Question

This question may looks silly, but please guide me
I have a function to convert long data to char array

void ConvertLongToChar(char *pSrc, char *pDest)
{
    pDest[0] = pSrc[0];
    pDest[1] = pSrc[1];
    pDest[2] = pSrc[2];
    pDest[3] = pSrc[3];
}

And I call the above function like this

long lTemp = (long) (fRxPower * 1000);
ConvertLongToChar ((char *)&lTemp, pBuffer);

Which works fine.
I need a similar function to reverse the procedure. Convert char array to long.
I cannot use atol or similar functions.

Answer 1

Leaving the burden of matching the endianness with your other function to you, here’s one way:

unsigned long int l = pdest[0] | (pdest[1] << 8) | (pdest[2] << 16) | (pdest[3] << 24);

Just to be safe, here’s the corresponding other direction:

unsigned char pdest[4];
unsigned long int l;
pdest[0] = l         & 0xFF;
pdest[1] = (l >>  8) & 0xFF;
pdest[2] = (l >> 16) & 0xFF;
pdest[3] = (l >> 24) & 0xFF;

Going from char[4] to long and back is entirely reversible; going from long to char[4] and back is reversible for values up to 2^32-1.

Note that all this is only well-defined for unsigned types.

(My example is little endian if you read pdest from left to right.)

Addendum: I’m also assuming that CHAR_BIT == 8. In general, substitute multiples of 8 by multiples of CHAR_BIT in the code.