opengroup dup and dup2

NAME

dup, dup2 - duplicate an open file descriptor

SYNOPSIS

#include <unistd.h>
int dup(int fildes);
int dup2(int fildes, int fildes2);

EXAMPLES

Redirecting Standard Output to a File

The following example closes standard output for the current processes, re-assigns standard output to go to the file referenced by pfd, and closes the original file descriptor to clean up.

#include <unistd.h>
// ...
int pfd;
// ...
close(1);
dup(pfd);
close(pfd);
// ...

NOTE:

由于file descriptor的scope是process，所以file descriptor不存在进程间的race condition；并且dup函数能够在当前可用文件描述符中的最小值上对pfd进行赋值，所以在执行完成close(1)后，则当前可用文件描述符中的最小值就是1，所以上述能够保证dup(pfd)在1上进行dup；

Redirecting Error Messages

The following example redirects messages from stderr to stdout.

#include <unistd.h>
...
dup2(1, 2);
...

APPLICATION USAGE

Implementations may use file descriptors that must be inherited into child processes for the child process to remain conforming(一致性), such as for message catalog(消息目录) or tracing purposes. Therefore, an application that calls dup2() with an arbitrary integer for fildes2 risks non-conforming(不相符的) 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 streams. In order to avoid a race condition of leaking an unintended file descriptor into a child process, an application should consider opening all file descriptors with the FD_CLOEXEC bit set unless the file descriptor is intended to be inherited across exec.

NOTE: 实现可以使用必须继承到子进程的文件描述符，以使子进程保持一致，例如用于消息目录或跟踪目的。 因此，为fildes2调用带有任意整数的dup2的应用程序存在不符合行为的风险，而dup2只能用于覆盖应用程序通过显式操作获取的文件描述符值，或者用于三个文件描述符 对应于标准文件流。 为了避免将非预期文件描述符泄漏到子进程中的竞争条件，应用程序应考虑打开所有文件描述符并设置FD_CLOEXEC位，除非文件描述符是要跨exec继承的。