This seem just dummy question.
What is a problem. I have some old project, that works on 8 bit prosecos. Now I reforming for arm7 procesor.
Ok, I find out, that in old procesor is int 16 bit type, in arm is 32 bit type, no problem. In rtl.h are definitions like U32 as unsigned int, ... and on the and U8 as unsigned char. Sizeof(U8) is 1 - 1 byte (8 bit).
Than i define my own structure for exaple:
typedef struct { U8 byte_1; U16 bytes_2; } MY_STRUCTURE;
sizeof(MY_STRUCTURE) is 4!!! How could be this posible?
Alignment. The U16 member is 16-bits wide, and so has to start on an even address. Because of this, the compiler has to add a hidden pad byte between byte_1 and bytes_2.
Most modern microprocessors requires data to be aligned. Some because of the increased speed of getting the data when it is aligned, allowing access with a single memory read, and some because the hardware is totally
unable to process unaligned integers (other than possibly emulated in an exception handler).
32-bit processors normally want their 16-bit variables on an even address, and their 32-bit or larger variables to be 4-byte aligned, i.e. have an address with the last two bits zero. Some processors wants (or requires) 64-bit integers, doubles etc to be 8-byte aligned.
Besides the width of the memory bus, most modern processors also makes use of a processor cache, and the cache is normally way wider than the memory interface, so a cache word may contain two, four or more memory words.
Thanke you for your quick reply.
... soo, the sizeof will not work, or can be sizeof somehow change? someone write own code like sizeof that works in this case?
sizeof() will return the true size. It has to include the padding byte because the padding does consume space.
"the sizeof will not work"
Yes, sizeof will work precisely as defined!
The key is a correct understanding of how sizeof works!
A decent 'C' textbook should explain this; See also: c-faq.com/.../endpad.html
I tried like I seid, sizeof(MY_STRUCTURE), but didne't work. I get value 4 not 3! Tested again. This is not such big problem, real problem is that I read data from SD card, for example 3 bytes, but I can't use memcpy(&my_structure, &readed_data[0], 3).
My real structures are 190 bytes big, have 20 subvariables.
Now I using just dummy way. Copy to buffer, than copy to my structure, subvariable by subvariable