C Pocket Reference

Functions with Variable Numbers of Arguments

Functions that can be called with a variable number of arguments always expect a fixed number of mandatory arguments—at least one is required—and a variable number of optional arguments. A well-known example is the function printf(): the format string argument is mandatory, while all other arguments are optional. Internally, printf() determines the number and type of the other arguments from the information in the format string.

In the function declarator, optional arguments are indicated by three dots (...). For example:

int printf( char *str, ... );     // Prototype

In the function definition, the optional arguments are accessed through an object with the type va_list , which contains the argument information. This type is defined in the header file stdarg.h , along with the macros va_start , va_arg , and va_end , which are used to manage the arguments.

In order to read the optional arguments, the function must carry out the following steps:

1. Declare an object of type va_list. In the following example, this object is named arglist.

2. Invoke the macro va_start to prepare the arglist object to return the first optional argument. The parameters of va_start are the arglist object and the name of the last mandatory parameter.

3. Invoke the macro va_arg with the initialized arglist object to obtain each of the optional arguments in sequence. The second parameter of va_arg is the type of the optional argument that is being obtained.

   After each invocation ...