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A microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC).
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stdout(4) [freebsd man page]

FD(4)							   BSD Kernel Interfaces Manual 						     FD(4)

NAME
fd, stdin, stdout, stderr -- file descriptor files DESCRIPTION
The files /dev/fd/0 through /dev/fd/# refer to file descriptors which can be accessed through the file system. If the file descriptor is open and the mode the file is being opened with is a subset of the mode of the existing descriptor, the call: fd = open("/dev/fd/0", mode); and the call: fd = fcntl(0, F_DUPFD, 0); are equivalent. Opening the files /dev/stdin, /dev/stdout and /dev/stderr is equivalent to the following calls: fd = fcntl(STDIN_FILENO, F_DUPFD, 0); fd = fcntl(STDOUT_FILENO, F_DUPFD, 0); fd = fcntl(STDERR_FILENO, F_DUPFD, 0); Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored. IMPLEMENTATION NOTES
By default, /dev/fd is provided by devfs(5), which provides nodes for the first three file descriptors. Some sites may require nodes for additional file descriptors; these can be made available by mounting fdescfs(5) on /dev/fd. FILES
/dev/fd/# /dev/stdin /dev/stdout /dev/stderr SEE ALSO
tty(4), devfs(5), fdescfs(5) BSD
June 9, 1993 BSD

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FD(4)							   BSD Kernel Interfaces Manual 						     FD(4)

NAME
fd, stdin, stdout, stderr -- file descriptor files DESCRIPTION
The files /dev/fd/0 through /dev/fd/# refer to file descriptors which can be accessed through the file system. If the file descriptor is open and the mode the file is being opened with is a subset of the mode of the existing descriptor, the call: fd = open("/dev/fd/0", mode); and the call: fd = fcntl(0, F_DUPFD, 0); are equivalent. Opening the files /dev/stdin, /dev/stdout and /dev/stderr is equivalent to the following calls: fd = fcntl(STDIN_FILENO, F_DUPFD, 0); fd = fcntl(STDOUT_FILENO, F_DUPFD, 0); fd = fcntl(STDERR_FILENO, F_DUPFD, 0); Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored. IMPLEMENTATION NOTES
By default, /dev/fd is provided by devfs(5), which provides nodes for the first three file descriptors. Some sites may require nodes for additional file descriptors; these can be made available by mounting fdescfs(5) on /dev/fd. FILES
/dev/fd/# /dev/stdin /dev/stdout /dev/stderr SEE ALSO
tty(4), devfs(5), fdescfs(5) BSD
June 9, 1993 BSD

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