If I have a text file with the following content opened as binary
1234567890
a call like this:
fseek(fp, 5L, SEEK_SET);
give me 6 when I call (char)fgetc(fp) because I offset 5 byte from byte 0 (not start from 1 but from 2)
but If I do:
fseek(fp, -3L, SEEK_END);
give me 8 and not 7 when I call (char)fgetc(fp).
Why? It seems as with SEEK_END the offset doesn't start from the previous byte after the last.
Answer
SEEK_END searches from the one-past last byte of the file:
1234567890 <--- bytes from the file
0123456789A <--- SEEK_SET-relative position
A9876543210 <--- SEEK_END-relative position (absolute value)
^
This is the (0, SEEK_END) byte
With this in mind, the very last byte of the file is the one found at (-1, SEEK_END) and thus the (-3, SEEK_END) byte is the 8.
Note that this is consistent with how C usually handles this kind of thing. For example a pointer to the end of a memory block will usually point to one-past the last byte of that block.
This also has the nice feature that you can get the size of the file with a call to fseek(SEEK_END) plus ftell(). No need to add or substract 1!
The man page from fseek() is a bit ambiguous about this issue, but compare with the man lseek that contains the same issue:
If whence is SEEK_END, the file offset shall be set to the size of the file plus offset.
In your example the size of the file is 10, and the offset is -3, so the final position is 10-3 = 7. And in offset 7 there is an 8.