usr: libc: Add initial vsnprintf() impl

- Support %x specifier for base 16 strings
- Support %d specifier for base 10 strings
- Support %p specifier for pointer strings
- Allow padding of %d strings

Signed-off-by: Ian Moffett <ian@osmora.org>

1 files changed, 135 insertions, 0 deletions

diff --git a/lib/libc/src/stdio/vsnprintf.c b/lib/libc/src/stdio/vsnprintf.c
new file mode 100644
index 0000000..c122459
--- /dev/null
+++ b/lib/libc/src/stdio/vsnprintf.c
@@ -0,0 +1,135 @@
+/*
+ * Copyright (c) 2023-2025 Ian Marco Moffett and the Osmora Team.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of Hyra nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/types.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <string.h>
+
+/* TODO FIXME: Use stdarg.h */
+#define __va_arg(ap, type)  __builtin_va_arg(ap, type)
+
+static inline void
+printc(char *buf, size_t size, size_t *off, char c)
+{
+    if (*off < size - 1) {
+        buf[(*off)++] = c;
+    }
+    buf[*off] = 0;
+}
+
+static void
+printstr(char *buf, size_t size, size_t *off, const char *s)
+{
+    while (*off < size - 1 && *s != '\0') {
+        buf[(*off)++] = *(s)++;
+    }
+    buf[*off] = 0;
+}
+
+int
+vsnprintf(char *s, size_t size, const char *fmt, va_list ap)
+{
+    size_t tmp_len, num_len, off = 0;
+    ssize_t num = 0;
+    char c, c1, num_buf[256] = {0};
+    const char *tmp_str;
+    uint8_t pad_width = 0;
+
+    while (off < (size - 1)) {
+        while (*fmt && *fmt != '%') {
+            printc(s, size, &off, *fmt++);
+        }
+        if (*(fmt)++ == '\0' || off == size - 1) {
+            return off;
+        }
+
+        /*
+         * Handle a case where we have "%0N". For example:
+         * "%04d"
+         */
+        if (*fmt == '0') {
+            ++fmt;
+            while (*fmt >= '0' && *fmt <= '9') {
+                pad_width = pad_width * 10 + (*fmt - '0');
+                ++fmt;
+            }
+        }
+
+        c = *fmt++;
+        switch (c) {
+        case 'c':
+            c1 = (char )__va_arg(ap, int);
+            printc(s, size, &off, c1);
+            break;
+        case 'd':
+            num = __va_arg(ap, int);
+            itoa(num, num_buf, 10);
+
+            if (pad_width > 0) {
+                num_len = strlen(num_buf);
+                for (size_t i = num_len; i < pad_width; ++i)
+                    printc(s, size, &off, '0');
+            }
+            printstr(s, size, &off, num_buf);
+            break;
+        case 'p':
+            num = __va_arg(ap, uint64_t);
+            itoa(num, num_buf, 16);
+            tmp_len = strlen(num_buf);
+
+            /* Add '0x' prefix */
+            printc(s, size, &off, '0');
+            printc(s, size, &off, 'x');
+            /*
+             * Now we pad this.
+             *
+             * XXX TODO: This assumes 64-bits, should be
+             *           cleaned up.
+             */
+            for (size_t i = 0; i < 18 - tmp_len; ++i) {
+                printc(s, size, &off, '0');
+            }
+            printstr(s, size, &off, num_buf + 2);
+            break;
+        case 'x':
+            num = __va_arg(ap, uint64_t);
+            itoa(num, num_buf, 16);
+            tmp_len = strlen(num_buf);
+            printstr(s, size, &off, num_buf + 2);
+            break;
+        case 's':
+            tmp_str = __va_arg(ap, const char *);
+            printstr(s, size, &off, tmp_str);
+            break;
+        }
+    }
+
+    return 0;
+}