We have often used the functions type printf (), scanf (),…
These functions are unique, they accept a variable number of parameters!
Can we also create such functions?
Presentation of a library of the language C “stdarg. h “.
Explanation with examples and exercises.
Functions with variable number of arguments
The C language allows you to define functions whose number of arguments is not fixed and can vary from one call to another. We have often used the following functions:
These functions are unique: They accept a variable number of parameters!
/* a parameter */
printf ( “Hello! n”);
/* 2 Parameters */
printf ( “Cube of% ld is: n”, value);
/* 4 Parameters */
printf ( “% ld *% ld =%ldn”, I, value, product);
Can we also create such functions?
Declaration and syntax
The declaration is made in the following way:
type fonction(type1 arg1, type2 arg2, …)
Call to function:
function (arg1, arg2);
Rules to be respected
Rule 1: If the prototype contains p formal parameters, then the call to function must be done with at least p parameters.
The following statement is considered:
function type (type1 arg1, type2 arg2, Type3 Arg3,…);
When calling this function we must provide at least 3 arguments:
function (Arg1, arg2, arg3)//appel correct
function (Arg1, arg2)//appel incorrect
function (Arg1, arg2, Arg3, arg4)//appel correct
Rule 2: A function with a variable number of parameters must have at least one fixed parameter.
int sum (…); Incorrect statement
int sum (int a,…); Correct statement
Rule 3: Scoring… (Mandatory at the end of the parameter list) specifies that the function has a variable number of parameters. This variable list of parameters must always appear in the last position among the formal parameters.
void error (int n, char * msg,…);
This statement says that the error () function is defined in such a way that calls must provide at least two arguments, one of type int and one of type char *, but they can provide additional arguments.
Error (3, “correct call”);
Access to arguments
To access the arguments after the last fixed argument, you must use some
Functions (or rather macros) of the stdarg. h file
This library contains all the functions needed to access the arguments. It defines a type to be used to process variable lists of parameters,
Va_list array containing the argument information as well as 3 macros to retrieve the value of the parameters.
void Va_start (Va_list ap, last);
Makes AP point to the first variable argument provided to the function.
Type Va_arg (va_list ap, type);
Returns the first variable argument and points AP to the next argument.
Type is the type of the argument that is going to be read and Va_arg generates an expression of this void Va_end (Va_list ap);
returns to normal before return & #224; the calling function.
Va_start must be applied before va_arg or va_end.
Va_end must be called after Va_arg has read all the arguments, otherwise the program behavior will be unpredictable.
Progress in the argument list the first call to Va_arg actually returns the first argument in the list
Each subsequent call to Va_arg allows you to retrieve an argument.
-Va_start and Va_end do not return a value.
-Va_arg Returns the current argument of the list (the one on which AP points).
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Char, unsigned char, or float types are forbidden with va_arg.
Parameters passing rules:
For integer-type parameters
-Char, shorts are converted to int.
-long remains in long format.
For real-type parameters
-they are converted to double