使用strncpy的ofstream中的垃圾值

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.