From 864ecde569ce9fad636abe1135de33fdc94e6981 Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Thu, 5 Sep 2024 19:49:07 -0400
Subject: [PATCH 1/1] src/vecteur.cc: skip undefined behavior with a bounds
 check

---
 src/vecteur.cc | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/vecteur.cc b/src/vecteur.cc
index de10197..119d454 100644
--- a/src/vecteur.cc
+++ b/src/vecteur.cc
@@ -8490,6 +8490,17 @@ namespace giac {
 	  }
 	  else {
 	    int C=col+1;
+	    // mjo: C can be equal to cmax here, which makes buffer[C]
+	    // illegal. In that case, however, nothing will happen
+	    // below:
+	    //
+	    // 1. C = cmax ==> C >= (cmax-4), so the "for" loop is skipped
+	    // 2. After the "for" loop, C += ptr-&buffer[C] sets C to zero
+	    // 3. Now C = cmax means that the second "for" loop is skipped
+	    //
+	    // As a result, we can comment out this whole thing when
+	    // C = cmax to avoid a crash.
+	    if (C < cmax) {
 	    longlong * ptr= &buffer[C],*ptrend=&buffer[0]+cmax-4;
 	    const int *ptrN=&Nline[C];
 	    for (;ptr<ptrend;ptrN+=4,ptr+=4){
@@ -8502,6 +8513,7 @@ namespace giac {
 	    for (;C<cmax;++C){
 	      buffer[C] -= coeff*Nline[C];   
 	    }
+	    }
 	  }
 	}
 	// copy back buffer to N[l]
-- 
2.44.2