unions and structs

I can't understand why I get the following errors.
Here is my code:

Code:

union header{ /* block header [LINE 11] */ struct{ unsigned int size; /* size of this block */ int prev_check = 0; union header *ptr; /* next block if on free list */ } s; Align x; /* force alignment of blocks */ }; typedef union header Header; /*[LINE 20]*/

These are the errors that I get:

Code:

mymalloc.c:14: warning: no semicolon at end of struct or union mymalloc.c:14: error: syntax error before '=' token mymalloc.c:14: warning: no semicolon at end of struct or union mymalloc.c:17: error: syntax error before '}' token mymalloc.c:17: warning: data definition has no type or storage class mymalloc.c:19: error: syntax error before '}' token

I am getting this error just by declaring a variable: int prev_check. I wont get these errors when I remove that line. Please advise.

You can't initialize variables in a union declaration. In other words: no = 0 allowed.

Just took a baton and hit my head.

What should I be searching for if I get the following:

Code:

warning: implicit declaration of function `morecore' warning: assignment makes pointer from integer without a cast At top level: error: conflicting types for 'morecore' error: previous implicit declaration of 'morecore' was here

This is the snippet of code

Code:

if (p == freeptr) /*p & freeptr are pointers */ if ((p = morecore(nunits)) == NULL) /*morecore returns a pointer */ return NULL; /* none left */

You might be missing a header file -- apparently there was no declaration of morecore in scope at the first part. (The second one is probably your definition of morecore, with the proper types, which don't match the implicit int.)

The implicit declaration warning (and the assignment one) means that the function isn't declared at the point where you call it; you probably forgot to put a prototype in or include a header file.

On the other hand, the other errors mean that your prototype and function declaration don't match.

At a guess, you probably changed morecore() somehow and forgot to update the prototype, and the prototype itself occurs after the code you posted or in a different file.

Perhaps you have two functions called morecore as well, but that's somewhat less likely . . . .

[edit] Here I was assuming that you wrote morecore() yourself. tabstop's idea is probably more likely. [/edit]

Well you were right, I forgot to add it in the header file. But after adding it (same function statement) I get 2 error for previous declaration and conflicting type errors for the same line in the header file as well as c file.

Code:

mymalloc.c:69: error: conflicting types for 'morecore' mymalloc.h:6: error: previous declaration of 'morecore' was here mymalloc.c:69: error: conflicting types for 'morecore' mymalloc.h:6: error: previous declaration of 'morecore' was here

So now there's a conflict between your prototype in your header file and reality (as expressed in your c file). Just copy the top line of your function, change the curly-brace to a semicolon, and make that your prototype -- it will save heartache later on.

As I had said in my earlier mail, I had done so. If you look at the earlier errors mentioned in this forum. Im thinking does it have anything to do with the warning : assignment makes pointer from integer without a cast.

Quote:

Originally Posted by Ron

As I had said in my earlier mail, I had done so. If you look at the earlier errors mentioned in this forum. Im thinking does it have anything to do with the warning : assignment makes pointer from integer without a cast.

You just said you aren't getting that warning any more.

But no, what I mean is that your prototype might be something like

Code:

int * morecore(int something);

but your code actually has

Code:

int morecore(int something) { /* stuff */ }

thank you 'tabstop', but that is what I had done. Still the errors.

Quote:

Originally Posted by Ron

thank you 'tabstop', but that is what I had done. Still the errors.

What I'm trying to get you to do is show us line 69 of mymalloc.c and line 6 of mymalloc.h, which are the two lines mentioned in the last error message you posted.

Code:

line 6 static Header *morecore(unsigned long nunits); line 69 static Header *morecore(unsigned long nunits) { }

Well, I've tried to recreate the error, but can't. (I had at first thought that a forward declaration of Header might cause problems, but that seemed to work just fine.) The only thing I can think of at this point is that Header might mean different things at different times? That's the best guess I can make right now.

Did you use the function before its declaration?

static functions are only visible in the file they are declared/defined in. The two morecore declaration and definitions are not the same function.

Well, I'm assuming he's not compiling his .h file separately (although perhaps I shouldn't). I had tried it (as mentioned above) with the prototype in a #included header, and that works just fine (as it should).

Yes I am running the static function in the same file.
And , am I not right in this. As I have included the . h in the beginnning of my program; wont the compiler run the .h file before running the program file.
Hence I have not compiled my .h file seperately. Even if I did, it should not hurt. As the compiler will anyway do the same.

