C Code Acting Differently to C++ on Lookup - c++

Question

I have the following code block (NOT written by me), which performs mapping and recodes ASCII characters to EBCDIC.
// Variables.
CodeHeader* tchpLoc = {};
...
memset(tchpLoc->m_ucpEBCDCMap, 0xff, 256);
for (i = 0; i < 256; i++) {
if (tchpLoc->m_ucpASCIIMap[i] != 0xff) {
ucTmp2 = i;
asc2ebn(&ucTmp1, &ucTmp2, 1);
tchpLoc->m_ucpEBCDCMap[ucTmp1] = tchpLoc->m_ucpASCIIMap[i];
}
}
The CodeHeader definition is
typedef struct {
...
UCHAR* m_ucpASCIIMap;
UCHAR* m_ucpEBCDCMap;
} CodeHeader;
and the method that seems to be giving me problems is
void asc2ebn(char* szTo, char* szFrom, int nChrs)
{
while (nChrs--)
*szTo++ = ucpAtoe[(*szFrom++) & 0xff];
}
[Note, the unsigned char array ucpAtoe[256] is copied at the end of the question for reference].
Now, I have an old C application and my C++11 conversion running side-by-side, the two codes write a massive .bin file and there is a tiny discrepancy which I have traced to the above code. What is happening for both codes is that the block
...
if (tchpLoc->m_ucpASCIIMap[i] != 0xff) {
ucTmp2 = i;
asc2ebn(&ucTmp1, &ucTmp2, 1);
tchpLoc->m_ucpEBCDCMap[ucTmp1] = tchpLoc->m_ucpASCIIMap[i];
}
gets entered into for i = 32 and the asc2ebn method returns ucTmp1 as 64 or '#' for both C and C++ variants great. The next entry is for i = 48, for this value the asc2ebn method returns ucTmp1 as 240 or 'ð' and the C++ code returns ucTmp1 as -16 or 'ð'. My question is why is this lookup/conversion producing different results for exactly the same input and look up array (copied below)?
In this case the old C code is taken as correct, so I want the C++ to produce the same result for this lookup/conversion. Thanks for your time.
static UCHAR ucpAtoe[256] = {
'\x00','\x01','\x02','\x03','\x37','\x2d','\x2e','\x2f',/*00-07*/
'\x16','\x05','\x25','\x0b','\x0c','\x0d','\x0e','\x0f',/*08-0f*/
'\x10','\x11','\x12','\xff','\x3c','\x3d','\x32','\xff',/*10-17*/
'\x18','\x19','\x3f','\x27','\x22','\x1d','\x35','\x1f',/*18-1f*/
'\x40','\x5a','\x7f','\x7b','\x5b','\x6c','\x50','\xca',/*20-27*/
'\x4d','\x5d','\x5c','\x4e','\x6b','\x60','\x4b','\x61',/*28-2f*/
'\xf0','\xf1','\xf2','\xf3','\xf4','\xf5','\xf6','\xf7',/*30-37*/
'\xf8','\xf9','\x7a','\x5e','\x4c','\x7e','\x6e','\x6f',/*38-3f*/
'\x7c','\xc1','\xc2','\xc3','\xc4','\xc5','\xc6','\xc7',/*40-47*/
'\xc8','\xc9','\xd1','\xd2','\xd3','\xd4','\xd5','\xd6',/*48-4f*/
'\xd7','\xd8','\xd9','\xe2','\xe3','\xe4','\xe5','\xe6',/*50-57*/
'\xe7','\xe8','\xe9','\xad','\xe0','\xbd','\xff','\x6d',/*58-5f*/
'\x79','\x81','\x82','\x83','\x84','\x85','\x86','\x87',/*60-67*/
'\x88','\x89','\x91','\x92','\x93','\x94','\x95','\x96',/*68-6f*/
'\x97','\x98','\x99','\xa2','\xa3','\xa4','\xa5','\xa6',/*70-77*/
'\xa7','\xa8','\xa9','\xc0','\x6a','\xd0','\xa1','\xff',/*78-7f*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*80-87*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*88-8f*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*90-97*/
'\xff','\xff','\xff','\x4a','\xff','\xff','\xff','\xff',/*98-9f*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*a0-a7*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*a8-af*/
'\xff','\xff','\xff','\x4f','\xff','\xff','\xff','\xff',/*b0-b7*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*b8-bf*/
'\xff','\xff','\xff','\xff','\xff','\x8f','\xff','\xff',/*c0-c7*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*c8-cf*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*d0-d7*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*d8-df*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*e0-e7*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff',/*e8-ef*/
'\xff','\xff','\xff','\x8c','\xff','\xff','\xff','\xff',/*f0-f7*/
'\xff','\xff','\xff','\xff','\xff','\xff','\xff','\xff' };

Answer 1

In both C and C++, the standard doesn't require char to be a signed or unsigned type. It's implementation defined, and apparently, your C compiler decided char to be unsigned char, while your C++ compiler decided it to be signed char.
For GCC, the flag to make char to be unsigned char is -funsigned-char. For MSVC, it's /J.