Claude-skill-registry C Ecosystem

<purpose> Provide comprehensive patterns for Modern C language (C11/C17/C23), memory management, toolchain configuration, and CLI tool development. </purpose>

<c_language> <standards_evolution> <standard name="C11"> <description>ISO/IEC 9899:2011 - Major modernization with threading and type-generic features</description> <features> <feature name="_Generic">Type-generic selection for polymorphic macros</feature> <feature name="_Atomic">Atomic types and operations for lock-free programming</feature> <feature name="_Thread_local">Thread-local storage duration</feature> <feature name="_Static_assert">Compile-time assertions</feature> <feature name="_Alignas">Alignment specifier for variables and types</feature> <feature name="_Alignof">Query alignment requirements</feature> <feature name="_Noreturn">Function that does not return</feature> <feature name="anonymous_structs_unions">Unnamed struct/union members</feature> </features> </standard>

<standard name="C17">
  <description>ISO/IEC 9899:2018 - Bug fix release with no new features</description>
  <changes>
    <change>__STDC_VERSION__ updated to 201710L</change>
    <change>Defect reports resolved</change>
    <change>ATOMIC_VAR_INIT deprecated</change>
  </changes>
</standard>

<standard name="C23">
  <description>ISO/IEC 9899:2024 - Significant modernization with C++ alignment</description>
  <features>
    <feature name="nullptr">Null pointer constant with nullptr_t type</feature>
    <feature name="typeof">Type inference operator</feature>
    <feature name="constexpr">Compile-time constant objects</feature>
    <feature name="auto">Type inference for variables</feature>
    <feature name="binary_literals">0b prefix for binary literals</feature>
    <feature name="digit_separators">Single quote separator in numeric literals</feature>
    <feature name="attributes">[[nodiscard]], [[maybe_unused]], [[deprecated]], [[fallthrough]], [[noreturn]]</feature>
    <feature name="bool_true_false">bool, true, false as keywords</feature>
    <feature name="embed">embed directive for binary resource inclusion</feature>
    <feature name="static_assert">Single argument form</feature>
  </features>
</standard>

</standards_evolution>

<type_system> <concept name="integer_types"> <description>Fixed-width integer types from stdint.h</description> <types> <type name="int8_t, int16_t, int32_t, int64_t">Exact-width signed integers</type> <type name="uint8_t, uint16_t, uint32_t, uint64_t">Exact-width unsigned integers</type> <type name="intptr_t, uintptr_t">Pointer-sized integers</type> <type name="size_t">Unsigned size type for memory operations</type> <type name="ptrdiff_t">Signed pointer difference type</type> </types> <example> #include <stdint.h> #include <stddef.h>

uint32_t compute_hash(const uint8_t *data, size_t len); </example> </concept>

<concept name="type_generic_selection">
  <description>_Generic for type-based dispatch (C11)</description>
  <example>

#define print_value(x) _Generic((x),
int: print_int,
double: print_double,
char *: print_string,
default: print_unknown
)(x) </example> <use_case>Implement type-safe generic interfaces without void pointers</use_case> </concept>

<concept name="compound_literals">
  <description>Anonymous compound literals for in-place struct/array creation</description>
  <example>

struct point { int x, y; }; void draw(struct point p);

draw((struct point){.x = 10, .y = 20});

int *arr = (int[]){1, 2, 3, 4, 5}; </example> </concept>

<concept name="designated_initializers">
  <description>Named field initialization for structs and arrays</description>
  <example>

struct config { int timeout; bool verbose; const char *name; };

struct config cfg = { .name = "myapp", .timeout = 30, .verbose = true, }; </example> </concept> </type_system>

<concurrency> <decision_tree name="when_to_use"> <question>Do you need thread-safe operations?</question> <if_yes>Use _Atomic for simple counters, pthreads for complex synchronization</if_yes> <if_no>Single-threaded sequential execution</if_no> </decision_tree>

<concept name="atomics">
  <description>Lock-free atomic operations (C11)</description>
  <example>

#include <stdatomic.h>

_Atomic int counter = 0;

void increment(void) { atomic_fetch_add(&counter, 1); }

int get_count(void) { return atomic_load(&counter); } </example> </concept>

<concept name="thread_local">
  <description>Thread-local storage (C11)</description>
  <example>

