How to specify the libpq correct connection string URL for Unix domain socket connection in the shell?

https://www.postgresql.org/docs/current/libpq-example.html
compile the 1st example source code.

gcc test.c -L/usr/local/pgsql15/lib -I/usr/local/pgsql15/include -lpq

Cannot figure out the connection string. All the following failed.

./a.out postgresql:///test15?host=/tmp&port=5415
./a.out postgresql://test15?host=/tmp:5415
./a.out postgresql://test15?host=/tmp&port='5415'
./a.out postgresql://test15?host=/tmp:'5415'
./a.out postgresql://test15?host=/tmp&port=/tmp
./a.out postgresql:///tmp:5415/test15?

The error is almost similar:

connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such file or directory
        Is the server running locally and accepting connections on that socket?

[1]+  Exit 1                  ./a.out postgresql:///test15?host=/tmp

cat /usr/local/pgsql15/data_42091266/postgresql.conf | rg unix return

unix_socket_directories = '/tmp'        # comma-separated list of directories
#unix_socket_group = ''                 # (change requires restart)
unix_socket_permissions = 0777          # begin with 0 to use octal notation

Thanks to jjanes
The following two way works.

./a.out postgresql:///test15?host=/tmp\&port=5415

./a.out "postgresql:///test15?host=/tmp&port=5415"

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.htmlhttps://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

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