DIAGNOSTICS
Kernel functions that indicate success or failure by means of either 0 or an
errno(2) value sometimes have a need to indicate that “special” handling is required at an upper layer or, in the case of
ioctl(2) processing, that “nothing was wrong but the request was not handled”. To handle these cases, some negative
errno(2) values are defined which are handled by the kernel before returning a different
errno(2) value to userland or simply zero.
The following is a list of the defined names and their meanings as given in <errno.h>. It is important to note that the value -1 is not used, since it is commonly used to indicate generic failure and leaves it up to the caller to determine the action to take.
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-2 EJUSTRETURN Modify regs, just return.
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No more work is required and the function should just return.
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-3 ERESTART Restart syscall.
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The system call should be restarted. This typically means that the machine dependent system call trap code will reposition the process's instruction pointer or program counter to re-execute the current system call with no other work required.
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-4 EPASSTHROUGH Operation not handled by this layer.
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The operation was not handled and should be passed through to another layer. This often occurs when processing ioctl(2) requests since lower layer processing may not handle something that subsequent code at a higher level will.
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-5 EDUPFD Duplicate file descriptor.
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This error is returned from the device open routine indicating that the l_dupfd field contains the file descriptor information to be returned to the caller, instead of the file descriptor that has been opened already. This error is used by cloning device multiplexors. Cloning device multiplexors open a new file descriptor and associate that file descriptor with the appropriate cloned device. They set l_dupfd to that new file descriptor and return EDUPFD. vn_open(9) takes the file descriptor pointed to by l_dupfd and copies it to the file descriptor that the open call will return.
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-6 EMOVEFD Move file descriptor.
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This error is similar to EDUPFD except that the file descriptor in l_dupfd is closed after it has been copied.
