Forward Variadic Argument List to ncurses printw function in c

Question

This has been asked in several different flavors. Yet I can still not get it to work. Here is my function definition.

void                                                                           
ncurses_add_line(const char *fmt, ...)                                         
{                                                                              
  if (ncurses_window) {                                                        
    va_list args;                                                              
    va_start(args, fmt);                                                       
    printw(fmt, args);                                                           
    printw("\n");                                         
    va_end(args);                                                              
  }                                                                            
}

When I call this function I get gibberish in the variadic print out of my function. If I call printw directly it all works. For example, if I call ncurses_add_line like ncurses_add_line("Hello %d", var) I get a value not store in var. However, if I call printw("Hello %d", var), I see the value of var displayed next to "Hello" as in, if var == 1 then "Hello 1" is printed with printw but this is not the case for ncurses_add_line.

What do I need to change?

My reason for wrapping this up is because I don't want to include in my header file, only in my c file.

jdigital · Accepted Answer · 2016-11-19T05:31:11.087

Try vwprintw instead of printw. vwprintw takes a va_list as its argument.

The idiom you're trying to use -- passing a va_list to a function that takes a variable number of arguments -- won't work. One solution is to find a variant of the function that will work (in this case, vwprintw). An alternative is to "flatten" the va_list: in this case, you could use vsprintf to create a formatted string, and then pass that into curses.

score 1 · Answer 2 · answered Nov 19 '16 at 05:25

args is not something like an array of arguments. It is an internal structure. You have to read out every single argument by passing the type. Please keep in mind, that in C there is no runtime reflection, so you have to add the types in your code.

void ncurses_add_line(const char *fmt, ...)                                         
{                                                                              
if (ncurses_window) 
{            
  va_list args; 
  va_start(args, fmt);       
  char *arg = va_arg( args, int ); // take out one arg by giving the type (int)
  printw(fmt, arg);   
  printw("\n");                                         
  va_end(args);                                                              
  }                                                                            
}

Forward Variadic Argument List to ncurses printw function in c

2 Answers2