#include <threads.h>

_Thread_local int errno_local; </example> </concept> </concurrency>

<undefined_behavior> <description>Common undefined behavior to avoid in C</description> <categories> <category name="memory">Null pointer dereference, use-after-free, buffer overflow, double-free</category> <category name="arithmetic">Signed integer overflow, division by zero, shift beyond type width</category> <category name="aliasing">Strict aliasing violations, type punning without union</category> <category name="sequence">Modifying variable twice between sequence points</category> </categories> <mitigation>Use sanitizers (ASan, UBSan) during development to detect UB</mitigation> </undefined_behavior>

<anti_patterns> <avoid name="void_pointer_abuse"> <description>Overusing void* for generic programming</description> <instead>Use _Generic macros or code generation</instead> </avoid>

<avoid name="magic_numbers">
  <description>Hardcoded numeric values without explanation</description>
  <instead>Use named constants with define or enum</instead>
</avoid>

<avoid name="strcpy_strcat">
  <description>Using unbounded string functions</description>
  <instead>Use snprintf for strings. Note: strncpy does NOT null-terminate if source exceeds size</instead>
</avoid>

<avoid name="sprintf">
  <description>Using sprintf without bounds checking</description>
  <instead>Use snprintf with explicit buffer size</instead>
</avoid>

<avoid name="gets">
  <description>Using gets() which has no bounds checking</description>
  <instead>Use fgets() with explicit buffer size</instead>
</avoid>

<avoid name="scanf_unbounded">
  <description>Using scanf with unbounded %s format</description>
  <instead>Use %Ns with explicit width or fgets followed by sscanf</instead>
</avoid>

<avoid name="unchecked_malloc">
  <description>Not checking malloc return value</description>
  <instead>Always check for NULL after allocation</instead>
</avoid>

<avoid name="format_string_vuln">
  <description>Using user input as format string (printf(user_input))</description>
  <instead>Always use printf("%s", user_input) or puts()</instead>
</avoid>

</anti_patterns> </c_language>

<memory_management> <decision_tree name="allocation_strategy"> <question>What is the lifetime and access pattern of the memory?</question> <branch condition="Short-lived, function-scoped">Use stack allocation or VLA (with size limit)</branch> <branch condition="Long-lived, single owner">Use malloc/free with clear ownership</branch> <branch condition="Many small allocations">Use arena allocator</branch> <branch condition="Fixed-size objects">Use pool allocator</branch> </decision_tree>

<pattern name="basic_allocation"> <description>Standard malloc/calloc/realloc/free patterns</description> <example> #include &lt;stdlib.h&gt; #include &lt;string.h&gt;

char *duplicate_string(const char *src) { if (!src) return NULL;

size_t len = strlen(src) + 1; char *dst = malloc(len); if (!dst) return NULL;

memcpy(dst, src, len); return dst;

} </example> <rules priority="critical"> <rule>Always check malloc/calloc/realloc return value for NULL</rule> <rule>Use calloc for zero-initialized memory</rule> <rule>After realloc, use the returned pointer (original may be invalid)</rule> <rule>Set pointer to NULL after free to prevent use-after-free</rule> </rules> </pattern>

<pattern name="arena_allocator"> <description>Bulk allocation with single-point deallocation</description> <example> typedef struct { char *base; size_t size; size_t offset; } Arena;

Arena arena_create(size_t size) { Arena a = {0}; a.base = malloc(size); if (a.base) a.size = size; return a; }

void *arena_alloc(Arena *a, size_t bytes) { size_t aligned = (bytes + 7) & ~7; // 8-byte alignment if (a->offset + aligned > a->size) return NULL; void *ptr = a->base + a->offset; a->offset += aligned; return ptr; }

void arena_reset(Arena *a) { a->offset = 0; }

void arena_destroy(Arena *a) { free(a->base); *a = (Arena){0}; } </example> <use_case>Parsing, compilers, request handling - many allocations freed together</use_case> </pattern>

<pattern name="pool_allocator"> <description>Fixed-size object allocation with O(1) alloc/free</description> <example> typedef struct PoolBlock { struct PoolBlock *next; } PoolBlock;

typedef struct { PoolBlock *free_list; char *memory; size_t object_size; size_t capacity; } Pool;

