Add xsnprintf() and remove some unnecessary strlen(3) calls

xsnprintf.c

@@ -0,0 +1,36 @@
+#include "xsnprintf.h"
+
+#include <assert.h>
+#include <limits.h>
+#include <stdio.h>
+
+size_t
+xvsnprintf(char *buf, size_t n, const char *format, va_list ap)
+{
+    assert(n <= INT_MAX);
+    int len = vsnprintf(buf, n, format, ap);
+
+    /*
+     * ISO C11 §7.21.6.5 states:
+     * "The snprintf function returns the number of characters that
+     * would have been written had n been sufficiently large, not
+     * counting the terminating null character, or a negative value
+     * if an encoding error occurred. Thus, the null-terminated output
+     * has been completely written if and only if the returned value
+     * is nonnegative and less than n."
+     */
+    assert(len >= 0);
+    assert(len < (int)n);
+
+    return (size_t)len;
+}
+
+size_t
+xsnprintf(char *buf, size_t n, const char *format, ...)
+{
+    va_list ap;
+    va_start(ap, format);
+    size_t len = xvsnprintf(buf, n, format, ap);
+    va_end(ap);
+    return len;
+}