In pure C how can I pass a variable number of parameters into a function?

Question

Hey,

How can i pass (and access) using C, not c++, variable parameters into a function?

void foo(char* mandatory_param, char* optional_param, char* optional_param2...)

thanks

/fmsf

Answer 1

Use stdarg.h

You need to use va_list and then use the macros va_start, va_arg, and va_end.

For more information, see http://www.acm.uiuc.edu/webmonkeys/book/c_guide/2.10.html

Answer 2

It sounds like you are looking for varargs.

#include <stdarg.h>
void foo(const char *fmt, ...)
{
  va_list argp;
  va_start(argp, fmt);
  int i = va_arg(argp, int);
  // Do stuff...
  va_end(argp);
}

Answer 3

Read about Variable Arguments in C

Answer 4

In a language that does not support optional parameters directly, there are a few ways to achieve a similar effect. I will list them in order from the least versatile to the most:

1. Create multiple overloads of the same function. As I recall, you cannot do this in C.

2. Use variadic functions. Just Google this: http://www.google.com/search?q=variadic+function+c

3. I recommend this: Create a "params" or "args" class (or struct in C), like this:

)

// untested C code
struct FooArgs {
    char * mandatory_param;
    char * optional_param;
    char * optional_param2;
    // add other params here;
};

and then make your method call take in a single argument:

// untested
void foo(struct fooArgs * args)

This way, as needs change, you can add parameters to fooArgs without breaking anything.

Answer 5

#include <stdarg.h>

void do_sth (int foo, ...)
{
    int baz = 7;             /* "baz" argument */
    const char *xyz = "xyz"; /* "xyz" argument */

    /* Parse named parameters */
    va_list ap;
    va_start (ap, foo);
    for (;;) {
        const char *key = va_arg (ap, char *);
        if (key == NULL) {
            /* Terminator */
            break;
        } else if (strcmp (key, "baz") == 0) {
            baz = va_arg (ap, int);
        } else if (strcmp (key, "xyz") == 0) {
            xyz = va_arg (ap, char *);
        } else {
            /* Handle error */
        }
    }
    va_end (ap);

    /* do something useful */
}

do_sth (1, NULL);                             // no named parameters
do_sth (2, "baz", 12, NULL);                  // baz = 12
do_sth (3, "xyz", "foobaz", NULL);            // xyz = "foobaz"
do_sth (4, "baz", 12, "xyz", "foobaz", NULL); // baz = 12, xyz = "foobaz"

http://stackoverflow.com/questions/2826128/variadic-functions-and-arguments-assignment-in-c-c/2826411#2826411