diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 431e7b5..5189c76 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -18,7 +18,21 @@ jobs:
     container:
       image: fedora:latest
 
-    name: Test compile
+    # Build and test under each supported C compiler. CTest, checksec, and the
+    # coverage target in test/CMakeLists.txt already branch on the compiler, so
+    # selecting CC here exercises the full pipeline for both toolchains.
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler:
+          - gcc
+          - clang
+
+    env:
+      # Forward the chosen compiler to CMake via CC.
+      CC: ${{ matrix.compiler }}
+
+    name: Test compile (${{ matrix.compiler }})
 
     steps:
       - name: Install build dependencies
@@ -37,6 +51,10 @@ jobs:
             bats \
             hostname \
             lcov
+          # clang + LLVM coverage tools.
+          if [ "${{ matrix.compiler }}" = "clang" ]; then
+            dnf -y install clang clang-devel llvm
+          fi
 
       - name: Checkout
         uses: actions/checkout@v6
diff --git a/src/C/CMakeLists.txt b/src/C/CMakeLists.txt
index 6022dab..2a117c7 100644
--- a/src/C/CMakeLists.txt
+++ b/src/C/CMakeLists.txt
@@ -114,6 +114,8 @@ add_compile_options(
   -Walloca                               # Warn on suspicious alloca usage
   -Wcast-qual                            # Warn on cast removing const/volatile
   -Wcast-align                           # Warn on alignment-affecting casts
+  -Wdouble-promotion                     # Implicit float-to-double promotion
+  -Wmissing-field-initializers           # Struct fields left uninitialized
   -Wwrite-strings                        # String literals are const
   -Wundef                                # Undefined macro in #if
   -Wmissing-prototypes                   # Missing function prototypes
@@ -140,6 +142,9 @@ if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
         -Wduplicated-branches              # Duplicated branches
         -Wnull-dereference                 # Potential NULL dereferences
         -Wjump-misses-init                 # Jump skips variable initialization
+        -Warith-conversion                 # Implicit arithmetic conversions
+        -Wstringop-overflow                # String ops writing past buffer end
+        -Wtrampolines                      # Nested functions needing exec stack
     )
 
   check_c_compiler_flag(-fplugin=annobin SET_ANNOBIN)
@@ -222,12 +227,20 @@ else ()
   message(WARNING "Stack clash protection not supported by compiler")
 endif(STACK_CLASH)
 
-# Disable stack slot reuse
+# Disable stack slot reuse.
+# This is a GCC-only hardening flag: it prevents the compiler from
+# reusing a single stack frame for nested calls, which would let a
+# stack-clash attack defeat -fstack-clash-protection. Clang provides
+# no equivalent option, so on Clang -fstack-clash-protection (enabled
+# above) is the sole defense and this flag is skipped instead of aborting.
 CHECK_C_COMPILER_FLAG(-fstack-reuse=none STACK_REUSE)
 if (STACK_REUSE)
   add_compile_options(-fstack-reuse=none)
-else ()
+  message(STATUS "Stack slot reuse disabled: enabled")
+elseif(CMAKE_C_COMPILER_ID STREQUAL "GNU")
   message(FATAL_ERROR "Required compiler flag '-fstack-reuse=none' not supported")
+else ()
+  message(WARNING "Clang does not support '-fstack-reuse=none'; relying on -fstack-clash-protection")
 endif(STACK_REUSE)
 
 # Zero call-used registers on return
diff --git a/src/C/init-kerberos-client-dir.c b/src/C/init-kerberos-client-dir.c
index ce181ae..91da78b 100644
--- a/src/C/init-kerberos-client-dir.c
+++ b/src/C/init-kerberos-client-dir.c
@@ -89,7 +89,11 @@ static void free_buffers(char *keytab, char *keytab_dirname, char *keytab_filena
   }
 }
 
