tabs and tab overview

xsnprintf.c

@@ -0,0 +1,55 @@
+#include "xsnprintf.h"
+
+#include <errno.h>
+#include <limits.h>
+#include <stdio.h>
+#include "debug.h"
+#include "macros.h"
+
+/*
+ * ISO C doesn't require vsnprintf(3) to set errno on failure, but
+ * POSIX does:
+ *
+ * "If an output error was encountered, these functions shall return
+ * a negative value and set errno to indicate the error."
+ *
+ * The mandated errors of interest are:
+ *
+ * - EILSEQ: A wide-character code does not correspond to a valid character
+ * - EOVERFLOW: The value of n is greater than INT_MAX
+ * - EOVERFLOW: The value to be returned is greater than INT_MAX
+ *
+ * ISO C11 states:
+ *
+ * "The vsnprintf 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."
+ *
+ * See also:
+ *
+ * - ISO C11 §7.21.6.12p3
+ * - https://pubs.opengroup.org/onlinepubs/9699919799/functions/vsnprintf.html
+ * - https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html
+ */
+static size_t
+xvsnprintf(char *restrict buf, size_t n, const char *restrict format, va_list ap)
+{
+    int len = vsnprintf(buf, n, format, ap);
+    if (unlikely(len < 0 || len >= (int)n)) {
+        FATAL_ERROR(__func__, (len < 0) ? errno : ENOBUFS);
+    }
+    return (size_t)len;
+}
+
+size_t
+xsnprintf(char *restrict buf, size_t n, const char *restrict format, ...)
+{
+    va_list ap;
+    va_start(ap, format);
+    size_t len = xvsnprintf(buf, n, format, ap);
+    va_end(ap);
+    return len;
+}