convert decimal to hex, add to byte array

Question

I am working on some image conversion in objective C, for iOS, and having some difficulties in saving the total image size in a byte array..

I did some searching, and stackoverflow has many questions with conversion from decimal to hex, but all of them explain how to print hex value using %x and printf(or NSLog), which is not what I want.

I have an image with size 2500

int totalSize  = 2500;

I want to add this size to specific index in image header

UInt8 headerData[HEADER_SIZE];
memset(headerData, 0, HEADER_SIZE);

//2500 dec  = 9C4 hex
headerData[2]   = 0xC4;
headerData[3]   = 0x09;
headerData[4]   = 0x00;
headerData[5]   = 0x00;    //image size total 4 bytes

I found this thread and tried it out

char hexval[5];
memset(hexval, 0, 5);
if (totalSize <= 0xFFFF){
    sprintf(&hexval[0], "%0x", totalSize);
}

printf("%x %x %x %x", hexval[0], hexval[1], hexval[2], hexval[3]);

and it printed

36 36 33 32

which is not the correct value, or am I missing something obvious here?

EDIT: Sorry, forgot put the endianness part.. I want to save in little endian format.

Note : I am adding a C tag also, since a C solution is applicable here too..

Answer 1

Revised : so this is not byte order issue, but here's code to properly get the byte values in little endian order, byte by byte:

int totalSize = 2500;

headerData[2] = totalSize >> 0 & 0xFF; // 0xC4
headerData[3] = totalSize >> 8 & 0xFF; // 0x09
headerData[4] = totalSize >> 16 & 0xFF; // 0x00
headerData[5] = totalSize >> 24 & 0xFF; // 0x00

---

That 2nd piece of code is just wrong though in many ways. What is it's purpose?

Do you perhaps just want this (no loops or anything):

printf("%04x", totalSize);

Or to get it to hexval C string and print that:

char hexval[5]; // actually, since totalSize can be > 0xFFFF, 9 would be better size
snprintf(hexval, sizeof hexval, "%04x", totalSize); // note: sizeof hexval only works if hexval is char array with static size
printf("%s", hexval);

Or do you really want the ASCII codes of individual characters, when value is printed out as hex string?

---

Hmm, also, when you say "with conversion from decimal to hex", what do you mean? Storing numeric value to a binary file (I presume your image file is binary file and not XML or something) has nothing to do with decimal/hexadecimal conversion. It is storing a number in memory to bytes in file, and decimal/hexadecimal does not really enter into it at all (except to print values for humans to read for debugging purposes).

Answer 2

If you are doing this under iOS, which is little endian, and you want little endian order, it's as simple as:

*(uint32_t *)(header + 2) = totalSize;

If you want a cross-platform solution, then the byte-by-byte solution provided by @hyde is the way to go.