relocation truncated | Address overflow in relocation | Use -mcmodel=large; restructure image |
version 'GLIBC_2.33' not found | Binary needs newer glibc | Link statically or rebuild on older host |
circular reference | Archives mutually depend | Use --start-group/--end-group |
9. Linker map file
bash
# Generate a map file (shows symbol → section → file)
gcc -Wl,-Map=prog.map -o prog foo.o bar.o
less prog.mapUseful for debugging binary size and symbol placement.
For a comprehensive linker and LTO flags reference, see references/flags.md.
Related skills
- Use
skills/binaries/elf-inspectionfor examining the resulting binary - Use
skills/compilers/gccorskills/compilers/clangfor compile-phase LTO flags - Use
skills/binaries/binutilsforar,strip,objcopy
