1376 words
7 minutes
C23_Finding_variable_errors_that_are_not_initialized
2026-08-11
z_c
c
/
c23
/
clang
/
gcc
/
cmake

link#

test version check|🔝|#

# Linux OS
OS: openSUSE Tumbleweed x86_64
# test 커널 버젼
Kernel: Linux 7.1.6-1-default

# clang 버젼
$ clang --version
clang version 22.1.8
Target: x86_64-suse-linux
Thread model: posix
InstalledDir: /usr/bin

# gcc 버젼
$ gcc --version
gcc (SUSE Linux) 16.1.1 20260731
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

gcc or clang#

  • clang
# 이거 에러를 못잡네
 /usr/bin/clang -std=c23 -Wall -Wextra -pedantic -Werror -O1 -ggdb -o ./target/a02_uninitialized_error_check src/main.c

# 이게 최고네
$ /usr/bin/clang --analyze -std=c23 -Xanalyzer -analyzer-output=text src/main.c

src/main.c:24:5: warning: 2nd function call argument is an uninitialized
      value [core.CallAndMessage]
   24 |     printf("the temp is %u\n", tmp);
      |     ^                          ~~~
src/main.c:8:5: note: 'tmp' declared without an initial value
    8 |     unsigned tmp;
      |     ^~~~~~~~~~~~
src/main.c:12:5: note: 'Default' branch taken. Execution continues on line
      24
   12 |     switch ((unsigned)argc) {
      |     ^
src/main.c:24:5: note: 2nd function call argument is an uninitialized value
   24 |     printf("the temp is %u\n", tmp);
      |     ^                          ~~~
1 warning generated.
  • gcc
# gcc 는 버젼 16이상 되야하는듯
❯ gcc -std=c23 -Wmaybe-uninitialized -Wall -Wextra -pedantic -Werror -O1 -ggdb -o ./target/a02_uninitialized_error_check src/main.c
src/main.c: In function ‘main’:
src/main.c:24:5: error: ‘tmp’ may be used uninitialized [-Werror=maybe-uninitialized]
   24 |     printf("the temp is %u\n", tmp);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.c:8:14: note: ‘tmp’ was declared here
    8 |     unsigned tmp;
      |              ^~~
cc1: all warnings being treated as errors

Cmake를 활용한 Build 활용법|🔝|#

This example is deliberately broken: it demonstrates a switch that fails to assign a value on every path, so the compiler rejects it.

기본 컴파일러가 gcc로 되어 있는 상태#

$  cmake -S . -B target && cmake --build target
-- The C compiler identification is GNU 16.1.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: ./a03_uninitialized_error_part2/target
[ 50%] Building C object CMakeFiles/a03_uninitialized_error_part2.dir/src/main.c.o
./a03_uninitialized_error_part2/src/main.c: In function ‘main’:
./a03_uninitialized_error_part2/src/main.c:25:32: error: ‘tmp’ undeclared (first use in this function)
   25 |     printf("the temp is %u\n", tmp);
      |                                ^~~
./a03_uninitialized_error_part2/src/main.c:25:32: note: each undeclared identifier is reported only once for each function it appears in
./a03_uninitialized_error_part2/src/main.c:4:26: warning: unused parameter ‘argv’ [-Wunused-parameter]
    4 | int main(int argc, char *argv[argc + 1]) {
      |                    ~~~~~~^~~~~~~~~~~~~~
./a03_uninitialized_error_part2/src/main.c:13:18: warning: statement will never be executed [-Wswitch-unreachable]
   13 |         unsigned tmp = 45;
      |                  ^~~
gmake[2]: *** [CMakeFiles/a03_uninitialized_error_part2.dir/build.make:79: CMakeFiles/a03_uninitialized_error_part2.dir/src/main.c.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:87: CMakeFiles/a03_uninitialized_error_part2.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2

clang 으로 강제 세팅#

$ cmake -S . -B target -D CMAKE_BUILD_TYPE=Debug -D CMAKE_C_COMPILER=/usr/bin/clang && cmake --build target

