216 lines
4.4 KiB
HTML
216 lines
4.4 KiB
HTML
<HTML
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>mmap()</TITLE
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TITLE="System calls"
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TITLE="close()"
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>The Linux SCSI Generic (sg) HOWTO</TH
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><TD
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>Chapter 7. System calls</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="MMAP">7.6. mmap()</H1
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><DIV
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CLASS="FORMALPARA"
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><P
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><B
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>mmap(void * start, size_t length, int prot,
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int flags, int sg_fd, off_t offset). </B
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>This system call returns a pointer to the beginning of the reserved buffer
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associated with the sg file descriptor 'sg_fd'. The 'start' argument is
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a hint to the kernel and is ignored by this driver; best set it to 0.
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The 'length' argument should be less than or equal to the size of the
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reserved buffer associated with 'sg_fd'. If it exceeds the
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reserved buffer size (after 'length' has been rounded up to a page size
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multiple) then MAP_FAILED is returned and ENOMEM is placed in errno. The
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'prot' argument should either be PROT_READ or (PROT_READ | PROT_WRITE).
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The 'flags' argument should contain MAP_SHARED. In a sense, the user
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application is "sharing" data with the sg driver. The MAP_PRIVATE flag
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does not play well with compiler optimization flags such as '-O2'.
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The 'offset' argument must be set to 0 (or NULL).</P
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></DIV
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><P
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>The mmap() system call can be made multiple times on the same sg_fd.
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The munmap() system call is not required if close() is called on
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sg_fd. Mmap-ed IO is well-behaved when a process is fork()-ed
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(or the equivalent finer grained clone() system call is made).
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In the case of a fork(), 2 processes will be sharing the same memory mapped
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area together with the sg driver for a sg_fd and the last one to
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close the sg_fd (or exit) will cause the shared memory to be freed.</P
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><P
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>It is assumed that if the default reserved buffer size of 32 KB is
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not sufficient then a ioctl(SG_SET_RESERVED_SIZE) call is made
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prior to any calls to mmap(). If the required size is not a multiple
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of the kernel's page size (returned by getpagesize() system call) then
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the size passed to ioctl(SG_SET_RESERVED_SIZE) should be rounded up
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to the next page size multiple.</P
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><P
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>Mmap-ed IO is requested by setting (or or-ing in) the SG_FLAG_MMAP_IO
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constant into the flag member of the the sg_io_hdr structure prior to
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a call to write() or ioctl(SG_IO). The logic to do mmap-ed IO _assumes_
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that an appropriate mmap() call has been made by the application. In
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other words it does not check.
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<A
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NAME="AEN477"
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HREF="#FTN.AEN477"
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>[1]</A
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></P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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WIDTH="5%"
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><A
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NAME="FTN.AEN477"
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HREF="mmap.html#AEN477"
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>[1]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>The sg driver does record that the mmap() system call has been invoked
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at least once on a file descriptor. This is not sufficient because the
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given 'length' may be too short for the current IO. Also the driver is
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unaware of munmap() calls so it could easily be tricked.</P
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></TD
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>Next</A
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WIDTH="33%"
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VALIGN="top"
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>close()</TD
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><TD
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WIDTH="34%"
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VALIGN="top"
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VALIGN="top"
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>fcntl(sg_fd, F_SETFL, oflags | FASYNC)</TD
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