Hi Folks, I am trying to use enum to define a set of named constants with values so that they can be used by multiple C files in my project. For example. I use the following snipet to define Months in a headers file externals.h, and then If I then try to use it as follows in any of those files, it does not work.
enum MONTHS {M1 = 'Jan', M2 = 'Feb', M3 = 'Mar'}; .... .... extern enum MONTHS; // should it be extern enum MONTHS months; ? .... ..... SendByte (M1);//(This routine display M1 on my serial port).
I get error ---> (...error 230 MONTH unknown struct/union/enum tag)
What is the problem and how can I fix it ? I would like to get a better understanding of this enum business, if possible..
Thanks.
Bhal Tulpule
const char *p = (const char*)&var3; printf("first byte: %02x, second byte: %02x.\n",p[0],p[1]);
It is here that architecture really starts to matter.
So can we use the above code to test Endianess?
It is more obvious what you do, if you skip non-portable multi-character constants and write:
uint16_t endian_val = 0x1234; uint8_t *endian_test = (uint8_t*)&endian_val; if (endian_test[0] == 0x12 && endian_test[1] == 0x34) { printf("Found most significant byte on low address - big-endian machine.\n"); } else if (endian_test[0] = 0x34 && endian_test[1] == 0x12) { printf("Found least significant byte on low address - little-endian machine.\n"); } else { printf("Something weird is going on. Should normally only have two alternatives for a 16-bit variable...\n"); }
But it is way more common to do the reverse - create an array and then view it as an integer:
uint8_t endial_val[4] = {0x12, 0x34, 0x45, 0x67}; if (0x12345678 == *(uint32_t*)&endian_val) { ... } else if (0x78563412 == *(uint32_t*)&endian_val) { ... } else { ... }
Obviously, a four byte large integer would have more options, since it could be little-endian for 16-bit sub-groups but big-endian betwen the two 16-bit sub-groups.
ya, the above methods are the widely known method.
const char *p = (const char*)&var3; printf("first byte: %02x, second byte: %02x.\n",p[0],p[1]); This can be used as a new way around of doing it. also the var3 can be a 4byte long int for more options.
...it could be little-endian for 16-bit sub-groups but big-endian betwen the two 16-bit sub-groups.
do controller have such architectures? May be 16bit controllers?
A controller must obviously work with a word size larger than two bytes, to be able to be partially big-endian and partially little-endian.
The most common is a 16-bit controller that are big or little endian for the native word size. But have dedicated instructions that allows two 16-bit registers to be saved/loaded as one 32-bit value. Like when 16'16 multiply produces a 32-bit result or 32/16 produces a 16-bit result. In that case, the instruction set may write the two registers in the wrong order.
Similar issues is of course possible when looking at 64-bit processors, or 32-bit processors with partial 64-bit support.
"...to be able to be partially big-endian and partially little-endian"
Or it could be implemented in software by the compiler