basic: add STRERROR() wrapper for strerror_r()

src/basic/errno-util.h

@@ -6,6 +6,16 @@
 
 #include "macro.h"
 
+/* strerror(3) says that glibc uses a maximum length of 1024 bytes. */
+#define ERRNO_BUF_LEN 1024
+
+/* Note: the lifetime of the compound literal is the immediately surrounding block,
+ * see C11 §6.5.2.5, and
+ * https://stackoverflow.com/questions/34880638/compound-literal-lifetime-and-if-blocks
+ *
+ * Note that we use the GNU variant of strerror_r() here. */
+#define STRERROR(errnum) strerror_r(abs(errnum), (char[ERRNO_BUF_LEN]){}, ERRNO_BUF_LEN)
+
 static inline void _reset_errno_(int *saved_errno) {
         if (*saved_errno < 0) /* Invalidated by UNPROTECT_ERRNO? */
                 return;

src/test/meson.build

@@ -615,6 +615,8 @@ tests += [
         [files('test-errno-list.c') +
          generated_gperf_headers],
 
+        [files('test-errno-util.c')],
+
         [files('test-ip-protocol-list.c') +
          shared_generated_gperf_headers],
 

src/test/test-errno-util.c

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "errno-util.h"
+#include "stdio-util.h"
+#include "string-util.h"
+#include "tests.h"
+
+TEST(strerror_not_threadsafe) {
+        /* Just check that strerror really is not thread-safe. */
+        log_info("strerror(%d) → %s", 200, strerror(200));
+        log_info("strerror(%d) → %s", 201, strerror(201));
+        log_info("strerror(%d) → %s", INT_MAX, strerror(INT_MAX));
+
+        log_info("strerror(%d), strerror(%d) → %p, %p", 200, 201, strerror(200), strerror(201));
+
+        /* This call is not allowed, because the first returned string becomes invalid when
+         * we call strerror the second time:
+         *
+         * log_info("strerror(%d), strerror(%d) → %s, %s", 200, 201, strerror(200), strerror(201));
+         */
+}
+
+TEST(STRERROR) {
+        /* Just check that STRERROR really is thread-safe. */
+        log_info("STRERROR(%d) → %s", 200, STRERROR(200));
+        log_info("STRERROR(%d) → %s", 201, STRERROR(201));
+        log_info("STRERROR(%d), STRERROR(%d) → %s, %s", 200, 201, STRERROR(200), STRERROR(201));
+
+        const char *a = STRERROR(200), *b = STRERROR(201);
+        assert_se(strstr(a, "200"));
+        assert_se(strstr(b, "201"));
+
+        /* Check with negative values */
+        assert_se(streq(a, STRERROR(-200)));
+        assert_se(streq(b, STRERROR(-201)));
+
+        const char *c = STRERROR(INT_MAX);
+        char buf[DECIMAL_STR_MAX(int)];
+        xsprintf(buf, "%d", INT_MAX);  /* INT_MAX is hexadecimal, use printf to convert to decimal */
+        log_info("STRERROR(%d) → %s", INT_MAX, c);
+        assert_se(strstr(c, buf));
+}
+
+DEFINE_TEST_MAIN(LOG_INFO);