lib: enforce consistent `index()` behavior with empty needle argument

On macOS, the `memmem()` function returns `NULL` instead of the expected
start of the haystack string when given a zero-length needle argument.

Add special case handling for a zero-length needle argument to ensure
that the expected offset `0` is returned on all systems.

Ref: #176
Suggested-by: Erwan MAS <erwan@mas.nom.fr>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 4 insertions, 1 deletions

diff --git a/lib.c b/lib.c
index a875d46..1d18204 100644
--- a/lib.c
+++ b/lib.c
@@ -451,12 +451,15 @@ uc_index(uc_vm_t *vm, size_t nargs, bool right)
 					}
 					while (--p != sstr);
 				}
-				else {
+				else if (nlen > 0) {
 					p = (const char *)memmem(sstr, slen, nstr, nlen);
 
 					if (p)
 						ret = (ssize_t)(p - sstr);
 				}
+				else {
+					ret = 0;
+				}
 			}
 		}