Loading 32-bit hexadecimal into char array

Question

I am trying to load 32-bit hexadecimal into char array.

#define NUC 0xA8051701

unsigned char debug_msg[100];
sprintf (debug_msg, "%08x", NUC);

But it is loading only "A805" that to as ASCII character instead of hexadecimal. Can any one suggest what could be the problem.

What I'm actually looking for is:

debug_msg[0]=0xA8
debug_msg[1]=0x05
debug_msg[2]=0x17
debug_msg[3]=0x01

Instead of the correct value, it is loading as below:

debug_msg[0]=0x30
debug_msg[1]=0x30
debug_msg[2]=0x30
debug_msg[3]=0x30
debug_msg[4]=0x61
debug_msg[5]=0x38
debug_msg[6]=0x30
debug_msg[7]=0x35

Effectively it is loading 0x0000a805 that to in ASCII.

What is the size of int on your machine? Is it a big-endian architecture? What do you actually want as the result? (I guess that sizeof(int) == 2 — 16 bits, and it is big-endian, and you want debug_msg[0] = 0xA8; debug_msg[1] = 0x05; debug_msg[2] = 0x17; debug_msg[3] = 0x01; — but that's guesswork. You should certainly explain what you want and it might help to identify the processor type and so on.) — Jonathan Leffler
– Jonathan Leffler, Commented Mar 24, 2017 at 5:39
"But it is loading only "A805" that to as ascii character instead of hexadecimal." . How did u confirmed that? — jjm
– jjm, Commented Mar 24, 2017 at 5:55
@Vinodkumar Please let me know the output of printf("%s",debug_msg); — jjm
– jjm, Commented Mar 24, 2017 at 6:02
Replace NUC with a variable holding the value to convert. The whole thing macroizes easily. But you should have explained all this in the question without us having to dig it out tidbit by tidbit. — Jonathan Leffler
– Jonathan Leffler, Commented Mar 24, 2017 at 6:42

Jonathan Leffler · Accepted Answer · 2017-03-24 15:58:09Z

3

On the face of it, you need:

 debug_msg[0] = (NUC >> 24) & 0xFF;
 debug_msg[1] = (NUC >> 16) & 0xFF;
 debug_msg[2] = (NUC >>  8) & 0xFF;
 debug_msg[3] = (NUC >>  0) & 0xFF;

(where the >> 0 is optional but makes it look neater — if the compiler optimizes to omit any shift for that). If you want to handle different values in place of NUC, then:

unsigned long value = 0xA8051701;

debug_msg[0] = (value >> 24) & 0xFF;
debug_msg[1] = (value >> 16) & 0xFF;
debug_msg[2] = (value >>  8) & 0xFF;
debug_msg[3] = (value >>  0) & 0xFF;

Or in macro form:

#define MANGLE(value, debug_msg) \
    debug_msg[0] = (value >> 24) & 0xFF; \
    debug_msg[1] = (value >> 16) & 0xFF; \
    debug_msg[2] = (value >>  8) & 0xFF; \
    debug_msg[3] = (value >>  0) & 0xFF

used as:

MANGLE(0xA8051701, debug_msg)

or, if you want the values at arbitrary offsets in the array:

#define MANGLE(value, debug_msg, offset) \
    debug_msg[offset+0] = (value >> 24) & 0xFF; \
    debug_msg[offset+1] = (value >> 16) & 0xFF; \
    debug_msg[offset+2] = (value >>  8) & 0xFF; \
    debug_msg[offset+3] = (value >>  0) & 0xFF

Used as:

MANGLE(0xA8051701, debug_msg, 24);

There might be a need to wrap the body of the macro in a do { … } while (0) loop to make it work properly after an if statement, etc.

Or you could write an inline function to do the job. Or …

edited Mar 24, 2017 at 15:58

answered Mar 24, 2017 at 6:56

Jonathan Leffler

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2 Comments

Vinod kumar Over a year ago

excellent its working. But if i increase to 64-bit i could not get correct values ie. from 24 to 32 to 40 to 48

Jonathan Leffler Over a year ago

You're on an 8-bit micro. You'll need to read how the compiler handles 64-bit numbers, if it handles them at all. If it supports them, you can extend the macro to do more shifting, but you have to be careful about types and so on.

Swanand · Accepted Answer · 2017-03-24 06:52:03Z

0

You can use a Union here

#define NUMBER_OF_BYTES_FOR_UINT    4
Union Char_UInt
{
  U8 u8data[NUMBER_OF_BYTES_FOR_UINT];
  U32 u32data;
};

Now, you can assign "NUC" (or any 32-bit number) to "u32data" and then read "u8data", It will give you byte-by-byte values.

This is because union assigns same memory to character array and u32data. You write and read from same memory location but through different interfaces.

Note that, Endianness of system plays a role here.

answered Mar 24, 2017 at 6:52

Swanand

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2 Comments

Vinod kumar Over a year ago

The platform i'm workiing is 8051 microcontroller with keil as IDE.

Swanand Over a year ago

@Vinodkumar So you can use it as it is

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