example: inspect SECCOMP_MODE_STRICT return value

example/convert-to-nia/convert-to-nia.c

diff --git a/example/convert-to-nia/convert-to-nia.c b/example/convert-to-nia/convert-to-nia.c
index ef5abc7..c7eac6a 100644
--- a/example/convert-to-nia/convert-to-nia.c
+++ b/example/convert-to-nia/convert-to-nia.c
@@ -1098,8 +1098,11 @@
   // ----
 
 #if defined(WUFFS_EXAMPLE_USE_SECCOMP)
-  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
-  g_sandboxed = true;
+  g_sandboxed = prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT) >= 0;
+  if (!g_sandboxed) {
+    const char* msg = "SECCOMP_MODE_STRICT sandbox failed.\n";
+    ignore_return_value(write(STDERR_FD, msg, strlen(msg)));
+  }
 #endif
 
   int exit_code = compute_exit_code(main1(argc, argv));

example/jsonptr/jsonptr.cc

diff --git a/example/jsonptr/jsonptr.cc b/example/jsonptr/jsonptr.cc
index 7f9dc09..e96381b 100644
--- a/example/jsonptr/jsonptr.cc
+++ b/example/jsonptr/jsonptr.cc
@@ -1471,8 +1471,12 @@
   }
 
 #if defined(WUFFS_EXAMPLE_USE_SECCOMP)
-  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
-  g_sandboxed = true;
+  g_sandboxed = prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT) >= 0;
+  if (!g_sandboxed) {
+    const int stderr_fd = 2;
+    const char* msg = "SECCOMP_MODE_STRICT sandbox failed.\n";
+    ignore_return_value(write(stderr_fd, msg, strlen(msg)));
+  }
 #endif
 
   const char* z = main1(argc, argv);

example/mzcat/mzcat.c

diff --git a/example/mzcat/mzcat.c b/example/mzcat/mzcat.c
index c622da0..04195bc 100644
--- a/example/mzcat/mzcat.c
+++ b/example/mzcat/mzcat.c
@@ -397,8 +397,12 @@
 int  //
 main(int argc, char** argv) {
 #if defined(WUFFS_EXAMPLE_USE_SECCOMP)
-  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
-  g_sandboxed = true;
+  g_sandboxed = prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT) >= 0;
+  if (!g_sandboxed) {
+    const int stderr_fd = 2;
+    const char* msg = "SECCOMP_MODE_STRICT sandbox failed.\n";
+    ignore_return_value(write(stderr_fd, msg, strlen(msg)));
+  }
 #endif
 
   int exit_code = compute_exit_code(main1(argc, argv));

example/zcat/zcat.c

diff --git a/example/zcat/zcat.c b/example/zcat/zcat.c
index 8432f22..4323f9b 100644
--- a/example/zcat/zcat.c
+++ b/example/zcat/zcat.c
@@ -260,8 +260,12 @@
 int  //
 main(int argc, char** argv) {
 #if defined(WUFFS_EXAMPLE_USE_SECCOMP)
-  prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
-  g_sandboxed = true;
+  g_sandboxed = prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT) >= 0;
+  if (!g_sandboxed) {
+    const int stderr_fd = 2;
+    const char* msg = "SECCOMP_MODE_STRICT sandbox failed.\n";
+    ignore_return_value(write(stderr_fd, msg, strlen(msg)));
+  }
 #endif
 
   int exit_code = compute_exit_code(main1(argc, argv));