in order to better understanding the features and technical contents of the present invention , the present invention is hereinafter described in detail by incorporating with the accompanying drawings . however , the accompanying drawings are only for the convenience of illustration and description , no limitation is intended thereto . referring to fig2 and fig3 , an explosive view and a perspective view of an adjustable locking device in accordance with the first embodiment of the present invention are illustrated . the adjustable locking device can be applied to a connector for transmitting electronic signals or to a lock of luggage bag . in this particular embodiment , the locking device is applied to a connector . the connector includes a main body 10 , at least a link structure 20 and at least an actuating device 30 . the main body 10 can be a power plug , an ieee394 connector , a rj - 45 connector , a d - sub connector , a rca jack , a phone jack or an s terminal . in this particular embodiment , the main body 10 is a d - sub connector and is in a rectangular shape , which includes a connection port 11 , and a conductor portion 12 . the connection port 11 is extended from the front portion of the main body 10 , which includes a plurality of contact terminals disposed therein . a circular concave groove 111 ( shown in fig6 ) is formed at the outer periphery of the contact terminals . the concave grooves 111 provide a damp proof ring pad 13 to be inserted therein . a conductor portion 12 is disposed on the other end of the connection port 11 of the main body 10 . in addition , two semi - circular protrusive ears 14 are formed extending from the left and right sides of the main body 10 . a through hole 15 is formed on each of the protrusive ears 14 . furthermore , a sag region 18 is formed on the two protrusive ears 14 at the same side , where in this particular embodiment , the sag region 18 and the two protrusive ears 14 forming a connection portion 16 of the main body 10 . the link structure 20 can be appreciated as a single set or a double set structure being connected to the main body 10 . in this particular embodiment , the link structure 20 is of a double set structure . the link structure 20 includes a link element 21 and an adjusting element 22 . one end of the link element 21 can be connected to the connection portion 16 of the main body 10 . a horizontally arranged penetrating hole 211 and a groove hole 212 communicable with the penetrating hole 211 are formed on the link element 21 . the groove hole 212 provides a fastening element , such as a nut , to be disposed therein , while the penetrating hole 211 provides the adjusting element 22 to penetrate therethrough . in this particular embodiment , the adjusting element 22 is a screw , which penetrates through the through hole 15 of the main body 10 , the penetrating hole 211 of the link element 21 , the nut , and another through hole 15 of the main body 10 . the link element 21 is thus fastened in the sag region 18 of the connection portion 16 mentioned above . moreover , vertically arranged hole 213 is formed on the link element 21 . a locking hook 214 is formed extending from one end of the link element 21 . the actuating device 30 can also be appreciated as a single set or a double set structure that is pivotally connected to the link element 21 described above . should the actuating device 30 be a single set structure , the other end is then a fixed connection device ( not shown ). in this particular embodiment , the actuating device 30 is of a double set structure . the actuating device 30 includes a actuating element 31 and a buckling element 32 . the actuating element 31 is made of a metallic plate , which is directly curved into a u shape . a first through hole 311 and a second through hole 312 are formed on the plate of opposite ends . the first through hole 311 corresponds the hole 213 of the link element 21 for penetrating therethrough a shaft 33 , thereby pivotally connecting the link element 21 and the actuating element 31 together . the second through hole 312 provides the buckling element 32 to be pivotally connected . the buckling element 32 is made of a curved metallic wire , forming a rectangular frame that includes an opening end 321 and a closed end 322 . it is appreciated that the buckling element 32 can also be of semi - circular , triangular , trapezoidal or other geometrical shapes . a cylindrical shaft 323 is formed on each of the upper and lower opening ends 321 of the buckling element 32 . the cylindrical shaft 323 penetrates through the second through hole 312 of the actuating element . referring to fig4 and fig5 , the usage of the locking device in accordance with the first embodiment of the present invention are illustrated . referring also to fig6 , the sectional view of fig5 is illustrated . the combination of those elements set forth above provides the main body 10 to plug into a connection port 40 . the connection port 40 includes a socket 41 at the central portion thereof for inserting therein the main body 10 . a buckling hook 42 is formed at each of the left and right sides of the socket 41 . the buckling hooks 42 connect the closed end 322 of the locking element 32 . when using the locking device , the connection port 11 of the connector 10 is first disposed into the socket 41 of the connection port 40 . then , turning the actuating element 31 at two sides of the actuating device for the buckling element 32 to move forward . the closed end 322 of the buckling element 32 is then buckled on the corresponding buckling hook 42 of the connection port 40 . by turning the actuating element 31 backward about the shaft 33 and pressing the locking hook 214 of the link element 21 at the outer periphery end of the actuating element 31 , the connector main body 10 and the connection port 40 can then securely fastened and combined . referring to fig7 and fig8 , a perspective view and a sectional view of the locking device in accordance with the second embodiment of the present invention is illustrated . the connection portion 16 ′ of the connector main body 10 ′ is composed of a rectangular guiding hole 161 and two guiding tracks 162 outside of the guiding hole 161 . two guiding grooves 215 are formed on the link element 21 ′. the guiding grooves 215 cooperate with the guiding track 162 of the connection portion 16 ′ so as to dispose one end of the link element 21 ′ into the guiding hole 161 of the connection portion 16 ′. referring to fig9 and fig1 , a perspective view and a sectional view of the locking device in accordance with the third embodiment of the present invention are illustrated . the connection portion 16 ″ of the connector main body 10 ″ is composed of a rectangular guiding hole 161 , two guiding tracks 162 outside of the guiding hole 161 , and a vertical hole 17 communicable with the guiding hole 161 . a plurality of zigzag grooves 171 is formed at the top periphery of the vertical hole 17 . two protrusive bulks 216 are formed at one side of the link element 21 ″. a concave groove 217 is formed between the two protrusive bulks . the concave groove 217 is formed corresponding the vertical hole 17 of the connector main body 10 ″, and providing an adjusting element 22 ″ to penetrate therethrough . in this particular embodiment , the adjusting element is an eccentric shaft . a protrusive point 221 is formed at the top end of the adjusting element 22 ″. the protrusive point 221 can be disposed in the zigzag groove 171 of the vertical hole 17 . by rotating the adjusting element 22 ″, one can control the link element 21 ″ to move forward and backward . in summary , the locking device of the present invention indeed satisfies the utility , novelty and non - obviousness requirement of the patent law , a grant of letters patent is thus respectfully requested . however , since any person having ordinary skill in the art may readily find various equivalent alterations or modifications in light of the features as disclosed above , it is appreciated that the scope of the present invention is defined in the following claims . therefore , all such equivalent alterations or modifications without departing from the subject matter as set forth in the following claims is considered within the spirit and scope of the present invention .