Some system calls close by an if statement but others open after a variable is declared

Question

What's the difference with my question?

 fd = shm_open(argv[1], O_RDWR_O);
 if (fd == -1)
      errExit("shm_open");
 ....
 ....
 if(close(fd) == -1)
      errExit("close");     /* 'fd' is no longer needed */

I don't understand how the close statement closes the file descriptor with just an if statement?

Why don't they close it before-hand and then do the if statment?

Do you understand my question? Else I can try to think of how to rephrase it or elaborate..

Answer 1

You could write the second statement as;

int tmp; 

...

tmp = close(fd);
if(tmp == -1)
   errExit("close");

But since tmp serves no purpose after the check in the if (and a good compiler will indeed optimise it away), it has the same effect if you write it as the original code.

On the other hand, we can use assignments in if-statements, so:

 if ((fd = shm_open(argv[1], O_RDWR_O)) == -1)
      errExit("shm_open");

will also work.

The key here is that the first one does indeed store a value that we want to use later (if successfully opening the shared memory), so it's clearer to write it as two separate statements, rather than "hiding" it in the if-statement.

On the other hand, in the second case, we have no use for the result of close, so there's no purpose to writing an extra line and have an extra variable.