Pool pool_create(size_t object_size, size_t count) { Pool p = {0}; size_t size = object_size > sizeof(PoolBlock) ? object_size : sizeof(PoolBlock); p.memory = malloc(size * count); if (!p.memory) return p;

p.object_size = size; p.capacity = count;

// Build free list for (size_t i = 0; i < count; i++) { PoolBlock *block = (PoolBlock *)(p.memory + i * size); block->next = p.free_list; p.free_list = block; } return p;

}

void *pool_alloc(Pool *p) { if (!p->free_list) return NULL; PoolBlock *block = p->free_list; p->free_list = block->next; return block; }

void pool_free(Pool *p, void *ptr) { PoolBlock *block = ptr; block->next = p->free_list; p->free_list = block; }

void pool_destroy(Pool *p) { free(p->memory); *p = (Pool){0}; } </example> <use_case>Game entities, network connections, fixed-size records</use_case> </pattern>

<pattern name="goto_cleanup"> <description>Resource cleanup with goto for error handling</description> <example> int process_file(const char *path) { int result = -1; FILE *fp = NULL; char *buffer = NULL;

fp = fopen(path, "r"); if (!fp) goto cleanup;

buffer = malloc(BUFFER_SIZE); if (!buffer) goto cleanup;

// ... process file ...

result = 0; // Success

cleanup: free(buffer); if (fp) fclose(fp); return result; } </example> <note>Single cleanup point prevents resource leaks on error paths</note> </pattern>

<anti_patterns> <avoid name="memory_leak"> <description>Forgetting to free allocated memory</description> <instead>Use Valgrind/ASan, establish clear ownership rules</instead> </avoid>

<avoid name="double_free">
  <description>Freeing the same pointer twice</description>
  <instead>Set pointer to NULL after free, use ownership tracking</instead>
</avoid>

<avoid name="use_after_free">
  <description>Accessing memory after it has been freed</description>
  <instead>Clear pointers after free, use ASan for detection</instead>
</avoid>

<avoid name="buffer_overflow">
  <description>Writing beyond allocated buffer bounds</description>
  <instead>Always track buffer sizes, use bounded functions</instead>
</avoid>

</anti_patterns> </memory_management>

<cli_development> <decision_tree name="argument_parsing"> <question>What complexity of argument parsing is needed?</question> <branch condition="Simple flags and options">Use getopt()</branch> <branch condition="Long options needed">Use getopt_long()</branch> <branch condition="GNU-style with help generation">Use argp (GNU extension)</branch> </decision_tree>

<pattern name="getopt"> <description>POSIX standard option parsing</description> <example> #include &lt;unistd.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt;

int main(int argc, char *argv[]) { int verbose = 0; const char *output = NULL; int opt;

while ((opt = getopt(argc, argv, "vo:h")) != -1) { switch (opt) { case 'v': verbose = 1; break; case 'o': output = optarg; break; case 'h': printf("Usage: %s [-v] [-o output] [file...]\n", argv[0]); return 0; default: fprintf(stderr, "Usage: %s [-v] [-o output] [file...]\n", argv[0]); return 1; } }

// Remaining arguments: argv[optind] to argv[argc-1] for (int i = optind; i < argc; i++) { printf("Processing: %s\n", argv[i]); }

return 0;

} </example> </pattern>

<pattern name="getopt_long"> <description>GNU extension for long options</description> <example> #include &lt;getopt.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt;

static struct option long_options[] = { {"verbose", no_argument, NULL, 'v'}, {"output", required_argument, NULL, 'o'}, {"help", no_argument, NULL, 'h'}, {NULL, 0, NULL, 0 } };

int main(int argc, char *argv[]) { int verbose = 0; const char *output = NULL; int opt;

while ((opt = getopt_long(argc, argv, "vo:h", long_options, NULL)) != -1) { switch (opt) { case 'v': verbose = 1; break; case 'o': output = optarg; break; case 'h': printf("Usage: %s [--verbose] [--output FILE] [file...]\n", argv[0]); return 0; default: return 1; } }

return 0;

