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Name

Synopsis

#include <unistd.h>
int dup(int oldfd);int dup2(int oldfd, int newfd);
#define _GNU_SOURCE             /* See feature_test_macros(7) */#include
<fcntl.h>              /* Obtain O_* constant definitions */#include <unistd.h>
int dup3(int oldfd, int newfd, int flags);

Description

After a successful return, the old and new file descriptors may be used interchangeably. They refer to the same open file description (see open(2) ) and thus share file offset and file status flags; for example, if the file offset is modified by using lseek(2) on one of the descriptors, the offset is also changed for the other.

The two descriptors do not share file descriptor flags (the close-on-exec flag). The close-on-exec flag (FD_CLOEXEC; see fcntl(2) ) for the duplicate descriptor is off.

dup2()

The steps of closing and reusing the file descriptor newfd are performed atomically. This is important, because trying to implement equivalent functionality using close(2) and dup() would be subject to race conditions, whereby newfd might be reused between the two steps. Such reuse could happen because the main program is interrupted by a signal handler that allocates a file descriptor, or because a parallel thread allocates a file descriptor.

Note the following points:

*

If oldfd is not a valid file descriptor, then the call fails, and newfd is not closed.

*

If oldfd is a valid file descriptor, and newfd has the same value as oldfd, then dup2() does nothing, and returns newfd.

dup3()

*

The caller can force the close-on-exec flag to be set for the new file descriptor by specifying O_CLOEXEC in flags. See the description of the same flag in open(2) for reasons why this may be useful.

*

If oldfd equals newfd, then dup3() fails with the error EINVAL.

Return Value

Errors

EBADF

oldfd isn’t an open file descriptor.

EBADF

newfd is out of the allowed range for file descriptors (see the discussion of RLIMIT_NOFILE in getrlimit(2) ).

EBUSY

(Linux only) This may be returned by dup2() or dup3() during a race condition with open(2) and dup().

EINTR

The dup2() or dup3() call was interrupted by a signal; see signal(7) .

EINVAL

(dup3()) flags contain an invalid value.

EINVAL

(dup3()) oldfd was equal to newfd.

EMFILE

The process already has the maximum number of file descriptors open and tried to open a new one (see the discussion of RLIMIT_NOFILE in getrlimit(2) ).

Versions

Conforming to

dup3() is Linux-specific.

Notes

If newfd was open, any errors that would have been reported at close(2) time are lost. If this is of concern, then--unless the program is single-threaded and does not allocate file descriptors in signal handlers--the correct approach is not to close newfd before calling dup2(), because of the race condition described above. Instead, code something like the following could be used:

    /* Obtain a duplicate of ’newfd’ that can subsequently
       be used to check for close() errors; an EBADF error
       means that ’newfd’ was not open. */
    tmpfd = dup(newfd);
    if (tmpfd == -1 && errno != EBADF) {
        /* Handle unexpected dup() error */
    }
    /* Atomically duplicate ’oldfd’ on ’newfd’ */
    if (dup2(oldfd, newfd) == -1) {
        /* Handle dup2() error */
    }
    /* Now check for close() errors on the file originally
       referred to by ’newfd’ */
    if (tmpfd != -1) {
        if (close(tmpfd) == -1) {
            /* Handle errors from close */
        }
    }

See Also

Colophon

Table of Contents

• Name
• Synopsis
• Description
• dup2()
• dup3()
• Return Value
• Errors
• Versions
• Conforming to
• Notes
• See Also
• Colophon