So I have a very simple program that reads the 3 first bytes of a file:
int main(void)
{
FILE *fd = NULL;
int i;
unsigned char test = 0;
fd = fopen("test.bmp", "r");
printf("position: %ld\n", ftell(fd));
for (i=0; i<3; i++) {
fread(&test, sizeof (unsigned char), 1, fd);
printf("position: %ld char:%X\n", ftell(fd), test);
}
return (0);
}
When I try it with a text file it works fine:
position: 0
position: 1 char: 61
position: 2 char: 62
position: 3 char: 63
but when I run it with a PNG for example I get:
position: 0
position: 147 char:89
position: 148 char:50
position: 149 char:4E
Note that the 3 first bytes of the file are indeed 89 50 4E but I don't know where the 147 comes from.
With a bmp file I get:
position: 0
position: -1 char:42
position: 0 char:4D
position: 1 char:76
Do you know where these first positions come from?
Thanks a lot for your help
Answer
You need to open the file in binary mode:
fd = fopen("test.bmp", "rb");
If you try to read a binary file like a bitmap in text mode, the bytes corresponding to carriage returns and linefeeds confuse things.
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