printf causes code crash - c

Question

I am working on MODBUS creating library. Library is working fine for testing purpose I am using a Modbus based Power meter. I am facing a very strange problem.
Trying to print value on console, by using printf. By fetching first value side by side I am printing it, on second fetch value is coming in array correctly but on using pritnf code get crashed. :(
I have tried to debug the code also and came to know on using printf for a value second time code get crashed.
Here is part of code:
int wmain(void)
{
FLOAT dataFloat[5] = {0};
CHAR dataBuffer[200] = {0};
// Initializing Structure declared in library header
psModbusRTU pModbus =(psModbusRTU)malloc(sizeof(psModbusRTU));
// Array pointer to store data
BYTE *readData = (BYTE *) malloc(NUMBER_OF_READ_REGISTERS * sizeof(BYTE));
memset(readData, 0, NUMBER_OF_READ_REGISTERS * sizeof(BYTE));
// Function to read the voltage
if (!ModbusRTUReadRegister(pModbus, FC_READ_HOLDING_REGISTERS, VOLTAGE_REGISTER_ADDRESS, NUMBER_OF_READ_REGISTERS, readData))
{
printf("\nERROR Read register error %d", GetLastError());
getchar();
return 0;
}
dataFloat[0] = hexToFloatPointConversion(readData);
printf("\nvoltage : %f", dataFloat[0]); //(THIS IS FIRST PRINTF TILL HERE CODE IS FINE)
/// Function reads the frequnecy
if (!ModbusRTUReadRegister(pModbus, FC_READ_HOLDING_REGISTERS, FREQUENCY_REGISTER_ADDRESS, NUMBER_OF_READ_REGISTERS, readData))
{
printf("\nERROR Read register error %d", GetLastError());
getchar();
return 0;
}
dataFloat[1] = hexToFloatPointConversion(readData); // Saving frequency value in array
printf("\nFrequency : %f", dataFloat[1]); // (THIS IS WHERE PROBLEM ARISES CODE CRASH WITH NO REASON)
hexToFloatPointConversion Function:
FLOAT hexToFloatPointConversion(BYTE *readData)
{
DWORD dataHex = 0;
DWORD exponent = 0;
DWORD fraction = 0;
DWORD signBit = 0;
FLOAT dataFloat = 0;
/// Saving BYTE array data into one DWORD
dataHex = readData[0];
dataHex = dataHex << 8;
dataHex = dataHex + readData[1];
dataHex = dataHex << 8;
dataHex = dataHex + readData[2];
dataHex = dataHex << 8;
dataHex = dataHex + readData[3];
/// Getting sign bit, exponent and fraction
signBit = dataHex >> 31;
exponent = (dataHex >> 23 & 0xFF);
fraction = dataHex & 0x7FFFFF;
/// Calculation for converting data into float.
dataFloat = (pow(2, (exponent - SINGLE_PRECISION_CONSTANT)) * ( 1 + (fraction * pow(2, -23))));
return dataFloat;
}
psModbusRTU structure:
typedef struct sModbusRTU
{
HANDLE hModbus;
BYTE slaveNumber;
BYTE functionCode;
BOOL fModbusInitialize;
BOOL fSlaveAddress;
BOOL fModbusOpen;
BOOL fException;
WCHAR portName[16];
DWORD exceptionData;
DCB dcb;
}sModbusRTU, *psModbusRTU;
Tried to debug also in run time and looked into dis-assembly break point but did not get exactly why it is happening.
Here is the dis-assembly:
40050658 str r3, [sp, #0x14]
4005065C str r5, [sp, #8]
40050660 str r2, [sp]
40050664 str r4, [sp, #4]
40050668 bl 40031078
4005066C __debugbreak_ce //Break point
40050670 mvn r3, #0x37, 24
40050674 eor r3, r3, #0xAB
40050678 ldr r2, [r3]
4005067C movw r3, #0xF7F8
40050680 movt r3, #0xF101
Please help me out, my project is stuck in middle just because of this small problem.
Please respond back soon

Answer 1

Wrong size passed to malloc().
// was psModbusRTU pModbus =(psModbusRTU)malloc(sizeof(psModbusRTU));
// -----------------------------------------------v--------------
psModbusRTU pModbus = (psModbusRTU) malloc(sizeof(*psModbusRTU));
Suggest alternate malloc() idiom. Use sizeof(*target).
Also, unless you compiler requires it (which it shouldn't), recommend dropping the cast
// example
psModbusRTU *pModbus = malloc(sizeof(*pModbus));

Answer 2

The %f specifier is expecting to format a double (8 bytes) and not a float (4 bytes) (See this MSDN page for details. Try casting the values to double in your printf statements, e.g. printf("\nvoltage : %f", (double) dataFloat[0]);