A sample test run on my machine.

$ id ashwin
  uid=1000(ashwin) gid=1000(ashwin) groups=1000(ashwin),118(admin)

$ id ganti
  uid=1001(ganti) gid=1001(ganti) groups=1001(ganti),117(fuse)
  
$ ./test_caphash 1000@1001 randomstring
Capability successfully written to the /dev/caphash 

$ ./test_capuse 1000@1001@randomstring &
[1] 22677
$ Capability successfully written to the /dev/capuse 
The user id of the process is now changed to the target user specified. Please do a 'ps -aux | grep capuse' and verify. 

$ ps aux | grep capuse
ganti    22677  0.0  0.0   1504   316 pts/4    S    09:53   0:00 ./test_capuse 1001@1000@randomstring
ashwin   22679  0.0  0.0   2884   764 pts/4    S+   09:53   0:00 grep capuse

Note that the user name has changed to 'ganti'.