String Functions

The 825 can be programmed using C/C++ character array strings and the standard C library such as:

char szMsg[10];
char szID[10];
strcpy(szID, "123");
sprintf(szMsg, "ID: %s", szID);
PrintLCD(szMsg);

This is the traditional C programming way of handling strings. The C language has no true string type; strings are just arrays of characters. This is very fast and efficient because the compiler just generates code to read and write the memory directly. However, there is no automatic checking that pointers point to valid memory, or to prevent writing beyond memory that is allocated. It is very easy to crash an 825 indicator by programming this code incorrectly.

In the above example the szMsg and szID character arrays are each allocated 10 bytes. C strings require a null character at the end to tell various functions such as strcpy when to stop reading characters. This means szMsg and szID can have a maximum of 9 characters each. If the strcpy(szID, "123") is changed to strcpy(szID, "1234567890") the compiler will generate code that writes the null character into an invalid memory location. When the program is executed this may crash the indicator immediately, or it may not cause a crash until some other time in execution when that memory location becomes important. If the strcpy is changed to strcpy(szID, "123456") szID will not overflow, but szMsg will overflow in the sprintf statement that follows.

The 825 also supports a newer C++ string type std::string. Using std::string does not require as much diligence from the programmer. The string will automatically allocate more memory and grow if needed by an assignment. The operations are also more like the BASIC language so this is helpful for programmers who are not as skilled in C/C++.

string strMsg;
string strID;
strID = "123";
strMsg = "ID: ";
strMsg = strMsg + strID;
PrintLCD(strMsg);

The 825 standard library functions now accept string parameters. We are now recommending using strings in application programs as much as possible to reduce the possibility of pointers to invalid memory, or overflow bugs.

For more information about std::strings refer to: ​http://www.cplusplus.com/reference/string/string/

To further help with string operations some additional functions are now provided in the 825 library.