[SOLVED] new array in a function

NevemTeve · 07-07-2020, 09:53 PM

TL;DR there is now `new` in C, use malloc/free/realloc.

Code:

int myalloc(Elem **retp, unsigned elements) {
    Elem *p= malloc (sizeof(Elem)*elements);
    if (!p) exit(20);
    *retp= p;
    return 0;
}

int main (void) {
    Elem *arrayptr= NULL;
    myalloc(&arrayptr, 21);
    ...
    free(arrayptr);
    return 0;
}

pan64 · 07-08-2020, 01:44 AM

Quote:

Originally Posted by bkelly

Yes, it mostly makes sense. I think you explained it well. I was thinking along the lines of **. Combining the * & gives the compiler more information, I presume. And yes, the code does work for me.
Thank you for your time and patience.

I used to say & means (when using here) I pass the variable by its location, so the function will take/handle the original variable. Without & a copy will be passed to the function and that copy will be used inside the function (and will be lost at the end of the function).
All the other things (like: float *farr) are the type and the name of the variable.

pan64 · 07-08-2020, 01:58 AM

2 comments to post #16 (and in general) - although probably they are not really important here:
1: You can return with an error code from myalloc, and handle that error in main (exiting inside a function is not always fortunate).
2: before malloc the previous content of retp should be freed or will be leaked.

NevemTeve · 07-08-2020, 03:43 AM

Well, in most cases, there's no sensible action when malloc fails, so maybe one should go with some `emalloc/ecalloc/erealloc` functions:

Code:

void *erealloc (void *oldp, size_t newsize) {
    void *newp= realloc (p, newsize);
    if (newp!=NULL || newsize==0) return newp;
    fprintf (stderr, "*** realloc(%p, %lu) failed, exiting\n", oldp, (unsigned long)newsize);
    exit(20);
}

SoftSprocket · 07-08-2020, 10:06 AM

It can be useful if it is a server. Unwinding the stack, freeing memory as you go may keep the system up and allow for alarms to be issued and responded to - without the server crashing completely. It may also allow for freeing resources other then memory before the server shuts down. Personally I'd prefer to make the decision somewhere other then in an allocation routine for most applications.