Add a snprintf_safe() helper function

Returns true on success or false on error _or_ truncation. Since truncation is
almost always an error anyway, we might as well make this easier to check.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

meson.build

@@ -420,6 +420,11 @@ test(
     executable('test-compose', 'test/compose.c', dependencies: test_dep),
     env: test_env,
 )
+test(
+    'utils',
+    executable('test-utils', 'test/utils.c', dependencies: test_dep),
+    env: test_env,
+)
 test(
     'symbols-leak-test',
     find_program('test/symbols-leak-test.bash'),

src/utils.h

@@ -26,6 +26,7 @@
 
 #include <errno.h>
 #include <inttypes.h>
+#include <stdarg.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
@@ -290,4 +291,18 @@ int vasprintf(char **strp, const char *fmt, va_list ap);
 # endif /* !HAVE_VASPRINTF */
 #endif /* !HAVE_ASPRINTF */
 
+static inline bool
+ATTR_PRINTF(3, 4)
+snprintf_safe(char *buf, size_t sz, const char *format, ...)
+{
+    va_list ap;
+    int rc;
+
+    va_start(ap, format);
+    rc = vsnprintf(buf, sz, format, ap);
+    va_end(ap);
+
+    return rc >= 0 && (size_t)rc < sz;
+}
+
 #endif /* UTILS_H */

test/utils.c

@@ -0,0 +1,50 @@
+/*
+ * Copyright © 2019 Red Hat, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "config.h"
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "test.h"
+#include "utils.h"
+
+int
+main(void)
+{
+    char buffer[10];
+
+    assert(!snprintf_safe(buffer, 0, "foo"));
+    assert(!snprintf_safe(buffer, 1, "foo"));
+    assert(!snprintf_safe(buffer, 3, "foo"));
+
+    assert(snprintf_safe(buffer, 10, "foo"));
+    assert(streq(buffer, "foo"));
+
+    assert(!snprintf_safe(buffer, 10, "%s", "1234567890"));
+    assert(snprintf_safe(buffer, 10, "%s", "123456789"));
+
+    return 0;
+}