} </example> </pattern>

<pattern name="exit_codes"> <description>Standard exit code conventions</description> <codes> <code value="0" name="EXIT_SUCCESS">Successful execution</code> <code value="1" name="EXIT_FAILURE">General error</code> <code value="2">Command line usage error</code> <code value="126">Command found but not executable</code> <code value="127">Command not found</code> <code value="128+N">Terminated by signal N</code> </codes> <example> #include &lt;stdlib.h&gt; #include &lt;sysexits.h&gt; // EX_USAGE, EX_DATAERR, etc.

int main(int argc, char *argv[]) { if (argc < 2) { fprintf(stderr, "Usage: %s <file>\n", argv[0]); return EX_USAGE; // 64 }

FILE *fp = fopen(argv[1], "r"); if (!fp) { perror(argv[1]); return EX_NOINPUT; // 66 }

// ...

return EXIT_SUCCESS;

} </example> </pattern>

<pattern name="signal_handling"> <description>Graceful signal handling for clean shutdown</description> <example> #include &lt;signal.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;stdatomic.h&gt;

static atomic_int running = 1;

static void handle_signal(int sig) { (void)sig; running = 0; }

int main(void) { struct sigaction sa = {0}; sa.sa_handler = handle_signal; sigemptyset(&sa.sa_mask); sa.sa_flags = 0;

sigaction(SIGINT, &sa, NULL); sigaction(SIGTERM, &sa, NULL);

while (running) { // Main loop work }

printf("Shutting down gracefully...\n"); return 0;

} </example> <rules priority="critical"> <rule>Use sigaction() instead of signal() for portability</rule> <rule>Keep signal handlers minimal - set flag only</rule> <rule>Use volatile sig_atomic_t or atomic types for signal flags</rule> </rules> </pattern>

<pattern name="error_reporting"> <description>Consistent error message format</description> <example> #include &lt;errno.h&gt; #include &lt;stdarg.h&gt; #include &lt;stdio.h&gt; #include &lt;string.h&gt;

static const char *progname = "myapp";

void set_progname(const char *argv0) { const char *p = strrchr(argv0, '/'); progname = p ? p + 1 : argv0; }

void error(const char *fmt, ...) { fprintf(stderr, "%s: ", progname); va_list ap; va_start(ap, fmt); vfprintf(stderr, fmt, ap); va_end(ap); fprintf(stderr, "\n"); }

void error_errno(const char *fmt, ...) { int saved_errno = errno; fprintf(stderr, "%s: ", progname); va_list ap; va_start(ap, fmt); vfprintf(stderr, fmt, ap); va_end(ap); fprintf(stderr, ": %s\n", strerror(saved_errno)); } </example> </pattern> </cli_development>

<toolchain> <compilers> <compiler name="gcc"> <description>GNU Compiler Collection</description> <flags> <flag name="-std=c11">Enable C11 standard (or c17, c23)</flag> <flag name="-Wall -Wextra -Wpedantic">Comprehensive warnings</flag> <flag name="-Werror">Treat warnings as errors</flag> <flag name="-Wshadow">Warn on variable shadowing</flag> <flag name="-Wconversion">Warn on implicit conversions</flag> <flag name="-Wstrict-prototypes">Require function prototypes</flag> <flag name="-fanalyzer">Static analysis (GCC 10+)</flag> </flags> </compiler>

<compiler name="clang">
  <description>LLVM C compiler</description>
  <flags>
    <flag name="-std=c11">Enable C11 standard (or c17, c23)</flag>
    <flag name="-Wall -Wextra -Wpedantic">Comprehensive warnings</flag>
    <flag name="-Werror">Treat warnings as errors</flag>
    <flag name="-Weverything">All warnings (use selectively)</flag>
  </flags>
</compiler>

</compilers> <sanitizers> <sanitizer name="AddressSanitizer"> <description>Memory error detection (buffer overflow, use-after-free)</description> <flags>-fsanitize=address -fno-omit-frame-pointer</flags> <example> gcc -fsanitize=address -fno-omit-frame-pointer -g -o myapp myapp.c </example> </sanitizer>

