C Language

Standards Evolution

C11 (ISO/IEC 9899:2011)

Major modernization with threading and type-generic features:

• _Generic - Type-generic selection for polymorphic macros
• _Atomic - Atomic types and operations for lock-free programming
• _Thread_local - Thread-local storage duration
• _Static_assert - Compile-time assertions
• _Alignas/_Alignof - Alignment specifier and query
• _Noreturn - Function that does not return
• Anonymous structs/unions

C17 (ISO/IEC 9899:2018)

Bug fix release with no new features. __STDC_VERSION__ = 201710L.

C23 (ISO/IEC 9899:2024)

• nullptr - Null pointer constant with nullptr_t type
• typeof - Type inference operator
• constexpr - Compile-time constant objects
• auto - Type inference for variables
• Binary literals (0b), digit separators (')
• Attributes: [[nodiscard]], [[maybe_unused]], [[deprecated]], [[fallthrough]], [[noreturn]]
• bool, true, false as keywords
• #embed for binary resource inclusion

Type System

Fixed-width integers (stdint.h):

• int8_t, int16_t, int32_t, int64_t / unsigned variants
• intptr_t, uintptr_t - Pointer-sized integers
• size_t - Unsigned size type; ptrdiff_t - Signed pointer difference

Type-generic selection (C11):

#define print_value(x) _Generic((x), \
    int: print_int, \
    double: print_double, \
    char *: print_string, \
    default: print_unknown)(x)

Compound literals and designated initializers:

struct config cfg = { .name = "myapp", .timeout = 30, .verbose = true };
draw((struct point){.x = 10, .y = 20});

Concurrency (C11)

Atomics:

#include <stdatomic.h>
_Atomic int counter = 0;
atomic_fetch_add(&counter, 1);

Thread-local storage: _Thread_local int errno_local;

Undefined Behavior to Avoid

• Memory: Null dereference, use-after-free, buffer overflow, double-free
• Arithmetic: Signed integer overflow, division by zero, shift beyond type width
• Aliasing: Strict aliasing violations, type punning without union
• Sequence: Modifying variable twice between sequence points

Use sanitizers (ASan, UBSan) during development to detect UB.

Anti-patterns

• void* abuse - Use _Generic macros or code generation
• Magic numbers - Use named constants with #define or enum
• strcpy/strcat/sprintf/gets - Use snprintf, fgets
• scanf with unbounded %s - Use %Ns with explicit width
• Unchecked malloc - Always check for NULL
• Format string vuln (printf(user_input)) - Use printf("%s", user_input)

Memory Management

Allocation Patterns

Basic allocation - Always check return values:

char *dst = malloc(len);
if (!dst) return NULL;

Arena allocator - Bulk allocation with single-point deallocation:

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

Use case: Parsing, compilers, request handling.

Pool allocator - Fixed-size object allocation with O(1) alloc/free. Use case: Game entities, network connections, fixed-size records.

Goto cleanup - Resource cleanup with single exit point:

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(SIZE); if (!buffer) goto cleanup;
    // ... process ...
    result = 0;
cleanup:
    free(buffer);
    if (fp) fclose(fp);
    return result;
}

CLI Development

getopt - POSIX standard option parsing:

while ((opt = getopt(argc, argv, "vo:h")) != -1) {
    switch (opt) {
    case 'v': verbose = 1; break;
    case 'o': output = optarg; break;
    case 'h': /* help */ return 0;
    default: return 1;
    }
}
// Remaining args: argv[optind] to argv[argc-1]

getopt_long - GNU extension for --long-options.

Exit codes: 0=success, 1=error, 2=usage error, 126=not executable, 127=not found, 128+N=signal N. See <sysexits.h> for EX_USAGE, EX_NOINPUT, etc.

Signal handling:

static atomic_int running = 1;
static void handle_signal(int sig) { (void)sig; running = 0; }
// Use sigaction() instead of signal() for portability

Error reporting: Include program name, use strerror(errno) for system errors.

Toolchain

Compiler Flags

# GCC/Clang recommended
-std=c11 -Wall -Wextra -Wpedantic -Werror -Wshadow -Wconversion -Wstrict-prototypes
# GCC 10+ static analysis
-fanalyzer

Sanitizers

• AddressSanitizer: -fsanitize=address -fno-omit-frame-pointer
• UndefinedBehaviorSanitizer: -fsanitize=undefined
• ThreadSanitizer: -fsanitize=thread (cannot combine with ASan)
• MemorySanitizer (Clang only): -fsanitize=memory

Static Analysis

• clang-tidy: clang-tidy src/*.c -- -std=c11
• cppcheck: cppcheck --enable=all --error-exitcode=1 src/
• valgrind: valgrind --leak-check=full --show-leak-kinds=all ./myapp

Testing

Check or Unity frameworks for unit testing.

Build Systems

Makefile (simple):

CC := gcc
CFLAGS := -std=c11 -Wall -Wextra -Wpedantic -g
SRCS := $(wildcard src/*.c)
OBJS := $(SRCS:.c=.o)
$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)

CMake and Meson for cross-platform builds. See cplusplus-ecosystem skill for detailed CMake patterns.

Context7 Integration

• C reference: /websites/cppreference_com

Best Practices

• Always check malloc/calloc/realloc return values
• Enable -Wall -Wextra -Werror for all builds
• Run with AddressSanitizer during development
• Use Valgrind before release
• Use fixed-width integer types from stdint.h
• Prefer snprintf over sprintf
• Use designated initializers for struct clarity
• Document ownership semantics in function comments