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UIO(9) Kernel Developer's Manual UIO(9) NAME uio, uiomove, uiomove_frombuf, uiomove_nofault -- device driver I/O routines SYNOPSIS #include <sys/types.h> #include <sys/uio.h> struct uio { struct iovec *uio_iov; /* scatter/gather list */ int uio_iovcnt; /* length of scatter/gather list */ off_t uio_offset; /* offset in target object */ ssize_t uio_resid; /* remaining bytes to copy */ enum uio_seg uio_segflg; /* address space */ enum uio_rw uio_rw; /* operation */ struct thread *uio_td; /* owner */ }; int uiomove(void *buf, int howmuch, struct uio *uiop); int uiomove_frombuf(void *buf, int howmuch, struct uio *uiop); int uiomove_nofault(void *buf, int howmuch, struct uio *uiop); DESCRIPTION The functions uiomove(), uiomove_frombuf(), and uiomove_nofault() are used to transfer data between buffers and I/O vectors that might possi- bly cross the user/kernel space boundary. As a result of any read(2), write(2), readv(2), or writev(2) system call that is being passed to a character-device driver, the appropriate driver d_read or d_write entry will be called with a pointer to a struct uio being passed. The transfer request is encoded in this structure. The driver itself should use uiomove() or uiomove_nofault() to get at the data in this structure. The fields in the uio structure are: uio_iov The array of I/O vectors to be processed. In the case of scatter/gather I/O, this will be more than one vector. uio_iovcnt The number of I/O vectors present. uio_offset The offset into the device. uio_resid The remaining number of bytes to process, updated after transfer. uio_segflg One of the following flags: UIO_USERSPACE The I/O vector points into a process's ad- dress space. UIO_SYSSPACE The I/O vector points into the kernel ad- dress space. UIO_NOCOPY Do not copy, already in object. uio_rw The direction of the desired transfer, either UIO_READ or UIO_WRITE. uio_td The pointer to a struct thread for the associated thread; used if uio_segflg indicates that the transfer is to be made from/to a process's address space. The function uiomove_nofault() requires that the buffer and I/O vectors be accessible without incurring a page fault. The source and destina- tion addresses must be physically mapped for read and write access, re- spectively, and neither the source nor destination addresses may be pageable. Thus, the function uiomove_nofault() can be called from con- texts where acquiring virtual memory system locks or sleeping are pro- hibited. The uiomove_frombuf() function is a convenience wrapper around uiomove() for drivers that serve data which is wholly contained within an existing buffer in memory. It validates the uio_offset and uio_resid values against the size of the existing buffer, handling short transfers when the request partially overlaps the buffer. When uio_offset is greater than or equal to the buffer size, the result is success with no bytes transferred, effectively signaling EOF. RETURN VALUES On success uiomove(), uiomove_frombuf(), and uiomove_nofault() will re- turn 0; on error they will return an appropriate error code. EXAMPLES The idea is that the driver maintains a private buffer for its data, and processes the request in chunks of maximal the size of this buffer. Note that the buffer handling below is very simplified and will not work (the buffer pointer is not being advanced in case of a partial read), it is just here to demonstrate the uio handling. /* MIN() can be found there: */ #include <sys/param.h> #define BUFSIZE 512 static char buffer[BUFSIZE]; static int data_available; /* amount of data that can be read */ static int fooread(struct cdev *dev, struct uio *uio, int flag) { int rv, amnt; rv = 0; while (uio->uio_resid > 0) { if (data_available > 0) { amnt = MIN(uio->uio_resid, data_available); rv = uiomove(buffer, amnt, uio); if (rv != 0) break; data_available -= amnt; } else tsleep(...); /* wait for a better time */ } if (rv != 0) { /* do error cleanup here */ } return (rv); } ERRORS uiomove() and uiomove_nofault() will fail and return the following er- ror code if: [EFAULT] The invoked copyin(9) or copyout(9) returned EFAULT In addition, uiomove_nofault() will fail and return the following error code if: [EFAULT] A page fault occurs. SEE ALSO read(2), readv(2), write(2), writev(2), copyin(9), copyout(9), sleep(9) HISTORY The uio mechanism appeared in some early version of Unix. AUTHORS This manual page was written by Jorg Wunsch. FreeBSD 13.2 March 11, 2017 UIO(9)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | ERRORS | SEE ALSO | HISTORY | AUTHORS
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