This section describes how you can perform various other operations on file descriptors, such as inquiring about or setting flags describing the status of the file descriptor, manipulating record locks, and the like. All of these operations are performed by the function fcntl.
The second argument to the fcntl function is a command that specifies which operation to perform. The function and macros that name various flags that are used with it are declared in the header file fcntl.h. Many of these flags are also used by the open function; see the section called “Opening and Closing Files”. int function>fcntl/function> (int filedes, int command, …) The fcntl function performs the operation specified by command on the file descriptor filedes. Some commands require additional arguments to be supplied. These additional arguments and the return value and error conditions are given in the detailed descriptions of the individual commands.
Briefly, here is a list of what the various commands are.
Duplicate the file descriptor (return another file descriptor pointing to the same open file). the section called “Duplicating Descriptors”.
Get flags associated with the file descriptor. the section called “File Descriptor Flags”.
Set flags associated with the file descriptor. the section called “File Descriptor Flags”.
Get flags associated with the open file. the section called “File Status Flags”.
Set flags associated with the open file. the section called “File Status Flags”.
Get a file lock. the section called “File Locks”.
Set or clear a file lock. the section called “File Locks”.
Like F_SETLK, but wait for completion. the section called “File Locks”.
Get process or process group ID to receive SIGIO signals. the section called “Interrupt-Driven Input”.
Set process or process group ID to receive SIGIO signals. the section called “Interrupt-Driven Input”.
This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time fcntl is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to fcntl should be protected using cancellation handlers.