Man page 9 : snprintf

snprintf - Format a string and place it in a buffer

int snprintf(char *  buf , size_t  size , const char *  fmt , ... );

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 @...: Arguments for the format string

...
    variable arguments

The return value is the number of characters which would be generated for the given input, excluding the trailing null, as per ISO C99. If the return is greater than or equal to size, the resulting string is truncated.

See the vsnprintf documentation for format string extensions over C99.