int igraph_strvector_set2(igraph_strvector_t *sv, long int idx, 
			  const char *value, int len);

This is almost the same as igraph_strvector_set, but the new value is not a zero terminated string, but its length is given.

Arguments: 

Returns: 

Time complexity: O(l), the length of the new string. Maybe more, depending on the memory management, if reallocation is needed.