Byte length of a MySQL column in C++ - c++

Question

I am using C++ to write a program for a MySQL database. I am trying to check a condition by comparing the length of a column (in bytes) to pass/fail. Here is the code:
while (row = mysql_fetch_row(result))
{
lengths = mysql_fetch_lengths(result);
num_rows = mysql_num_rows(result);
for (i = 0; i < num_fields; i++)
{
if (strstr(fields[i].name, "RSSI") != NULL)
{
if (lengths[*row[i]] == ??)
printf("current value is %s \t", row[i]);
}
}
}
So basically what i am trying to do is to look for the string "RSSI" in the columns and if the string is present i want to print that value. The values in each column are 3 bytes in length if present . So how do i check if lengths [*rows[i]] is 3 bytes in length? Thanks

Answer 1

According to the official MySQL documentation mysql_fetch_lengths returns an array of unsigned long with the lengths of the columns of the current row. Although the description isn't clear whether it's in bytes or something else, the example shown clarifies it.
So you should be checking directly to 3.
Also, there are some syntactic and semantic errors, and a possible refactoring in your code, among them the following:
Given the lengths variable is an array with the current rows' lengths, the expression lengths[*row[i]] should just be lengths[i] because i is the index of the current column.
The two ifs inside the for could be merged with the && operator for better readability.
Some variables are not defined or used correctly.
The code would look like this:
// Properly assign a value to fields variable.
fields = mysq_fetch_fields(result);
// Getting the number of fields outside the loop is better.
num_fields = mysql_num_fields(result);
while (row = mysql_fetch_row(result))
{
lengths = mysql_fetch_lengths(row);
for (i = 0; i < num_fields; i++)
if (strstr(fields[i].name, "RSSI") != NULL && lengths[i] == 3)
printf("current value is %s \t", row[i]);
printf("\n"); // For better output print each row in a new line.
}
You should really read the documentation carefully in order to avoid compilation or logic errors for using the wrong function.

Answer 2

I think there is a typo:
dev docs states:
(http://dev.mysql.com/doc/refman/5.0/en/mysql-fetch-lengths.html)
...
num_fields = mysql_num_fields(result);
lengths = mysql_fetch_lengths(result);
for(i = 0; i < num_fields; i++)
NOT
lengths = mysql_fetch_lengths(row);