diff --git a/src/global.c b/src/global.c
index 3c6141a6a0adfd4c788dd830952bcea18ae48b7f..4c9f56c83a2147ddae36100579f2f963dc25d2e1 100644
--- a/src/global.c
+++ b/src/global.c
@@ -614,7 +614,7 @@ void qh_initflags(char *command) {
   if (command <= &qh qhull_command[0] || command > &qh qhull_command[0] + sizeof(qh qhull_command)) {
     if (command != &qh qhull_command[0]) {
       *qh qhull_command= '\0';
-      strncat( qh qhull_command, command, sizeof( qh qhull_command));
+      strncat(qh qhull_command, command, sizeof(qh qhull_command)-strlen(qh qhull_command)-1);
     }
     while (*s && !isspace(*s))  /* skip program name */
       s++;
diff --git a/src/rboxlib.c b/src/rboxlib.c
index 4f11d22727c2779ef804448c9cfcd9e1221b6868..17d01ac3d8ab847aa56de684fb98ddbac4a0b26e 100644
--- a/src/rboxlib.c
+++ b/src/rboxlib.c
@@ -124,7 +124,7 @@ int qh_rboxpoints(FILE* fout, FILE* ferr, char* rbox_command) {
   }
 
   *command= '\0';
-  strncat(command, rbox_command, sizeof(command));
+  strncat(command, rbox_command, sizeof(command)-strlen(command)-1);
 
   while (*s && !isspace(*s))  /* skip program name */
     s++;
@@ -346,8 +346,8 @@ int qh_rboxpoints(FILE* fout, FILE* ferr, char* rbox_command) {
     }
   }else if (israndom) {
     seed= (int)time(&timedata);
-    sprintf(seedbuf, " t%d", seed);  /* appends an extra t, not worth removing */
-    strncat(command, seedbuf, sizeof(command));
+    printf(seedbuf, " t%d", seed);  /* appends an extra t, not worth removing */
+    strncat(command, seedbuf, sizeof(command)-strlen(command)-1);
     t= strstr(command, " t ");
     if (t)
       strcpy(t+1, t+3); /* remove " t " */