-static int mkdir_if_missing(const char *dir, uid_t owner, gid_t group, mode_t mode) __attribute__((access(read_only, 1))) __attribute__((warn_unused_result));
+static int mkdir_if_missing(const char *dir, uid_t owner, gid_t group, mode_t mode)
+#if !defined(__clang__)
+    __attribute__((access(read_only, 1)))
+#endif
+    __attribute__((warn_unused_result));
 static int mkdir_if_missing(const char *dir, uid_t owner, gid_t group, mode_t mode) {
   const cap_value_t caps_dac_override[] = {CAP_DAC_OVERRIDE};
   const int num_caps_dac_override = sizeof(caps_dac_override) / sizeof(cap_value_t);
diff --git a/src/C/kcron_keytab_file.h b/src/C/kcron_keytab_file.h
index b09bb98..4ecbe46 100644
--- a/src/C/kcron_keytab_file.h
+++ b/src/C/kcron_keytab_file.h
@@ -91,7 +91,11 @@ int validate_client_dirname(const char *client_keytab_dirname) {
  * Security Note: lstat() is used first (not stat()) to avoid a TOCTOU symlink
  * race that could substitute an attacker-controlled path.
  */
-int validate_keytab_directory(const char *keytab_dirname) __attribute__((access(read_only, 1))) __attribute__((warn_unused_result));
+int validate_keytab_directory(const char *keytab_dirname)
+#if !defined(__clang__)
+    __attribute__((access(read_only, 1)))
+#endif
+    __attribute__((warn_unused_result));
 int validate_keytab_directory(const char *keytab_dirname) {
   struct stat lst = {0};
   struct stat st = {0};
diff --git a/src/C/kcron_seccomp.h b/src/C/kcron_seccomp.h
index a2cc59c..8b99fca 100644
--- a/src/C/kcron_seccomp.h
+++ b/src/C/kcron_seccomp.h
@@ -73,6 +73,13 @@
  *
  * Security Principle: Default deny with explicit allow. If it's not in the list,
  * it can't be used. This is defense-in-depth against exploitation.
+ *
+ * Note on argument-restricted rules: SCMP_A0()/SCMP_A1() build a
+ * "struct scmp_arg_cmp" from only three fields (arg, op, datum_a). datum_b is
+ * now supplied explicitly as 0 so the call compiles identically under GCC and
+ * Clang; Clang otherwise reports -Wmissing-field-initializers on the omitted
+ * trailing field (GCC tolerates it, which is exactly why the omission is not
+ * obvious on the host build). datum_b is unused for SCMP_CMP_EQ.
  */
 int set_kcron_seccomp(void) __attribute__((warn_unused_result)) __attribute__((flatten));
 int set_kcron_seccomp(void) {
@@ -151,14 +158,14 @@ int set_kcron_seccomp(void) {
    */
 
   /* Write to stdout - for printing keytab path */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1, SCMP_A0(SCMP_CMP_EQ, 1)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1, SCMP_A0(SCMP_CMP_EQ, 1, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'write' to stdout: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
   }
 
   /* Write to stderr - for error messages */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1, SCMP_A0(SCMP_CMP_EQ, 2)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1, SCMP_A0(SCMP_CMP_EQ, 2, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'write' to stderr: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
@@ -180,7 +187,7 @@ int set_kcron_seccomp(void) {
   }
 
   /* Close directory handle (fd 3) */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 1, SCMP_A0(SCMP_CMP_EQ, 3)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 1, SCMP_A0(SCMP_CMP_EQ, 3, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'close' for fd 3: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
@@ -192,21 +199,21 @@ int set_kcron_seccomp(void) {
    */
 
   /* Write to keytab file (fd 4) */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1, SCMP_A0(SCMP_CMP_EQ, 4)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1, SCMP_A0(SCMP_CMP_EQ, 4, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'write' to fd 4: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
   }
 
   /* Close keytab file (fd 4) */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 1, SCMP_A0(SCMP_CMP_EQ, 4)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 1, SCMP_A0(SCMP_CMP_EQ, 4, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'close' for fd 4: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
   }
 
