diff -r 7fd13b9f8f60 -r d315a068235a src/array.c
--- a/src/array.c	Sat Aug 10 11:12:49 2019 +0200
+++ b/src/array.c	Tue Sep 24 20:16:00 2019 +0200
@@ -35,6 +35,7 @@
 
 #include <string.h>
 #include <stdlib.h>
+#include <errno.h>
 
 #ifndef UCX_ARRAY_DISABLE_QSORT
 #ifdef __GLIBC__
@@ -67,6 +68,51 @@
     }
 }
 
+int ucx_array_util_set_a(UcxAllocator* alloc, void** array, size_t* capacity,
+    size_t elmsize, size_t index, ...) {
+    
+    if(!alloc || !capacity || !array) {
+        errno = EINVAL;
+        return 1;
+    }
+    
+    size_t newcapacity = *capacity;
+    while(index >= newcapacity) {
+        if(ucx_szmul(newcapacity, 2, &newcapacity)) {
+            errno = EOVERFLOW;
+            return 1;
+        }        
+    }
+
+    size_t memlen, offset;
+    if(ucx_szmul(newcapacity, elmsize, &memlen)) {
+        errno = EOVERFLOW;
+        return 1;
+    }
+    /* we don't need to check index*elmsize - it is smaller than memlen */
+    
+    
+    void* newptr = alrealloc(alloc, *array, memlen);
+    if(newptr == NULL) {
+        errno = ENOMEM; /* we cannot assume that every allocator sets this */
+        return 1;
+    }
+    *array = newptr;
+    *capacity = newcapacity;
+    
+    
+    char* dest = *array;
+    dest += elmsize*index;
+
+    va_list ap;
+    va_start(ap, index);
+    int elem = va_arg(ap, int);    
+    memcpy(dest, &elem, elmsize);
+    va_end(ap);
+    
+    return 0;
+}
+
 UcxArray ucx_array_new(size_t capacity, size_t elemsize) {
     return ucx_array_new_a(capacity, elemsize, ucx_default_allocator());
 }