dup（dup2/dup3）

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NAME
       dup, dup2, dup3 - duplicate a file descriptor

SYNOPSIS
       #include <unistd.h>

       int dup(int oldfd);
       int dup2(int oldfd, int newfd);

       #define _GNU_SOURCE
       #include <unistd.h>

       int dup3(int oldfd, int newfd, int flags);

DESCRIPTION
       These system calls create a copy of the file descriptor oldfd.

       dup() uses the lowest-numbered unused descriptor for the new descriptor.

       dup2() makes newfd be the copy of oldfd, closing newfd first if necessary, but note the following:

       *  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.

       After  a successful return from one of these system calls, 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.

       dup3() is the same as dup2(), except that:

       *  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
       On success, these system calls return the new descriptor.  On error, -1 is returned, and errno is set appropriately.
       
ERRORS
       EBADF  oldfd isnan open file descriptor, or newfd is out of the allowed range for file descriptors.

       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.  Or, oldfd was equal to newfd.

       EMFILE The process already has the maximum number of file descriptors open and tried to open a new one.

1、dup用于复制文件描述符，返回新的描述符；dup2区别是可指定目标描述符，通常用于重定向或新打开文件；dup3能够设置close-on-exec 标志位的开启和关闭，它通过在flag参数位设置 O_CLOEXEC 来强行设置 close-on-exec 标志位的开启，使进程就无法够使用该文件描述符。

2、dup的返回值newfd和原oldfd指向的是同一文件，共享锁、读写指针（偏移量）、权限，不共享close-on-exec标志。