Man page 9 : vscnprintf

vscnprintf - Format a string and place it in a buffer

int vscnprintf(char *  buf , size_t  size , const char *  fmt , va_list  args );

buf
    The buffer to place the result into

size
    The size of the buffer, including the trailing null space

fmt
    The format string to use

args
    Arguments for the format string

The return value is the number of characters which have been written into the buf not including the trailing Aq\0Aq. If size is == 0 the function returns 0.

If youAqre not already dealing with a va_list consider using scnprintf.

See the vsnprintf documentation for format string extensions over C99.