What does dup2 do in Linux?
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What does dup2 do in Linux?
The dup2() function duplicates an open file descriptor. Specifically, it provides an alternate interface to the service provided by the fcntl() function using the F_DUPFD constant command value, with fildes2 for its third argument. The duplicated file descriptor shares any locks with the original.
Why would you use dup2 ()?
The dup2() system function is used to create a copy of an existing file descriptor. In Linux, there are 3 standard file descriptors. They are: stdin: This is the standard input file descriptor.
What is the difference between dup and dup2?
The difference between dup and dup2 is that dup assigns the lowest available file descriptor number, while dup2 lets you choose the file descriptor number that will be assigned and atomically closes and replaces it if it’s already taken.
Does dup2 overwrite?
Therefore, an application that calls dup2() with an arbitrary integer for fildes2 risks non-conforming behavior, and dup2() can only portably be used to overwrite file descriptor values that the application has obtained through explicit actions, or for the three file descriptors corresponding to the standard file …
Why do we need duplicate system call?/?
dup() system call is used to duplicate a file descriptor. It creates a copy of the old file descriptor to a new file descriptor. The new file descriptor can be system-generated or a number of your choice depending upon either you use dup() or dup2().
What is #include Fcntl H?
Description. The /usr/include/fcntl. h file defines the values that can be specified for the Command and Argument parameters of the fcntl subroutine and for the Oflag parameter of the open subroutine. The file-status flags of an open file are described in the following information. Flag Values for open Subroutine.
What is Fcntl function What is the purpose of using it?
The fcntl is a system call. It allows the program to place a read or a write lock. This function can be used to amend the file properties that are either opened already or can be opened through any action applied to it. It is a versatile function and is used to modify files in many ways like open, read and write, etc.
How do I restore stdout after dup2?
How to re-open STDOUT after closing it?
- Use the dup() system call to make a backup of the STDOUT file descriptor.
- close the STDOUT file descriptor.
- Perform any operations if needed.
- Then restore the STDOUT file descriptor using dup2() system call.
Does dup2 close the file descriptor?
If fildes is a valid descriptor and is equal to fildes2, dup2() returns fildes2 without closing it. If fildes is not a valid descriptor, dup2() fails and does not close fildes2.
How do you use Fcntl?
To place a lock on a file, first create and zero out a struct flock variable. Set the l_type field of the structure to F_RDLCK for a read lock or F_WRLCK for a write lock. Then call fcntl, passing a file descriptor to the file, the F_SETLCKW operation code, and a pointer to the struct flock variable.
Why we use Fcntl h in C?
You can help to develop the work, or you can ask for assistance in the project room. fcntl. h is the header in the C POSIX library for the C programming language that contains constructs that refer to file control, e.g. opening a file, retrieving and changing the permissions of file, locking a file for edit, etc.
