Core files ... still trying

[Ken Warner]

In article <11954 at mimsy.UUCP> chris at mimsy.UUCP (Chris Torek) writes:
>In article <790 at scubed.UUCP> warner at scubed.UUCP (Ken Warner) writes:
>>Is there a way to run a core file?
>
>It cannot be done portably, but with certain restrictions, it can
>almost always be done.
[stuff deleted]
>-- 
>In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
>Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris

Well, I've looked at unexec() from emacs and then tried to write my own version
of a fuction that will create a new executable, saving the current data space.
It does work (sort of), but when the new executable is run, a segmentation
violation occurs in the clean up on exit.  I dunno how to deal with this.
Anyone care to comment?  Below is a stack trace from dbx showing the error. 
Below that is the fuction I've been working with.  

Basically, the function (my)unexec() copies the text segment from the 
executable file since that won't be changing, then copies the data segment 
from memory.  Then copies the symbol table and string table from the executable
file.  The result is an executable with a snapshot of the data segment.  Like
I said, it runs but dies a horrible death on exit.  ut.c was the name of
my main().

-------------- dbx stack dump -----------------------------------

signal SEGV (segmentation violation) in cfree at 0x4fe8
cfree+0x2c:             cmpl    a3@(8),a5
(dbx) where
cfree(0x23458, 0x0) at 0x4fe8
free(0x2345c) at 0x53eb
fclose(0x206f4) at 0x48b3
_fwalk(0x4848) at 0x4f35
_cleanup(0x1, 0x0) at 0x4841
exit(0x1) at 0x4ca9
main(argc = 1, argv = 0xefffb8c, 0xefffb94), line 78 in "ut.c"
(dbx)  

--------------- myunexec.c -------------------------------------

#include <a.out.h>
#include <sys/file.h>
#include <sys/types.h>
#include <stdio.h>
#include <sys/stat.h>
#include <errno.h>

extern int first(); /* this was the name of a little stub in main() */
extern char *sbrk ();
extern int end,edata,etext;
static struct exec hdr;

/* ****************************************************************
 * unexec(new_name,a_name) where new_name is the name of the new executable
 * and a_name is the name of the file containing the currently executing
 * program
 *
 */
unexec (new_name, a_name)
char *new_name, *a_name;
{

    int new, a_out = -1;
    if (a_name && (a_out = open (a_name, O_RDONLY)) < 0)
    {
	perror (a_name);
    }
    if ((new = open (new_name, O_WRONLY|O_CREAT|O_TRUNC,0755)) < 0)
    {
	perror (new_name);
    }

    if (make_a_out(new, a_out,new_name,a_name) < 0)
    {
	close (new);
	/* unlink (new_name);	    	/* Failed, unlink new a.out */
	return -1;	
    }

	close (new);
	if (a_out >= 0)
	    close (a_out);
	return 0;
}

/* ****************************************************************
 * make_a_out
 */
static int make_a_out (new, a_out,new_name,a_name)
int new, a_out;
char *new_name,*a_name;
{
	char buff[PAGSIZ];
	unsigned int i,numrd,pos,bss_end;
	char zbuf[4];

    bzero(buff,PAGSIZ);

/* read the header */
    if (read (a_out, &hdr, sizeof(hdr)) != sizeof hdr)
    {
      perror (a_out);
    }
    if (N_BADMAG (hdr))
    {
      printf("invalid magic number in %s", a_name);
      fflush(stdout);
      exit(-1);
    }

/* rewind a.out */
    lseek (a_out,0,0);

/*snarf and plop text from old a.out to new */
    for(i=0;i<hdr.a_text;i+=PAGSIZ)
    {
	bzero(buff,PAGSIZ);
	if (read (a_out,buff,PAGSIZ) != PAGSIZ)
	{
	  perror ("make_hdr:#1 read(a.out)");
	  exit(-1);
	}
	if(write(new,buff,PAGSIZ) != PAGSIZ)
	{
		perror("make_hdr:#2 write(a.out)");
		exit(-1);
	}
    }

/*read and write the runtime data space */
    for(i=N_DATADDR(hdr);i<N_DATADDR(hdr)+ hdr.a_data;i++) 
    {
	if(write(new,(char *)i,1) != 1)
	{
	    printf("i = %d\n",i);
	    perror("make_hdr:#3 write(new)");
	    fflush(stdout);
	    exit(-1);
	}
    }
/* jump over data in a_out and align to page boundry in new */
    lseek(a_out,N_SYMOFF(hdr),L_SET);
    pos = lseek(new,0,L_INCR);
    lseek(new,(PAGSIZ + ((pos - 1) & ~(PAGSIZ - 1))),L_SET);


#ifdef DEBUG
    printf("(PAGSIZ + ((pos - 1) & ~(PAGSIZ - 1))) = %x\n", PAGSIZ + ((pos - 1) & ~(PAGSIZ - 1)));
#endif
/* read symbol and text string space */ 
    while(1)
    {
	bzero(buff,PAGSIZ);
	if ((numrd = read (a_out,buff,PAGSIZ)) != PAGSIZ)
	{
	    printf("Last page read. \n");
	    for(i=0;i<numrd;i++)
		    write(new,buff[i],1);
	    break;
	}
	if(write(new,buff,PAGSIZ) != PAGSIZ)
	{
	    printf("Last page written.\n");
	}
    }
    bss_end = (unsigned)sbrk(0);

/*if you want to diddle with the break this will round to next page boundry */
/*
	if((brk(PAGSIZ + ((bss_end - 1) & ~(PAGSIZ - 1)))) == -1)
		perror("brk");
	bss_end = (unsigned)sbrk(0);
	printf("bss_end = %d\n",bss_end);

    hdr.a_bss = (int)bss_end - (int)N_BSSADDR(hdr);
	printf("hdr.a_bss = %d\n",hdr.a_bss);
*/
/*rewind to begining of file */
    lseek (new,0,0);
    /* you can set the entry point to anything you want in the new executable 
    * ... but you will get a bus error on return since main is not on the stack
    */
    /*hdr.a_entry = first; */

/*write out the header if you changed anything */
    if (write (new, &hdr, sizeof hdr) != sizeof hdr)
    {
	perror ("make_hdr:#6 write(new)");
    }

    return 0;

}