<sanitizer name="UndefinedBehaviorSanitizer">
  <description>Undefined behavior detection</description>
  <flags>-fsanitize=undefined</flags>
  <example>

gcc -fsanitize=undefined -g -o myapp myapp.c </example> </sanitizer>

<sanitizer name="ThreadSanitizer">
  <description>Data race detection</description>
  <flags>-fsanitize=thread</flags>
  <note>Cannot be combined with AddressSanitizer</note>
</sanitizer>

<sanitizer name="MemorySanitizer">
  <description>Uninitialized memory read detection (Clang only)</description>
  <flags>-fsanitize=memory</flags>
</sanitizer>

</sanitizers>

<static_analysis> <tool name="clang-tidy"> <description>Clang-based linter and static analyzer</description> <usage>clang-tidy src/.c -- -std=c11</usage> <configuration> <file_reference>.clang-tidy</file_reference> Checks: > -, bugprone-, clang-analyzer-, misc-, performance-, readability-*, -readability-identifier-length

WarningsAsErrors: '*' </configuration> </tool>

<tool name="cppcheck">
  <description>Static analysis tool for C/C++</description>
  <usage>cppcheck --enable=all --error-exitcode=1 src/</usage>
</tool>

<tool name="valgrind">
  <description>Runtime memory error detection</description>
  <usage>valgrind --leak-check=full --show-leak-kinds=all ./myapp</usage>
  <tools>
    <tool name="memcheck">Memory error detection (default)</tool>
    <tool name="helgrind">Thread error detection</tool>
    <tool name="cachegrind">Cache profiling</tool>
    <tool name="callgrind">Call graph profiling</tool>
  </tools>
</tool>

</static_analysis>

<testing> <decision_tree name="framework_selection"> <question>What testing style do you prefer?</question> <branch condition="Simple, minimal dependencies">Use Check or Unity</branch> <branch condition="BDD-style">Use cmocka</branch> <branch condition="Embedded/resource-constrained">Use Unity (smallest footprint)</branch> </decision_tree>

<framework name="Check">
  <description>Unit testing framework for C</description>
  <example>

#include <check.h>

START_TEST(test_addition) { ck_assert_int_eq(1 + 1, 2); } END_TEST

Suite *math_suite(void) { Suite *s = suite_create("Math"); TCase *tc = tcase_create("Core"); tcase_add_test(tc, test_addition); suite_add_tcase(s, tc); return s; }

int main(void) { Suite *s = math_suite(); SRunner *sr = srunner_create(s); srunner_run_all(sr, CK_NORMAL); int failed = srunner_ntests_failed(sr); srunner_free(sr); return failed ? EXIT_FAILURE : EXIT_SUCCESS; } </example> </framework> </testing> </toolchain>

<build_systems> <decision_tree name="build_system_selection"> <question>What is your project's complexity and portability needs?</question> <branch condition="Simple project, Unix-only">Use Make</branch> <branch condition="Cross-platform, dependencies">Use CMake or Meson</branch> <branch condition="Modern, fast builds">Use Meson</branch> </decision_tree>

<make> <pattern name="simple_makefile"> <description>Basic Makefile for C projects</description> <example> CC := gcc CFLAGS := -std=c11 -Wall -Wextra -Wpedantic -g LDFLAGS := LDLIBS :=

SRCS := $(wildcard src/*.c) OBJS := $(SRCS:.c=.o) TARGET := myapp

.PHONY: all clean

all: $(TARGET)

$(TARGET): $(OBJS) $(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)

%.o: %.c $(CC) $(CFLAGS) -c -o $@ $<

clean: rm -f $(OBJS) $(TARGET) </example> </pattern> </make>

<cmake> <pattern name="modern_cmake"> <description>Modern CMake for C projects</description> <example> cmake_minimum_required(VERSION 3.20) project(myapp VERSION 1.0.0 LANGUAGES C)

set(CMAKE_C_STANDARD 11) set(CMAKE_C_STANDARD_REQUIRED ON) set(CMAKE_C_EXTENSIONS OFF)

add_executable(myapp src/main.c src/utils.c) target_include_directories(myapp PRIVATE include)

target_compile_options(myapp PRIVATE $<$<C_COMPILER_ID:GNU,Clang>: -Wall -Wextra -Wpedantic -Werror > ) </example> <note>For detailed CMake patterns, see cplusplus-ecosystem skill</note> </pattern> </cmake>

