Display Negative Number

[ljbeng]

I am getting strings from an HTTP request that will have hex values and I must convert those strings to a signed decimal.

//typical string inside response: //0E1D052BFBB711C1002C0042007A014DFE44022B270F7FFF8000000000000000
//every 4 characters above are a signed decimal value

for (a = 0; a <= 63; a+=4){
	sprintf(vval,"0X%c%c%c%c",response[a],response[a+1],response[a+2],response[a+3]);
	ds = strtol(vval, NULL, 16);
	sprintf(vval,"%d",ds);
}

The problem is I never see a negative number. Decoding 0x8000 gives me 32768 but not -32768.

Thanks.

[JLBorges]

#include <iostream>
#include <cstring>
#include <string>
#include <cstdint>
#include <sstream>
#include <vector>

int main()
{
    const char* const typical_response = "0E1D" "052B" "FBB7" "11C1" "002C" "0042"
                "007A" "014D" "FE44" "022B" "270F" "7FFF" "8000" "0000" "00g0" "0000" ;

    constexpr std::size_t RESP_SZ = 64 ;
    char response[RESP_SZ] ;
    std::memcpy( response, typical_response, RESP_SZ ) ;

    std::vector<int> numbers ;

    // every 4 characters above are a signed decimal value
    constexpr std::size_t NCHARS = 4 ;
    for( std::size_t i = 0 ; i < RESP_SZ ; i += NCHARS ) // in 4 char chunks
    {
        std::istringstream stm( { response+i, response+i+NCHARS } ) ;

        using uint_type = std::uint16_t ;
        using sint_type = std::int16_t ;
        uint_type value ; // read in as unsigned [ 0,  0xffff ]
        if( stm >> std::hex >> value && stm.eof() ) 
            numbers.push_back( sint_type(value) ) ; // push_back as signed [ -32768, +32767 ]
        else 
        {
            std::cerr << "inalid hex number starting at position " << i << ": '" << stm.str() << "'\n" ;
            numbers.push_back(0) ;
        }
    }

    for( int n : numbers ) std::cout << n << ' ' ;
    std::cout << '\n' ;
}

http://coliru.stacked-crooked.com/a/61d357c0fe036d20

[doug4]

You didn't give us the type of vval. My guess is vval is an int with a size of 4 bytes.

The value 0x8000 is only 2 bytes (4 hex characters = 2 bytes).

If you make vval a signed 2-byte value (short on some systems), you will probably get the behavior you are looking for.

[tath]

0x8000 short is negative but 0x00008000 long is positive number.
You need to have 0xFFFF8000 to see it as negative. So as doug4 wrote: make vval short or sign extend short to 4 bytes:
vval = (vval<<16)>>16;
before using it in strtol

[ljbeng]

Thanks doug4, short worked.

[ljbeng]

Now I am trying to get 4 bytes into a floating point number. I am casting to float and can display positive numbers but negative numbers show 1.#R...

sprintf(vval,"0x%c%c%c%c%c%c%c%c",response[a+4],response[a+5],response[a+6],response[a+7],response[a],response[a+1],response[a+2],response[a+3]);
	dsl = strtol(vval, NULL, 16);
	printf("%d,%f\n",a, *(float *)&dsl);
	dsF = *(float *)&dsl;
	sprintf(fval,"%7.2f",dsF);

Thanks.

[ljbeng]

Here is the real problem, I did this little test:

long dsl;
char vval[16];

sprintf(vval,"0xbdcccccd");
dsl = strtol(vval, NULL, 16);

above, vval = "0xbdcccccd"
dsl = 0x7fffffff

[ljbeng]

got it.

unsigned int and strtoul fixed it.