tools/nolibc/printf: Prepend sign to converted number

Instead of appending the converted number to the sign, convert first
and then prepend the sign (or "0x").
Use the length returned by u64toh_r() instead of calling strlen().

Needed so that zero padding can be inserted between the sign and digits
in an upcoming patch.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260308113742.12649-12-david.laight.linux@gmail.com
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index a377f164c70d9d8d5b4adbf29c6157eb1889286e..3620aa643c814e180dffce6c7dc43e5b91159eb6 100644 (file)
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -345,9 +345,10 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
        long long signed_v;
        int written, width, len;
        unsigned int flags, ch_flag;
-       char outbuf[21];
+       char outbuf[2 + 22 + 1];
        char *out;
        const char *outstr;
+       unsigned int sign_prefix;
 
        written = 0;
        while (1) {
@@ -445,32 +446,49 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
                                goto do_strlen_output;
                        }
 
-                       out = outbuf;
+                       /* The 'sign_prefix' can be zero, one or two ("0x") characters.
+                        * Prepended least significant byte first stopping on a zero byte.
+                        */
+                       sign_prefix = 0;
 
                        if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'd', 'i')) {
                                /* "%d" and "%i" - signed decimal numbers. */
                                if (signed_v < 0) {
-                                       *out++ = '-';
+                                       sign_prefix = '-';
                                        v = -(signed_v + 1);
                                        v++;
                                }
                        }
 
+                       /* The value is converted offset into the buffer so that
+                        * the sign/prefix can be added in front.
+                        * The longest digit string is 22 + 1 for octal conversions, the
+                        * space is reserved even though octal isn't currently supported.
+                        */
+                       out = outbuf + 2;
+
                        /* Convert the number to ascii in the required base. */
                        if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'd', 'i', 'u')) {
                                /* Base 10 */
-                               u64toa_r(v, out);
+                               len = u64toa_r(v, out);
                        } else {
                                /* Base 16 */
                                if (_NOLIBC_PF_FLAGS_CONTAIN(ch_flag, 'p')) {
-                                       *(out++) = '0';
-                                       *(out++) = 'x';
+                                       /* "%p" needs "0x" prepending. */
+                                       sign_prefix = '0' << 8 | 'x';
                                }
-                               u64toh_r(v, out);
+                               len = u64toh_r(v, out);
                        }
 
-                       outstr = outbuf;
-                       goto do_strlen_output;
+                       /* Add the 0, 1 or 2 ("0x") sign/prefix characters at the front. */
+                       for (; sign_prefix; sign_prefix >>= 8) {
+                               /* Force gcc to increment len inside the loop. */
+                               _NOLIBC_OPTIMIZER_HIDE_VAR(len);
+                               len++;
+                               *--out = sign_prefix;
+                       }
+                       outstr = out;
+                       goto do_output;
                }
 
                if (ch == 'm') {