What is Fcntl in C?

The fcntl() function can be used to change the properties of a file that is already open. The fcntl() funciton is extremely versatile, and we will be looking at it more than once. The fcntl() function can be used for five different purposes, each purpose defined by an integer constant set as its second argument.

Is Fcntl a Posix?

With fcntl() (POSIX): you create a lock record on the file at filesystem level including process id. If the process dies or closes any filedescriptor to this file, the lock record gets removed by the system. A request for an exclusive lock shall fail if the file descriptor was not opened with write access.

What is the use of Fcntl?

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.

Is Fcntl a blocking call?

int flags = fcntl(fd, F_GETFL, 0); fcntl(fd, F_SETFL, flags | O_NONBLOCK); The code snippet above will configure such a descriptor for non-blocking access. If data is not available when you call read, then the system call will fail with a return value of -1 and errno is set to EAGAIN.

What is Fcntl API?

The fcntl (file control) API is used to retrieve or set the flags that are associrated with a socket or stream file.

What does Fcntl stand for?

What does fcntl stand for? fcntl stands for “file control”

What is O_rdwr?

The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file. The open() function will return a file descriptor for the named file that is the lowest file descriptor not currently open for that process. Open for reading only. O_WRONLY.

Is ioctl a blocking call?

The IOCTL call has many functions; establishing blocking mode is only one of its functions. The value in COMMAND determines which function IOCTL will perform. The REQARG of 0 specifies non-blocking (a REQARG of 1 would request that socket S be set to blocking mode).

What is O_ndelay?

The O_NDELAY flag causes read() or write() to return zero instead of blocking. Since read() also returns zero on end-of-file, it is difficult to distinguish the two cases. BSD also has an O_NDELAY flag that causes the error EWOULDBLOCK to be returned if the process would block.

What is O_creat in C?

What do you need to know about fcntl ( )?

The fcntl () commands that are supported are: Duplicates the descriptor. A third int argument must be specified. fcntl () returns the lowest descriptor greater than or equal to this third argument that is not already associated with an open file. This descriptor refers to the same object as descriptor and shares any locks.

How to duplicate file descriptors in fcntl ( 2 )?

Duplicated file descriptors (made with dup (2), fcntl (F_DUPFD), fork (2), etc.) refer to the same open file description, and thus share the same file status flags. The file status flags and their semantics are described in open (2) . F_GETFL ( void ) Return (as the function result) the file access mode and the file status flags; arg is ignored.

How are read and write locks in fcntl?

A read lock is placed on a readable file descriptor, and a write lock is placed on a writable file descriptor. More than one process may hold a read lock on the same file at the same time, but only one process may hold a write lock, and the same file may not be both locked for read and locked for write.

When to use F _ setsig in fcntl ( )?

If you set the O_ASYNC status flag on a file descriptor (either by providing this flag with the open (2) call, or by using the F_SETFL command of fcntl ()), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor. F_SETSIG can be used to obtain delivery of a signal other than SIGIO.