Using pointers to struct members as args to functions


 
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# 1  
Old 07-28-2011
Using pointers to struct members as args to functions

In a well-known book on the C language, there is an example of an efficient method for using a struct member as an argument to a function. (I'm a C noob, but I believe the correct terminology might be: use call-by-reference instead of call-by-value.) The function is printf.

Anyway, here's a similar example, to illustrate the technique:

Code:
struct a *pp;

struct a b;
b.1 = c; 
b.2 = cc; 
b.3 = ccc; 
b.4 = cccc; 
}

struct a b, *pp;

pp = &b;

printf("b.1 is %s\n",(*pp).b.1);

This is easy enough to understand.

But how do we do this with structure members that are themselves pointers or functions?

Let's say I have a struct that resembles the one below. And I want to print the value of lg.

Code:
const struct kelly {
const char *cindy;
const char *juliette;
void (*anna)(const char *);
void (*jennifer)(void);
void (*rachel)(void);
void (*lg)(int);
char *(*christine)(const char *, char *);
}

# 2  
Old 07-28-2011
Quote:
Originally Posted by uiop44
But how do we do this with structure members that are themselves pointers or functions?

Let's say I have a struct that resembles the one below. And I want to print the value of lg.

Code:
const struct kelly {
const char *cindy;
const char *juliette;
void (*anna)(const char *);
void (*jennifer)(void);
void (*rachel)(void);
void (*lg)(int);
char *(*christine)(const char *, char *);
}

Just learn to keep in mind when you're using a pointer as a value and when you're using the value it points to. To print the value of lg, use:

Code:
/* `const' is useless here! */
struct kelly {
    /* ... */
    void (*lg)(int);
};

struct kelly kelly_smith;
printf("lg of kelly_smith = %p\n", kelly_smith.lg);


Edit: Unless kelly_smith is declared as a pointer, in which case use:

Code:
struct kelly *kelly_smith;
printf("lg of kelly_smith = %p\n", kelly_smith->lg);

/* Or, equivalently... */
printf("lg of kelly_smith = %p\n", (*kelly_smith).lg);


Last edited by JohnGraham; 07-28-2011 at 09:05 AM..
# 3  
Old 07-28-2011
Quote:
Originally Posted by uiop44
But how do we do this with structure members that are themselves pointers or functions?

Let's say I have a struct that resembles the one below. And I want to print the value of lg.

Code:
const struct kelly {
const char *cindy;
const char *juliette;
void (*anna)(const char *);
void (*jennifer)(void);
void (*rachel)(void);
void (*lg)(int);
char *(*christine)(const char *, char *);
}

Struct member lg is a pointer to a function that takes an int and returns void...so i am not sure what you mean when you want to print its value. Value in lg will be address of the function it is pointing to and dereferencing it *lg is the same thing as calling that function..kind of like
Code:
void f(int);
int i;
struct kelly k;
k.lg=f;
printf("value in lg is %u\n", lg);   // which is the address of function f
*lg;   // same as calling f(i)

# 4  
Old 07-28-2011
Quote:
Originally Posted by JohnGraham

Code:
/* `const' is useless here! */
struct kelly {
    /* ... */
    void (*lg)(int);
};

JohnGraham: Would const be useful if the struct was as follows?

Code:
const struct kelly {
/* ... */
void (*lg)(int);
} friends[] = {
/* ... */
};

Shamrock: Many thanks.

The "kelly" struct is based on real code that works well, but I (sloppily) took it out of a reasonably complex context, where the function being called by dereferencing "lg" varies based on the program name, whether input is from stdin/file, the command line arguments, sanity checks, etc.

To understand this code, there's a fair amount of indirection to resolve. The reward if I can figure it out is that it's a workable example of how to parse an above average number of command line variations.
# 5  
Old 07-28-2011
Quote:
Originally Posted by uiop44
JohnGraham: Would const be useful if the struct was as follows?

Code:
const struct kelly {
/* ... */
void (*lg)(int);
} friends[] = {
/* ... */
};

Yes, (assuming you want all the elements of your array to be const) since that's qualifying the array. It seemed like you wanted to make a const type (which you could do with a typedef).
# 6  
Old 07-29-2011
JG: Would you believe I also omitted a typdef that did just that? For the kelly example, I took the struct out of a context where I still don't fully understand the context. Hence the omissions.

Cheers for the helpful insights.
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