Junk values in ofstream using strncpy - c++

Question

I am running the following program below. I am taking the first 63 char values in B.txt and then attaching the float values in A.txt, beginning at the 62nd column in A.txt, at the end of the lines of B.txt
So if B.txt contains:
I am running the following program below. I am taking the firstXXXXXXXX
and A.txt contains:
I am running the following program below. I am taking the fir3.14
I want B.txt to look like:
I am running the following program below. I am taking the first3.14
However, the output I'm getting instead is:
I am running the following program below. I am taking the firstBUNCH OF JUNK3.14
int main()
{
loadfileB("B.txt");
return 0;
}
void loadfileB(char* fileName)
{
FILE* fp = fopen(fileName, "r");
char line[82];
vector<int> rownum;
vector<float> temp;
temp = loadfileA("A.txt");
int i = 0;
ofstream fout("output.txt");
while (fgets(line, 81, fp) != 0)
{
radius=temp[i];
char buffer[64];
strncpy(buffer, line, 63);
fout << buffer<< " " << radius << endl;
i++;
}
fclose(fp);
}
vector<float> loadfileA(char* fileName)
{
FILE* fp = fopen(fileName, "r");
char line[82];
vector<int> rownum;
vector <float> tempvec;
int i = 0;
while (fgets(line, 81, fp) != 0)
{
float temp;
getFloat(line, &temp, 60, 6);
tempvec.push_back(temp);
}
fclose(fp);
return tempvec;
}
void getFloat(char* line, float* d, int pos, int len)
{
char buffer[80];
*d = -1;
strncpy(buffer, &line[pos], len);
buffer[len] = '\0';
sscanf(buffer, "%f", d);
}

Answer 1

strncpy is a bad function to use. This is because it does not null-terminate its output if the input did not fit in the buffer. The garbage you are seeing is the result of passing a non-null-terminated buffer to a function that expected a null-terminated string.
The simplest fix is to replace:
char buffer[64];
strncpy(buffer, line, 63);
with:
std::string buffer = line;
buffer.resize(63);
In your other usage you do null-terminate, however you never check that len is smaller than 80 either. Again the simpler fix would be:
std::string buffer( line + pos, len );
sscanf(buffer.c_str(), "%f", d);
The getFloat function should have some way of signaling error (either a return value; or throw an exception if sscanf does not return 1).
Of course, you could replace a lot of your other C-style code with C++-style code too and avoid buffer size issues entirely.