-- The C compiler identification is Clang 22.1.8
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: ./a03_uninitialized_error_part2/target
[ 50%] Building C object CMakeFiles/a03_uninitialized_error_part2.dir/src/main.c.o
clang: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
warning: unknown warning option '-Werror=maybe-uninitialized'; did you mean '-Werror=uninitialized'? [-Wunknown-warning-option]
./a03_uninitialized_error_part2/src/main.c:25:32: error:
      use of undeclared identifier 'tmp'
   25 |     printf("the temp is %u\n", tmp);
      |                                ^~~
./a03_uninitialized_error_part2/src/main.c:4:26: warning:
      unused parameter 'argv' [-Wunused-parameter]
    4 | int main(int argc, char *argv[argc + 1]) {
      |                          ^
2 warnings and 1 error generated.
gmake[2]: *** [CMakeFiles/a03_uninitialized_error_part2.dir/build.make:79: CMakeFiles/a03_uninitialized_error_part2.dir/src/main.c.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:87: CMakeFiles/a03_uninitialized_error_part2.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2

fishshell에서 multi line으로 입력해서 눈에 보기 좋게 입력(clang 으로 컴파일러 강제 세팅)#

$ cmake -S . \
        -B target \
        -D CMAKE_BUILD_TYPE=Debug \
        -D CMAKE_C_COMPILER=/usr/bin/clang \
        && cmake --build target
-- The C compiler identification is Clang 22.1.8
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: ./a03_uninitialized_error_part2/target
[ 50%] Building C object CMakeFiles/a03_uninitialized_error_part2.dir/src/main.c.o
clang: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
warning: unknown warning option '-Werror=maybe-uninitialized'; did you mean '-Werror=uninitialized'? [-Wunknown-warning-option]
./a03_uninitialized_error_part2/src/main.c:25:32: error:
      use of undeclared identifier 'tmp'
   25 |     printf("the temp is %u\n", tmp);
      |                                ^~~
./a03_uninitialized_error_part2/src/main.c:4:26: warning:
      unused parameter 'argv' [-Wunused-parameter]
    4 | int main(int argc, char *argv[argc + 1]) {
      |                          ^
2 warnings and 1 error generated.
gmake[2]: *** [CMakeFiles/a03_uninitialized_error_part2.dir/build.make:79: CMakeFiles/a03_uninitialized_error_part2.dir/src/main.c.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:87: CMakeFiles/a03_uninitialized_error_part2.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2

  • tmp is declared without an initializer and only assigned in some switch cases (case 0, case 1). There is no default, so when the selector matches no case the variable is left indeterminate, yet it is read by printf afterward.

    • tmp는 초기화 없이 선언되고 일부 switch 케이스(case 0, case 1)에서만 값을 대입받습니다. default가 없으므로 선택자가 어떤 케이스와도 맞지 않으면 변수가 미정 상태로 남는데, 그 직후 printf에서 이를 읽습니다.

  • Three knobs in CMakeLists.txt make the warning into a hard error:

    1. -Wall -Wextra — enables -Wmaybe-uninitialized.
    2. -O1 — the may be uninitialized data-flow analysis only runs at -O1 and above (at -O0 it is silently skipped).
    3. -Werror=maybe-uninitialized — promotes that specific warning to an error so the build fails.
    • CMakeLists.txt의 세 설정이 이 경고를 컴파일 에러로 바꿉니다:
      1. -Wall -Wextra — -Wmaybe-uninitialized를 켭니다.
      2. -O1 — 초기화되지 않았을 수 있음 데이터 흐름 분석은 -O1 이상에서만 동작합니다(-O0에서는 생략됨).
      3. -Werror=maybe-uninitialized — 해당 경고를 에러로 격상시켜 빌드를 실패시킵니다.

