Chapter: Other topics
...Section: Variable number of arguments

You've already seen printf(). How many arguments does it expect? There is no fixed number. C lets you define functions to take a variable number of arguments. For example:

#include < stdio.h >
#include < string.h >
#include < stdarg.h >    // Required include for variable number of arguments.

// Function prototype for a function with variable number of arguments:
// This function will take in an integer and any number of strings.
char * concatStrings (int numStrings, ...);


int main ()
{
  char *str;

  // Example with 2 strings.
  str = concatStrings (2, "Hello", " World!");
  printf ("%s\n", str);

  // Example with 6 strings.
  str = concatStrings (6, "So,", " how're", " we", " doing", " today,", " eh?");
  printf ("%s\n", str);
}


// The function with definition and body.

char * concatStrings (int numStrings, ...)
{
  va_list argList;                // The var-arg package requires this variable
                                  // to be defined.

  // Variables for our use:
  char *resultString = "";        // Resulting string after concatenation.
  char *nextString;               // A variable to hold the next argument.
  int numStringsExtracted = 0;    

  // This call initializes the var-arg package:
  va_start (argList, numStrings);

  // Extract arguments one by one:
  while (numStringsExtracted < numStrings) {

    // Note: first argument is the required argList variable, and
    // the second argument is simply the type of the next argument:
    nextString = va_arg (argList, char*);

    numStringsExtracted ++;

    // Process argument: append the string to resultString

    // ...
  }

  // Required call to signal end of var-args:
  va_end (argList);

  // Return result.
  return resultString;
}

You should note that:

• The package stdarg.h needs to be included.
• The first argument must be known in advance: char * concatStrings (int numStrings, ...);
• The three periods act as an operator, indicating a variable number of arguments.
• The stdarg package requires you to define a variable to hold the unknown arguments:

  char * concatStrings (int numStrings, ...)
  {
    va_list argList;                // The var-arg package requires this variable
  
    // ...
  }  
  

• The unknown arguments themselves are extracted in a loop:

    va_start (argList, numStrings);
    while (numStringsExtracted < numStrings) {
      nextString = va_arg (argList, char*);
      numStringsExtracted ++;
      // ...  
    }
  

Exercise 13