<meson> <pattern name="meson_build"> <description>Meson build for C projects</description> <example> # meson.build project('myapp', 'c', version: '1.0.0', default_options: [ 'c_std=c11', 'warning_level=3', 'werror=true', ] )

src = files('src/main.c', 'src/utils.c') inc = include_directories('include')

executable('myapp', src, include_directories: inc) </example> </pattern> </meson> </build_systems>

<context7_integration> <description>Use Context7 MCP for up-to-date C documentation</description>

<c_libraries> <library name="cppreference" id="/websites/cppreference_com" /> </c_libraries>

<usage_patterns> <pattern name="language_reference"> <step>resolve-library-id libraryName="cppreference"</step> <step>get-library-docs context7CompatibleLibraryID="/websites/cppreference_com" topic="stdatomic.h"</step> </pattern>

<pattern name="standard_library">
  <step>get-library-docs context7CompatibleLibraryID="/websites/cppreference_com" topic="malloc"</step>
</pattern>

</usage_patterns> </context7_integration>

<best_practices> <practice priority="critical">Always check return values of malloc/calloc/realloc</practice> <practice priority="critical">Enable -Wall -Wextra -Werror for all builds</practice> <practice priority="critical">Run with AddressSanitizer during development</practice> <practice priority="critical">Use Valgrind before release</practice> <practice priority="high">Use fixed-width integer types from stdint.h</practice> <practice priority="high">Prefer snprintf over sprintf for buffer safety</practice> <practice priority="high">Use designated initializers for struct clarity</practice> <practice priority="high">Document ownership semantics in function comments</practice> <practice priority="medium">Use static for file-local functions and variables</practice> <practice priority="medium">Prefer const for read-only parameters</practice> <practice priority="medium">Use enum for related constants instead of define</practice> <practice priority="medium">Include what you use - minimize header dependencies</practice> </best_practices>

<workflow> <phase name="analyze"> <objective>Understand C code requirements</objective> <step>1. Check for existing code patterns and conventions</step> <step>2. Identify memory ownership requirements</step> <step>3. Review header dependencies</step> </phase> <phase name="implement"> <objective>Write safe, portable C code</objective> <step>1. Use appropriate C standard (C11 minimum recommended)</step> <step>2. Follow memory management patterns</step> <step>3. Handle all error conditions</step> </phase> <phase name="validate"> <objective>Verify C code correctness</objective> <step>1. Compile with warnings enabled</step> <step>2. Run with sanitizers</step> <step>3. Test with Valgrind</step> </phase> </workflow>

<error_escalation> <level severity="low"> <example>Compiler warning about unused variable</example> <action>Fix warning, maintain clean build</action> </level> <level severity="medium"> <example>Valgrind reports minor memory leak</example> <action>Track down and fix leak, verify with Valgrind</action> </level> <level severity="high"> <example>AddressSanitizer detects buffer overflow</example> <action>Stop, fix immediately - this is a security issue</action> </level> <level severity="critical"> <example>Use-after-free or double-free detected</example> <action>Block operation, require immediate fix and review</action> </level> </error_escalation>

<constraints> <must>Check all allocation return values</must> <must>Use bounded string functions (snprintf, strncpy)</must> <must>Enable compiler warnings</must> <must>Run sanitizers during development</must> <avoid>Using gets(), sprintf(), or other unsafe functions</avoid> <avoid>Raw pointer arithmetic without bounds checking</avoid> <avoid>Implicit type conversions that may lose data</avoid> <avoid>Global mutable state when possible</avoid> </constraints>

<related_agents> <agent name="design">Memory architecture, data structure design</agent> <agent name="execute">C implementation with proper memory management</agent> <agent name="security">Buffer overflow detection, input validation</agent> <agent name="performance">Cache optimization, memory layout, profiling with Valgrind</agent> </related_agents>

<related_skills> <skill name="serena-usage">Navigate C codebases and header hierarchies</skill> <skill name="context7-usage">C documentation via /websites/cppreference_com</skill> <skill name="cplusplus-ecosystem">CMake patterns and clang-tidy configuration</skill> <skill name="investigation-patterns">Debugging with Valgrind, GDB, and sanitizers</skill> </related_skills>