Move decode_hex into its own file

- Error detection against strotul(3) has been improved, as done in other
places.
- New function encode_hex has been implemented, which will be used
by future commits.

1 files changed, 49 insertions, 0 deletions

diff --git a/hex.c b/hex.c
new file mode 100644
index 0000000..61cf55f
--- /dev/null
+++ b/hex.c
@@ -0,0 +1,49 @@
+#include "hex.h"
+#include <errno.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int hex_encode(const void *const b, char *hex, const size_t buflen,
+    size_t hexlen)
+{
+    const char *buf = b;
+
+    for (size_t i = 0; i < buflen; i++)
+    {
+        const int r = snprintf(hex, hexlen, "%02hhx", *(const char *)buf++);
+
+        if (r < 0 || r >= hexlen)
+            return -1;
+
+        hexlen -= r;
+        hex += 2;
+    }
+
+    return 0;
+}
+
+int hex_decode(const char *const hex, void *const b, size_t n)
+{
+    unsigned char *buf = b;
+
+    for (const char *s = hex; *s; s += 2)
+    {
+        const char nibble[sizeof "00"] = {*s, *(s + 1)};
+
+        if (!n)
+            return -1;
+
+        char *end;
+
+        errno = 0;
+        *buf++ = strtoul(nibble, &end, 16);
+
+        if (errno || *end)
+            return -1;
+
+        n--;
+    }
+
+    return n ? -1 : 0;
+}