First of all, I'd like to thank the people and the moderators who have helped me by answering my questions. Hopefully When I get better, I will help others in return for your favors.

my question is regarding the fcntl expression in the <fcntl.h> library.
the describtion of this expression is:

//-----------------------------------------------------------------------------------------
The fcntl() function shall perform the operations described below on open files. The fildes argument is a file descriptor.
//-----------------------------------------------------------------------------------------


what is a file descriptor? It was also mentioned for the expression read/write, as follow:


//-----------------------------------------------------------------------------------------
The read() function shall attempt to read nbyte bytes from the file associated with the open file descriptor, fildes, into the buffer pointed to by buf. The behavior of multiple concurrent reads on the same pipe, FIFO, or terminal device is unspecified.
//-----------------------------------------------------------------------------------------



Thank you.