# Gcc

> GCC compiler skill for C/C++ projects. Use when selecting optimization levels, warning flags, debug builds, LTO, sanitizer instrumentation, or diagnosing compilation errors with GCC. Covers flag selection for debug vs release, ABI concerns, preprocessor macros, profile-guided optimization, and integration with build systems. Activates on queries about gcc flags, compilation errors, performance tu…

- **Type:** Skill
- **Install:** `agentstack add skill-mohitmishra786-low-level-dev-skills-gcc`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [mohitmishra786](https://agentstack.voostack.com/s/mohitmishra786)
- **Installs:** 0
- **Category:** [Developer Tools](https://agentstack.voostack.com/c/developer-tools)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [mohitmishra786](https://github.com/mohitmishra786)
- **Source:** https://github.com/mohitmishra786/low-level-dev-skills/tree/main/skills/compilers/gcc
- **Website:** https://www.lowleveldevskills.com

## Install

```sh
agentstack add skill-mohitmishra786-low-level-dev-skills-gcc
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# GCC

## Purpose

Guide agents through GCC invocation: flag selection, build modes, warning triage, PGO, LTO, and common error patterns. Assume the project uses GNU Make, CMake, or a shell script.

## Triggers

- "What flags should I use for a release build?"
- "GCC is giving me a warning/error I don't understand"
- "How do I enable LTO / PGO with GCC?"
- "How do I compile with `-fsanitize`?"
- "My binary is too large / too slow"
- Undefined reference errors, ABI mismatch, missing symbols

## Workflow

### 1. Choose a build mode

| Goal | Recommended flags |
|------|-------------------|
| Debug | `-g -O0 -Wall -Wextra` |
| Debug + debuggable optimisation | `-g -Og -Wall -Wextra` |
| Release | `-O2 -DNDEBUG -Wall` |
| Release (max perf, native only) | `-O3 -march=native -DNDEBUG` |
| Release (min size) | `-Os -DNDEBUG` |
| Sanitizer (dev) | `-g -O1 -fsanitize=address,undefined` |

Always pass `-std=c11` / `-std=c++17` (or the required standard) explicitly. Never rely on the implicit default.

### 2. Warning discipline

Start with `-Wall -Wextra`. For stricter standards compliance add `-Wpedantic`. To treat all warnings as errors in CI: `-Werror`.

Suppress a specific warning only in a narrow scope:

```c
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
// ...
#pragma GCC diagnostic pop
```

Do not pass `-w` (silences everything) except as a last resort for third-party headers.

### 3. Debug information

- `-g` — DWARF debug info, default level 2
- `-g3` — includes macro definitions (useful with GDB `macro expand`)
- `-ggdb` — DWARF extensions optimal for GDB
- `-gsplit-dwarf` — splits `.dwo` files; reduces link time, needed for `debuginfod`

Pair `-g` with `-Og` (not `-O0`) when you need readable optimised code in GDB.

### 4. Optimisation decision tree

```text
Need max throughput on a fixed machine?
  yes -> -O3 -march=native -flto
  no  -> profiling available?
           yes -> -O2 -fprofile-use
           no  -> -O2
Size-constrained (embedded, shared lib)?
  yes -> -Os  (or -Oz with clang)
```

`-O3` vs `-O2`: `-O3` adds aggressive loop transformations (`-funswitch-loops`, `-fpeel-loops`, `-floop-interchange`) and more aggressive inlining. Use `-O3` only after benchmarking; it occasionally regresses due to i-cache pressure.

`-Ofast`: enables `-ffast-math` which breaks IEEE 754 semantics (NaN handling, associativity). Avoid unless the numerical domain explicitly permits it.

### 5. Link-time optimisation (LTO)

```bash
# Compile
gcc -O2 -flto -c foo.c -o foo.o
gcc -O2 -flto -c bar.c -o bar.o
# Link (must pass -flto again)
gcc -O2 -flto foo.o bar.o -o prog
```

Use `gcc-ar` / `gcc-ranlib` instead of `ar` / `ranlib` when archiving LTO objects into static libs.

For parallel LTO: `-flto=auto` (uses `make`-style jobserver) or `-flto=N`.

See [references/flags.md](references/flags.md) for full flag reference. See `skills/binaries/linkers-lto` for linker-level LTO configuration.

### 6. Profile-guided optimisation (PGO)

```bash
# Step 1: instrument
gcc -O2 -fprofile-generate prog.c -o prog_inst
# Step 2: run with representative workload
./prog_inst &1 | grep -A20 '#include '

# Check if a flag is supported
gcc -Q --help=target | grep march
```

For a complete flag cheatsheet, see [references/flags.md](references/flags.md).
For common error patterns and examples, see [references/examples.md](references/examples.md).

## Related skills

- Use `skills/runtimes/sanitizers` to add `-fsanitize=*` builds
- Use `skills/compilers/clang` when switching to clang/LLVM
- Use `skills/binaries/linkers-lto` for advanced LTO linker flags
- Use `skills/debuggers/gdb` for debugging GCC-built binaries

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [mohitmishra786](https://github.com/mohitmishra786)
- **Source:** [mohitmishra786/low-level-dev-skills](https://github.com/mohitmishra786/low-level-dev-skills)
- **License:** MIT
- **Homepage:** https://www.lowleveldevskills.com

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-mohitmishra786-low-level-dev-skills-gcc
- Seller: https://agentstack.voostack.com/s/mohitmishra786
- Browse the marketplace: https://agentstack.voostack.com/browse

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Listed on AgentStack — the marketplace for AI agent skills and MCP servers. Every listing is security-reviewed. Creators keep 70%.