The interface __vsprintf_chk() shall function in the same way as the interface vsprintf(), except that __vsprintf_chk() shall check for stack overflow before computing a result, depending on the value of the flag parameter. If an overflow is anticipated, the function shall abort and the program calling it shall exit.
In general, the higher the value of flag, the more security measures this interface shall take in the form of checking the stack, parameter values, and so on.
The parameter slen specifies the size of the object pointed to by s. If its value is zero, the function shall abort and the program calling it shall exit.
The __vsprintf_chk() function is not in the source standard; it is only in the binary standard.