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   vsprintf    ( 3 )

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Возвращаемое значение (Return value)

Upon successful return, these functions return the number of
       characters printed (excluding the null byte used to end output to
       strings).

       The functions snprintf() and vsnprintf() do not write more than
       size bytes (including the terminating null byte ('\0')).  If the
       output was truncated due to this limit, then the return value is
       the number of characters (excluding the terminating null byte)
       which would have been written to the final string if enough space
       had been available.  Thus, a return value of size or more means
       that the output was truncated.  (See also below under NOTES.)

       If an output error is encountered, a negative value is returned.