Passing variable number of arguments around

cvariadic-functions

Say I have a C function which takes a variable number of arguments: How can I call another function which expects a variable number of arguments from inside of it, passing all the arguments that got into the first function?

Example:

void format_string(char *fmt, ...);

void debug_print(int dbg_lvl, char *fmt, ...) {
    format_string(fmt, /* how do I pass all the arguments from '...'? */);
    fprintf(stdout, fmt);
 }

Best Answer

To pass the ellipses on, you initialize a va_list as usual and simply pass it to your second function. You don't use va_arg(). Specifically;

void format_string(char *fmt,va_list argptr, char *formatted_string);


void debug_print(int dbg_lvl, char *fmt, ...) 
{    
 char formatted_string[MAX_FMT_SIZE];

 va_list argptr;
 va_start(argptr,fmt);
 format_string(fmt, argptr, formatted_string);
 va_end(argptr);
 fprintf(stdout, "%s",formatted_string);
}

Related Solutions

C++ – How to pass variable number of arguments to printf/sprintf

void Error(const char* format, ...)
{
    va_list argptr;
    va_start(argptr, format);
    vfprintf(stderr, format, argptr);
    va_end(argptr);
}

If you want to manipulate the string before you display it and really do need it stored in a buffer first, use vsnprintf instead of vsprintf. vsnprintf will prevent an accidental buffer overflow error.

Php – How to pass variable number of arguments to a PHP function

If you have your arguments in an array, you might be interested by the call_user_func_array function.

If the number of arguments you want to pass depends on the length of an array, it probably means you can pack them into an array themselves -- and use that one for the second parameter of call_user_func_array.

Elements of that array you pass will then be received by your function as distinct parameters.

For instance, if you have this function :

function test() {
  var_dump(func_num_args());
  var_dump(func_get_args());
}

You can pack your parameters into an array, like this :

$params = array(
  10,
  'glop',
  'test',
);

And, then, call the function :

call_user_func_array('test', $params);

This code will the output :

int 3

array
  0 => int 10
  1 => string 'glop' (length=4)
  2 => string 'test' (length=4)

ie, 3 parameters ; exactly like iof the function was called this way :

test(10, 'glop', 'test');

Best Answer

Related Solutions

C++ – How to pass variable number of arguments to printf/sprintf

Php – How to pass variable number of arguments to a PHP function

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