test.c

size_t size = 0;
char* buffer = NULL;
while ( true )
{
	size = size ? 2 * size : 4096;
	buffer = malloc(size);
	if ( !buffer )
		err(1, "malloc");
	ssize_t used = readlink(...);
	/* error check */
	if ( (size_t) used == size )
	{
		free(buffer);
		continue;
	}
	/* nul terminate */
	break;
}

printf("%s\n", buffer);