what's the purpose of fcntl with parameter F_DUPFD

Question

I　traced an oracle process, and find it first open a file /etc/netconfig as file handle 11, and then duplicate it as 256 by calling fcntl with parameter F_DUPFD, and then close the original file handle 11. Later it read using file handle 256. So what's the point to duplicate the file handle? Why not just work on the original file handle?

12931:   0.0006 open("/etc/netconfig", O_RDONLY|O_LARGEFILE)    = 11
12931:   0.0002 fcntl(11, F_DUPFD, 0x00000100)                  = 256
12931:   0.0001 close(11)                                       = 0
12931:   0.0002 read(256, " # p r a g m a   i d e n".., 1024)   = 1024
12931:   0.0003 read(256, " t s           t p i _ c".., 1024)   = 215
12931:   0.0002 read(256, 0x106957054, 1024)                    = 0
12931:   0.0001 lseek(256, 0, SEEK_SET)                         = 0
12931:   0.0002 read(256, " # p r a g m a   i d e n".., 1024)   = 1024
12931:   0.0003 read(256, " t s           t p i _ c".., 1024)   = 215
12931:   0.0003 read(256, 0x106957054, 1024)                    = 0
12931:   0.0001 close(256)                                      = 0

Answer 1

On some systems, like Solaris, standard I/O with FILE only works with file descriptors 0-255 because its implementation of the FILE structure uses an 8-bit integer instead of int. If a program uses a lot of file descriptors, it's useful to reserve file descriptors 3-255 using fnctl(fd, F_DUPFD, 256). Otherwise, functions like fopen(), freopen() and fdopen() will fail if you have 256 files open.

Answer 2

Here is another example when a technique of reserving low-numbered file descriptors is needed.

Assume that a process opens a large number of file descriptor e.g. it accepts more than 1024 simultaneous socket connections. At the same time the process also uses third party library that opens socket connections and uses select() to see if sockets are ready for reading or writing. Additionally the third party library was compiled with __FD_SETSIZE set to 1024 (default value).

If the library opens a socket when all file descriptors below 1024 are in use then it will get a descriptor that select() and associated FD_* macros can not cope with. This will result in process crashing or undefined behaviour.