   /* Sync keytab file to disk (fd 4) */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fsync), 1, SCMP_A0(SCMP_CMP_EQ, 4)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fsync), 1, SCMP_A0(SCMP_CMP_EQ, 4, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'fsync' for fd 4: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
@@ -216,7 +223,7 @@ int set_kcron_seccomp(void) {
    * Set permissions on keytab file (fd 4) to 0600.
    * Restricts to exactly mode 0600 (user read/write only).
    */
-  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchmod), 2, SCMP_A0(SCMP_CMP_EQ, 4), SCMP_A1(SCMP_CMP_EQ, _0600)) != 0) {
+  if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchmod), 2, SCMP_A0(SCMP_CMP_EQ, 4, 0), SCMP_A1(SCMP_CMP_EQ, _0600, 0)) != 0) {
     (void)fprintf(stderr, "%s: Cannot allowlist 'fchmod' for mode 0600: %s\n", __PROGRAM_NAME, strerror(errno));
     (void)seccomp_release(ctx);
     exit(EXIT_FAILURE);
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 21b2167..38eb4af 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -25,10 +25,12 @@ set(TEST_INCLUDES "${CMAKE_SOURCE_DIR}/src/C"
 # ============================================================================
 set(HAVE_COVERAGE_TOOLS OFF)
 
-if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
+# genhtml ships with lcov and is required by both toolchains, so search it
+# once here (not only inside the GCC branch) so the Clang branch can use it.
+find_program(LCOV_EXEC lcov)
+find_program(GENHTML_EXEC genhtml)
 
-  find_program(LCOV_EXEC lcov)
-  find_program(GENHTML_EXEC genhtml)
+if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
 
   if(LCOV_EXEC AND GENHTML_EXEC)
     set(HAVE_COVERAGE_TOOLS ON)
@@ -256,14 +258,26 @@ if(BUILD_TESTING AND HAVE_COVERAGE_TOOLS)
 
   elseif(CMAKE_C_COMPILER_ID MATCHES "Clang")
 
-    set(PROFILE_DATA ${CMAKE_BINARY_DIR}/default.profdata)
-    set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_CLEAN_FILES ${PROFILE_DATA})
+    set(PROFILE_DATA ${CMAKE_BINARY_DIR}/coverage/default.profdata)
+    # Instrumented processes write their sparse .profraw here. CTest changes
+    # into the subdirectory that holds its CTestTestfile.cmake, so a relative
+    # LLVM_PROFILE_FILE would land in build/test (not build) and the merge step
+    # below could not find it; use an absolute pattern instead.
+    set(COVERAGE_PROF_RAW ${CMAKE_BINARY_DIR}/coverage-prof)
+    set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_CLEAN_FILES ${PROFILE_DATA}
+                                               ${COVERAGE_PROF_RAW})
 
     add_custom_target(
       coverage
-      COMMAND ${CMAKE_COMMAND} -E env LLVM_PROFILE_FILE=default.profraw
+      COMMAND ${CMAKE_COMMAND} -E rm -rf ${COVERAGE_PROF_RAW}
+      COMMAND ${CMAKE_COMMAND} -E make_directory ${COVERAGE_PROF_RAW}
+      COMMAND ${CMAKE_COMMAND} -E make_directory ${COVERAGE_OUTPUT_DIR}
+      # %p-%m gives each process (and each child spawned under it, e.g. the
+      # instrumented client-keytab-name) a unique file so runs never clobber
+      # one another, and the absolute path makes the cwd irrelevant.
+      COMMAND ${CMAKE_COMMAND} -E env LLVM_PROFILE_FILE=${COVERAGE_PROF_RAW}/default-%p-%m.profraw
               ${CMAKE_CTEST_COMMAND} --output-on-failure
-      COMMAND ${LLVM_PROFDATA_EXEC} merge -sparse default.profraw -o ${PROFILE_DATA}
+      COMMAND ${LLVM_PROFDATA_EXEC} merge -sparse ${COVERAGE_PROF_RAW}/*.profraw -o ${PROFILE_DATA}
       COMMAND ${LLVM_COV_EXEC} show ${UNIT_TEST_BINARIES} -instr-profile=${PROFILE_DATA}
               -format=html -output-dir=${COVERAGE_OUTPUT_DIR}
       COMMAND ${LLVM_COV_EXEC} report ${UNIT_TEST_BINARIES}