malloc and structs

[warz]

I have a loop calling the dlopen() function and and setting the returned handle to a struct i have. the thing is, the number of calls to dlopen isn't static. it will change often. so i dont know how many times ill have to declare and instance of my structure for the dlopen() call. i need to use malloc() so i can create however many instances of my struct that i need. malloc only takes size as an argument though. how should i go about using malloc with my struct so after the loop im left with multiple structures?

[Maddox]

this is pretty basic. if you're going to be iterating through these a lot, you'll probably want to look into making a linked list.

anyways, first you'll need to count up the number of libraries are you are loading initially.

create your array of structs ( num is the number of initial libraries )

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int i;
struct mystruct * dlarray = (struct mystruct *)malloc(num * sizeof(struct mystruct));

/* populate it now */
for(i = 0; i < num; i++)
{
         dlarray[i].member = something;
}


when you want to add a library, you'll need to resize your array. ( newnum is the new number of libraries )

Code Select Expand


struct mystruct * oldarray = dlarray;
struct mystruct * newarray = (struct mystruct *)malloc(newnum * sizeof(struct mystruct));

/* remember num is the old size */
dlarray = (struct mystruct *)memcpy((void *)newarray, (void *)oldarray, num * sizeof(struct mystruct));
free(oldarray);

num = newsize;

/* populate new */
dlarray[num - 1].member = something;


[Zakath]

I recommend using new and delete rather than malloc and free, since their usage is so much simpler. It's up to you, of course, but I find that

Code Select Expand

mystruct *dlarray = new mystruct[ num ];is easier to work with than

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mystruct *dlarray = (mystruct *)malloc( num * sizeof( mystruct ));

[warz]

Ah, this is in C. Sorry, forgot to mention that.

[warz]

this is pretty basic. if you're going to be iterating through these a lot, you'll probably want to look into making a linked list.

anyways, first you'll need to count up the number of libraries are you are loading initially.

create your array of structs ( num is the number of initial libraries )

Code Select Expand


int i;
struct mystruct * dlarray = (struct mystruct *)malloc(num * sizeof(struct mystruct));

/* populate it now */
for(i = 0; i < num; i++)
{
         dlarray[i].member = something;
}


when you want to add a library, you'll need to resize your array. ( newnum is the new number of libraries )

Code Select Expand


struct mystruct * oldarray = dlarray;
struct mystruct * newarray = (struct mystruct *)malloc(newnum * sizeof(struct mystruct));

/* remember num is the old size */
dlarray = (struct mystruct *)memcpy((void *)newarray, (void *)oldarray, num * sizeof(struct mystruct));
free(oldarray);

num = newsize;

/* populate new */
dlarray[num - 1].member = something;


Perfect, Maddox. Works great. Thanks, brotha. hehe.

[Kp]

Maddox's code reflects a C++ user trying to think in C.   There's no need to explicitly malloc/free when growing the array, just use realloc(void *old_memory, int new_size);  realloc will then handle the copy/free details for you, leaving you with one memory block of the new size (possibly at a new address) which is guaranteed to have the same contents for the first min(oldsize, newsize) bytes.  C++'s memory allocation operators lack an equivalent call.

[Maddox]

Maddox's code reflects a C++ user trying to think in C.   There's no need to explicitly malloc/free when growing the array, just use realloc(void *old_memory, int new_size);  realloc will then handle the copy/free details for you, leaving you with one memory block of the new size (possibly at a new address) which is guaranteed to have the same contents for the first min(oldsize, newsize) bytes.  C++'s memory allocation operators lack an equivalent call.

this is true. I'm actually writing some C code right now, so this will come in handy.

[Zakath]

I would say that thinking in C++ isn't such a bad thing

C: myfunc( stuff ) int stuff; { /* function body */ }

C++: void myfunc( int stuff ) { /* function body */ }

[Kp]

I would say that thinking in C++ isn't such a bad thing

C: myfunc( stuff ) int stuff; { /* function body */ }

C++: void myfunc( int stuff ) { /* function body */ }

That's not at all relevant, and the latter style has been supported (and afaik encouraged) by gcc for ages.  Anyway, thinking in C++ is a bad idea when you're trying to write C code, since there's some C idioms that got lost in C++ (for instance, the idea of using realloc, which got lost because nobody thought to put a reallocation operator in C++).  You can make something that works, but it'll generally be less elegant.