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April 2005
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2005.04.11 14:17 "Re: TIFF_MAPPED", by Bob Friesenhahn
On Mon, 11 Apr 2005, katrina maramba wrote:
>
> Can anybody tell me what the TIFF_MAPPED macro means? It's comment
> says "file is mapped into memory". What does this mean? When can I
> have a 'memory mapped file'?
Libtiff normally memory maps input files on platforms where it makes
sense. You can re-inforce your desire for memory-mapped input by
adding 'M' to the TIFFOpen() option string, or prevent it by adding
'm' to the option string.
You can gain a bit more control over what libtiff does by using
TIFFClientOpen() to define the I/O functions it should use. One of
these functions is to memory map the file.
Please note that libtiff memory maps the entire file. It does not
attempt to do any "windowing" on the memory mapping. This means if
the input TIFF file is larger than the available process address
space, the mapping will fail.
/*
* Setup initial directory.
*/
switch (mode[0]) {
case 'r':
tif->tif_nextdiroff = tif->tif_header.tiff_diroff;
/*
* Try to use a memory-mapped file if the client
* has not explicitly suppressed usage with the
* 'm' flag in the open mode (see above).
*/
if ((tif->tif_flags & TIFF_MAPPED) &&
!TIFFMapFileContents(tif, (tdata_t*) &tif->tif_base, &tif->tif_size))
tif->tif_flags &= ~TIFF_MAPPED;
if (TIFFReadDirectory(tif)) {
tif->tif_rawcc = -1;
tif->tif_flags |= TIFF_BUFFERSETUP;
return (tif);
}
break;
Bob
======================================
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/