-----Original Message----- From: kernelnewbies-bounce@nl.linux.org [mailto:kernelnewbies-bounce@nl.linux.org]On Behalf Of Amith Sent: Wednesday, February 11, 2004 12:26 AM To: kernelnewbies Subject: Endianess . hi all, I had read about Little Endian and Big Endian byte ordering from some resources on the net. I had read that this doesnt affect char arrays and hence they can be used to overcome this problem. My question is char temp_char[2]; unsigned short temp_short; Don't they finally end up to 2 bytes of physical memory ? and so why is it that it doesnt affect char arrays . iam sure that i have missed something .Please throw some light on this one. thank you. cheers, amith Endianess refers to how any value larger than 8 bits (char) is stored in memory and is processor / device specific. The storage method or "endianess" is only an issue when the processor your code is running on uses a storage method different than that expected by another processor or bus device. Processors like Intel's Pentium use "little endian" storage. Motorola and PowerPC processors and most bus devices use what is called "big endian" storage. Though there are many devices that allow you to specify the endianess of a memory region they are accessing. Here are some specific examples: char foo[4] = {0, 1, 2, 3}; appears in memory as: 0x100: 0x00, 0x01, 0x02, 0x03 This is true of both big endian and little endian architectures because there is only 8 bits stored in each array element. short foo[4] = {0, 1, 2, 3}; appears in memory as: < foo[0] > < foo[1] > < foo[2] > < foo[3] > foo: 0x00, 0x00, 0x00, 0x01, 0x00, 0x02, 0x00, 0x03 foo: 0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x03, 0x00 < foo[0] > < foo[1] > < foo[2] > < foo[3] > The first representation of a short above uses "big endian" storage. The second uses "little endian" storage. As you can see from the illustration, if two devices are using a different storage technique and a common storage format for exchanged messages is not enforced, they will misinterpret each other's data. When networks were first being developed a common or "network order" was established for the transmission of information. This network order also matches "big endian". Most well written and portable network code will use macros to store values larger than 8 bits (one character) into memory for consumption by another bus device. These macros are compiled out on big endian systems and force the proper ordering in memory in little endian systems. I hope this explanation helps clear things up for you. - Dennis - -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
