The maximum system file descriptor, ordinarily 2. System file descriptors are passed toexec()ed processes, while higher file descriptors are not. Also, during anopen(), system file descriptors are preserved even if theopen() fails (ordinary file descriptors are closed before theopen() is attempted). The close-on-exec status of a file descriptor will be decided according to the value of$^F when the corresponding file, pipe, or socket was opened, not the time of theexec().
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