Langue: en

Version: 266345 (debian - 07/07/09)

Section: 2 (Appels système)

BSD mandoc


dup dup2 - duplicate an existing file descriptor


Lb libc


In unistd.h Ft int Fn dup int oldd Ft int Fn dup2 int oldd int newd


The Fn dup system call duplicates an existing object descriptor and returns its value to the calling process Fa ( newd = Fn dup oldd ) . The argument Fa oldd is a small non-negative integer index in the per-process descriptor table. The value must be less than the size of the table, which is returned by getdtablesize(2). The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.

The object referenced by the descriptor does not distinguish between Fa oldd and Fa newd in any way. Thus if Fa newd and Fa oldd are duplicate references to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open(2) system call. The close-on-exec flag on the new file descriptor is unset.

In Fn dup2 , the value of the new descriptor Fa newd is specified. If this descriptor is already in use and Fa oldd != Fa newd , the descriptor is first deallocated as if the close(2) system call had been used. If Fa oldd is not a valid descriptor, then Fa newd is not closed. If Fa oldd == Fa newd and Fa oldd is a valid descriptor, then Fn dup2 is successful, and does nothing.


The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.


The Fn dup and Fn dup2 system calls fail if:
The Fa oldd or Fa newd argument is not a valid active descriptor
Too many descriptors are active.


accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2)


The Fn dup and Fn dup2 system calls are expected to conform to St -p1003.1-90 .


The Fn dup and Fn dup2 functions appeared in AT&T System v7 .