CMakefile.txt|🔝|#

cmake_minimum_required(VERSION 4.0)

set(CMAKE_C_STANDARD 23)
set(CMAKE_C_STANDARD_REQUIRED ON)
set(CMAKE_C_EXTENSIONS OFF)
# set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

get_filename_component(ProjectId ${CMAKE_CURRENT_SOURCE_DIR} NAME)
string(REPLACE " " "_" ProjectId ${ProjectId})
project(${ProjectId} LANGUAGES C)

# Force GCC 15(LinuxOS)
# set(CMAKE_C_COMPILER "/opt/gcc-15/bin/gcc")
# Force GCC 15(macOS)
# set(CMAKE_C_COMPILER "/opt/homebrew/opt/gcc@15/bin/gcc-15")
# Force Clang 21(LinuxOS)
# set(CMAKE_C_COMPILER "/usr/bin/clang-21")
# Force Clang 21(macOS)
# set(CMAKE_CXX_COMPILER "/opt/homebrew/opt/llvm/bin/clang")

SET (CMAKE_C_FLAGS_INIT                "-Wall -std=c23")
SET (CMAKE_C_FLAGS_DEBUG_INIT          "-g")
SET (CMAKE_C_FLAGS_MINSIZEREL_INIT     "-Os -DNDEBUG")
SET (CMAKE_C_FLAGS_RELEASE_INIT        "-O3 -DNDEBUG")
SET (CMAKE_C_FLAGS_RELWITHDEBINFO_INIT "-O2 -g")

SET (CMAKE_CXX_FLAGS_INIT                "-Wall -std=c++26")
SET (CMAKE_CXX_FLAGS_DEBUG_INIT          "-g")
SET (CMAKE_CXX_FLAGS_MINSIZEREL_INIT     "-Os -DNDEBUG")
SET (CMAKE_CXX_FLAGS_RELEASE_INIT        "-O3 -DNDEBUG")
SET (CMAKE_CXX_FLAGS_RELWITHDEBINFO_INIT "-O2 -g")

# Common compile flags
add_compile_options(
    -pedantic
    -pthread
    -pedantic-errors
    -lm
    -Wall
    -Wextra
    -Werror=maybe-uninitialized
    -O1   # -Wmaybe-uninitialized analysis only runs at -O1 and above
    -ggdb
    # -std=c23
)

# Main executable with C sources
add_executable(${ProjectId}
    src/main.c
    # src/mandelbrot.c
)

target_link_options(${ProjectId} PRIVATE -pthread -lm)

# Output directory
set_target_properties(${ProjectId} PROPERTIES
    RUNTIME_OUTPUT_DIRECTORY
       "${CMAKE_BINARY_DIR}/$<LOWER_CASE:$<CONFIG>>"
)

c23 test code|🔝|#

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[argc + 1]) {
    (void)argv; // argc is used below; argv is not -> silence -Wunused-parameter
    // Deliberately NOT initialized. It is assigned only inside *some*
    // switch cases below; if no case matches, it is left indeterminate.
    unsigned tmp;

    // argc's value is not known at compile time, so the compiler cannot
    // prove which case (if any) will run -> 'tmp' may stay unset.
    switch ((unsigned)argc) {
    case 0:
        tmp = 0;
        break;
    case 1:
        tmp = 1;
        break;
        // NOTE: no `default` here, and no assignment for argc >= 2,
        //       so 'tmp' is not guaranteed to be set on every path.
    }

    // Error: on a path where no case matched, 'tmp' may be used uninitialized.
    printf("the temp is %u\n", tmp);

    return EXIT_SUCCESS;
}
C23_Finding_variable_errors_that_are_not_initialized
https://younghakim7.github.io/blog/posts/c23_finding_variable_errors_that_are_not_initialized/
Author
YoungHa
Published at
2026-08-11
© 2026 YoungHa. All Rights Reserved. / RSS / Sitemap
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