Man page 9 : scnprintf

scnprintf - Format a string and place it in a buffer

int scnprintf(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 written into buf not including the trailing Aq\0Aq. If size is == 0 the function returns 0.