The interface __vswprintf_chk() shall function in the same way as the interface vswprintf(), except that __vswprintf_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 slen is less than maxlen, the function shall abort and the program calling it shall exit.
The __vswprintf_chk() function is not in the source standard; it is only in the binary standard.