Converting int to bytes and switching endian efficiently - objective-c

Question

I have to do some int -> byte conversion and switch to big endian for some MIDI data I'm writing. Right now, I'm doing it like:
int tempo = 500000;
char* a = (char*)&tempo;
//reverse it
inverse(a, 3);
[myMutableData appendBytes:a length:3];
and the inverse function:
void inverse(char inver_a[],int j)
{
int i,temp;
j--;
for(i=0;i<(j/2);i++)
{
temp=inver_a[i];
inver_a[i]=inver_a[j];
inver_a[j]=temp;
j--;
}
}
It works, but it's not real clean, and I don't like that I'm having to specify 3 both times (since I have the luxury of knowing how many bytes it will end up).
Is there a more convenient way I should be approaching this?

Answer 1

Use the Core Foundation byte swapping functions.
int32_t unswapped = 0x12345678;
int32_t swapped = CFSwapInt32HostToBig(unswapped);
char* a = (char*) &swapped;
[myMutableData appendBytes:a length:sizeof(int32_t)];

Answer 2

This should do the trick:
/*
Quick swap of Endian.
*/
#include <stdio.h>
int main(){
unsigned int number = 0x04030201;
char *p1, *p2;
int i;
p1 = (char *) &number;
p2 = (p1 + 3);
for (i=0; i<2; i++){
*p1 ^= *p2;
*p2 ^= *p1;
*p1 ^= *p2;
}
return 0;
}
You can pack it into a function in whatever way you want to use it. The bitwise swap should compile into some pretty neat assembly :)
Hope it helps :)

Answer 3

int tempo = 500000;
//reverse it
inverse(&tempo);
[myMutableData appendBytes:(char*)tempo length:sizeof(tempo)];
and the inverse function:
void inverse(int *value)
{
char inver_a = (char*)value;
int j = sizeof(*value); //or u can put 2
int i,temp;
// commenting this j--;
for(i=0;i<(j/2);i++)
{
temp=inver_a[i];
inver_a[i]=inver_a[j];
inver_a[j]=temp;
j--;
}
}