id = semget(IPC_PRIVATE, 1, 0);

f = fopen("/tmp/miID", "w");
fwrite(f, id);
fclose(f);
.
.
.

f = fopen("/tmp/myID", "r");
fread(f, &tmp);
fclose(f);

id = semget(tmp, 1, 0);
.
.
.