diff --git a/.github/workflows/cmake-multi-platform.yml b/.github/workflows/cmake-multi-platform.yml
index 68197e71..57639632 100644
--- a/.github/workflows/cmake-multi-platform.yml
+++ b/.github/workflows/cmake-multi-platform.yml
@@ -196,3 +196,37 @@ jobs:
           exit 1
         fi
         echo "Exported symbols match the baseline."
+
+  missing-declarations:
+    # A function defined without a prior declaration is either missing
+    # from its header or should have been static. Both shapes reached
+    # master before, because the flag is in the project's clean set but
+    # nothing makes it fatal, and a warning in a build that passes is
+    # not read.
+    #
+    # Promoting this one flag needs no preparatory cleanup. The whole
+    # NIFTI_WARNINGS_AS_ERRORS set is still blocked by
+    # -Wmaybe-uninitialized in fslio.c, and waiting for that would
+    # leave this uncovered meanwhile.
+    name: no undeclared external functions (FSLSTYLE=${{ matrix.fslstyle }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        fslstyle: ["OFF", "ON"]
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install dependencies
+      run: sudo apt-get update && sudo apt-get install -y libexpat1-dev zlib1g-dev ninja-build
+    - name: Configure CMake
+      run: >
+        cmake -G Ninja -B ${{ github.workspace }}/build
+        -DCMAKE_BUILD_TYPE=Release
+        -DCMAKE_C_FLAGS=-Werror=missing-declarations
+        -DNIFTI_BUILD_APPLICATIONS=ON
+        -DUSE_CIFTI_CODE=ON
+        -DUSE_FSL_CODE=ON
+        -DFSLSTYLE=${{ matrix.fslstyle }}
+        -S ${{ github.workspace }}
+    - name: Build
+      run: cmake --build ${{ github.workspace }}/build
diff --git a/nifti2/nifti2_io.c b/nifti2/nifti2_io.c
index 58b20841..7bf82c88 100644
--- a/nifti2/nifti2_io.c
+++ b/nifti2/nifti2_io.c
@@ -7859,7 +7859,7 @@ znzFile nifti_image_write_hdr_img2(nifti_image *nim, int write_opts,
 
 #ifdef PIGZ
 #ifdef HAVE_ZLIB
-int doPigz2(nifti_image *nim, struct nifti_2_header nhdr, const nifti_brick_list * NBL) {
+static int doPigz2(nifti_image *nim, struct nifti_2_header nhdr, const nifti_brick_list * NBL) {
 	FILE *pigzPipe;
 	char command[768];
     strcpy(command, "pigz" );
@@ -7891,7 +7891,7 @@ int doPigz2(nifti_image *nim, struct nifti_2_header nhdr, const nifti_brick_list
 	return 0;
 }
 
-int doPigz(nifti_image *nim, struct nifti_1_header nhdr, const nifti_brick_list * NBL) {
+static int doPigz(nifti_image *nim, struct nifti_1_header nhdr, const nifti_brick_list * NBL) {
 	FILE *pigzPipe;
 	char command[768];
     strcpy(command, "pigz" );
diff --git a/niftilib/nifti1_io.c b/niftilib/nifti1_io.c
index 1992b188..68afb733 100644
--- a/niftilib/nifti1_io.c
+++ b/niftilib/nifti1_io.c
@@ -5776,6 +5776,7 @@ znzFile nifti_image_write_hdr_img2(nifti_image *nim, int write_opts,
 
 #ifdef PIGZ
 #ifdef HAVE_ZLIB
+#if 0  /* unused here: identical to doPigz below */
 int doPigz2(nifti_image *nim, struct nifti_1_header nhdr, const nifti_brick_list * NBL) {
 	FILE *pigzPipe;
 	char command[768];
@@ -5807,8 +5808,9 @@ int doPigz2(nifti_image *nim, struct nifti_1_header nhdr, const nifti_brick_list
 	free(fp);
 	return 0;
 }
+#endif
 
-int doPigz(nifti_image *nim, struct nifti_1_header nhdr, const nifti_brick_list * NBL) {
+static int doPigz(nifti_image *nim, struct nifti_1_header nhdr, const nifti_brick_list * NBL) {
 	FILE *pigzPipe;
 	char command[768];
     strcpy(command, "pigz" );