Question

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C/C++: Passing variable number of arguments around

According to the function declaration manner

return-type function-name(parameter-list, ...) {body ...}

Is the following code a kind of overloading?

(in A_FILE.h)

typedef VOID *FUNCTION(UINTN Number, ...);

It seems that I could just pass one Number parameter, or more parameters into the FUNCTION function, so does the number of parameters depends on the function implementation?

Answer 1

The ellipsis within a function declaration means that it will accept a number of arguments otherwise, variable parameters that are unknown at run-time, which by using the standard header file stdarg.h, the respective functions in that header file ‘stdarg.h’ can determine what each of the variable parameters are that makes up the argument passing into a function.

Consider this code example:

#define PANIC_BUF_LEN 256
void panic(const char *fmt, ...){
    char buf[PANIC_BUF_LEN];
    va_list argptr;
    va_start(argptr, fmt);
    vsprintf(buf, fmt, argptr);
    va_end(argptr);
    fprintf(stderr, buf);
    exit(errcode);
}

Typical invocation can be one of example:

panic("Error: %s failed! Due to unknown error, message is '%s'\n", "my_function", "Disk not ready");

Will produce an output on console in this manner:

Error: my_function failed! Due to unknown error, message is 'Disk not ready'

Notice the usage of how the functions va_start(...), va_end(...) and not to mention vsprintf(...) will take care of filling in the blanks within the “unknown” parameters provided va_list is initialized to point to the variable parameters which are unknown at run-time.

Edit: Just to emphasize, the invocation assumes that the string parameter in the form of a C string format is less than the maximum size represented by PANIC_BUF_LEN in the above example, nit-picky aside, that is to illustrate how a function can take in a number of standard C formatting strings used, for example, one can specify %d in the string format, and expect a int to match up the parameter.