Man page 9 : vsprintf

vsprintf - Format a string and place it in a buffer

int vsprintf(char *  buf , const char *  fmt , va_list  args );

buf
    The buffer to place the result into

fmt
    The format string to use

args
    Arguments for the format string

The function returns the number of characters written into buf. Use vsnprintf or vscnprintf in order to avoid buffer overflows.

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

See the vsnprintf documentation for format string extensions over C99.