Greetings,
I recently decided to install Ubuntu Version 9.10 on a new computer
and the installation came with gcc Version 4.4.1. I began moving a
number of programs that I have written over the years in "C" from my
old computer that runs Windows XP. I have written all of my programs
under the command line (DOS) window. I want to move them all to the
new computer and compile them to run under Ubuntu. Several of my
programs need to read and process image data from .BMP files. The
first 54 bytes in a .BMP file consist of the two structures (see
below) named WIN3XHEAD and followed immediately by
WIN3XINFOHEAD. The first thing that my programs do is to verify that
the file being read is a valid .BMP file and after compiling the
source code by gcc to run under Ubuntu, the .BMP file was flagged as
not being a valid .BMP file. Upon investigation, I determined that
gcc was padding the first structure to contain 16 bytes instead of 14
bytes and the extra two bytes of padding were located following the
first data item, short ImageFileType. The first two bytes that
should belong to the next data-item, long FileSize, were being stored
in the added 2 bytes of padding at the end of ImageFileType, and all
remaining bytes had been moved two bytes forward from where they
should be. This upset not only the first structure, but the second
structure and all of the image pixel data that follows.
I am an Electrical Engineer and spent the last 27 years of my career
employed as a Contract Software Engineer, writing industrial software
under contract for various Fortune 100 and 200 companies prior to
retiring in 2003 at the age of 70 years. I seem to recall that at
one time I encountered a similar problem while working on a project
for one of my clients. I don't recall what project it was or what
compiler or cpu I was using, but I seem to remember that there was a
compiler option that would enable the compiler to pad arrays to
32-byte boundaries versus 16-byte boundaries, presumably for the
purpose of allowing faster execution times at the expense of
increasing memory storage. I don't recall the name of the option,
but I suspect that this is a problem of that nature. I have spent
several days reading through gcc documentation but have not been able
to determine if there is such an option in gcc. In any case, I would
like to know if someone can suggest a solution to this problem. In
the mean time, I have temporarily resolved the problem by adding
extra code to measure the byte length of this first structure and if
it is 16 bytes, to fill the data fields by individually reading the
file data for each data-item within the structure. But I am hoping
that the discovery of an option will resolve the problem for my
remaining programs and for future use.
Sincerely,
Gene Creighton
www.charts-is-us.com
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typedef struct
{
short ImageFileType;
long FileSize;
short Reserved1;
short Reserved2;
long ImageDataOffset;
} WIN3XHEAD;
typedef struct
{
long HeaderSize;
long ImageWidth;
long ImageHeight;
short NumberOfImagePlanes;
short BitsPerPixel;
long CompressionMethod;
long SizeOfBitmap;
long HorzResolution;
long VertResolution;
long NumColorsUsed;
long NumSignificantColors;
} WIN3XINFOHEAD;
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