Does your prototype

Code:

static Header *morecore(unsigned long nunits);

occur before the definition of Header? (I can't see that resulting in your error, but still . . . .)

Could you post your complete code, .c and .h files?

Header file:

Code:

#ifndef MYMALLOC_H #define MYMALLOC_H void *mymalloc(unsigned long nbytes); static Header *morecore(unsigned long nunits); void myfree(void *p); #endif

C File:

Code:

#include <stdio.h> #include <stdlib.h> #include <assert.h> #include "mymalloc.h" #define MEM_SIZE 1024 * 1024 #define NALLOC 1024 typedef long Align; /* for alignment to long boundary */ union header{ /* block header */ struct{ unsigned long size; /* size of this block */ int prev_check; union header *ptr; /* next block if on free list */ } s; Align x; /* force alignment of blocks */ }; typedef union header Header; static Header base; /* empty list to get started */ static Header *freeptr = NULL; /* start of free list */ /* malloc: general-purpose storage allocator */ void *mymalloc(unsigned long nbytes) { Header *p, *prevptr; unsigned long nunits; nunits = (nbytes+sizeof(Header)-1)/sizeof(Header) + 1; if ((prevptr = freeptr) == NULL) { /* no free list yet */ base.s.ptr = (Header *)calloc(MEM_SIZE,sizeof(Header)); base.s.size = 0; /* allocase a memory from OS and then manage it */ base.s.ptr->s.size = MEM_SIZE; base.s.ptr->s.ptr = &base; freeptr = prevptr = &base; } for (p = prevptr->s.ptr; ; prevptr = p, p = p->s.ptr) { if (p->s.size >= nunits) { /* big enough */ if (p->s.size == nunits) /* exactly */ prevptr->s.ptr = p->s.ptr; else { /* allocate tail end */ p->s.size -= nunits; p += p->s.size; p->s.size = nunits; } freeptr = prevptr; base.s.prev_check =0; return (void *)(p+1); } if (p == freeptr) /* wrapped around free list */ if ((p = morecore(nunits)) == NULL) return NULL; /* none left */ } } static Header *morecore(unsigned long nunits) { char *cp, *sbrk(int); Header *up; if (nunits < NALLOC) nunits = NALLOC; cp = sbrk(nunits * sizeof(Header)); if (cp == (char *) -1) return NULL; up = (Header *) cp; up->s.size = nunits; free ((void *)(up+1)); return freeptr; } /* free: put block ap in free list */ void myfree(void *ap) { Header *bp, *p; bp = (Header *)ap - 1; /* point to block header */ printf("bp->s.size %d\n",bp->s.size); for (p = freeptr; !(bp > p && bp < p->s.ptr); p = p->s.ptr) if (p >= p->s.ptr && (bp > p || bp < p->s.ptr)) { printf("in the corner\n"); break; /* freed block at start or end of arena */ } bp->s.prev_check =1; if (bp + bp->s.size == p->s.ptr) { /* join to upper nbr */ bp->s.size += p->s.ptr->s.size; bp->s.ptr = p->s.ptr->s.ptr; } else bp->s.ptr = p->s.ptr; if (p + p->s.size == bp) { /* join to lower nbr */ p->s.size += bp->s.size; p->s.ptr = bp->s.ptr; } else p->s.ptr = bp; freeptr = p; }

Yes, you see, my guess was right. You can't use Header in the header file because it hasn't been defined yet.

Move this

Code:

typedef long Align; /* for alignment to long boundary */ union header{ /* block header */ struct{ unsigned long size; /* size of this block */ int prev_check; union header *ptr; /* next block if on free list */ } s; Align x; /* force alignment of blocks */ }; typedef union header Header;

into the header file, probably before your function prototypes.

Ok I wasnt aware of that. That has fixed the problem.
Now something I cant figure out.

Code:

Undefined first referenced symbol in file main /usr/lib/crt1.o ld: fatal: Symbol referencing errors. No output written to a.out collect2: ld returned 1 exit status

But how am I getting undefined symbol main when the main function is not in this file?

It looks like you invoked the linker and tried to create an executable.

When you're dealing with multiple source files, compile them with -c, and then link the object files together together without -c.

Code:

$ gcc -c source1.c $ gcc -c source2.c $ gcc source1.o source2.o -o executable $ ./executable

You can't get an executable out of a file that doesn't have a main. If this is just supposed to be a library, do "compile only" or something similar -- or add a "real" program to your project so that it will have something to do.

Thats a great sigh of relief. I didnt know about the -c functionality. I thought there was something wrong with my program.