Re: C programming: Segmentation fault within malloc?

[Rick Macdonald <rickmacd@home.com>]

Try breaking the mallocs into separate lines to see which one fails:

     int errflag = 0;
     if (!(node = (struct node *)malloc(sizeof(struct node)))) {
          errflag = 1;
     } else {
          if (!(node->data = (struct symbol *)malloc(sizeof(struct
symbol)))) {
               errflag = 2;
          }
     }
     if (errflag) {
          fprintf(stderr, "errflag=%d\n");
          fprintf(stderr, sym_tab_msg[MEMORY_ALLOCATION_FAILURE]);
          return FALSE;
     }

If the first one sets errflag without a segfault in malloc, it would seem
that your original compound statement tried to do the second malloc. The
second malloc would certainly fail if the first malloc results in "node"
being a null pointer, because it would try to set node->data. You can try
to test this with:

     if (!((node = (struct node *)malloc(sizeof(struct node))) &&
           (fprintf (stderr, "should not get here\n")))) {
     }


On Thu, 26 Jul 2001, Shaul Karl wrote:

> > Have you tried to look at the value sizeof(struct node)? It might be too big.
> > Otherwise, can you show us the backtrace in gdb.
> > 
> 
> 
> gdb says sizeof(struct node) == 20. It is mostly a couple of pointers:
> 
>  struct node {
>     enum colors   color;        
>     struct node  *left, *right, *parent;
>     void         *data; 
> };
> 
> 
> Breakpoint 1, insert_symbol (sym=0xbfffe25c) at symbols.c:197
> 197         if (!((node = (struct node *)malloc(sizeof(struct node)))  &&
> (gdb) p sizeof(struct node)
> $1 = 20
> (gdb) c
> Continuing.
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x400af19e in malloc () from /lib/libc.so.6
> (gdb) bt
> #0  0x400af19e in malloc () from /lib/libc.so.6
> #1  0x400ae844 in malloc () from /lib/libc.so.6
> #2  0x804c60a in insert_symbol (sym=0xbfffe25c) at symbols.c:197
> #3  0x8048c6f in tmp_variable (tmp=0xbfffe398, sym_data=0xbfffe2ec)
>     at actions.c:115
> #4  0x80497be in sym_for_const (tok=0xbfffe4f8, sym=0xbfffe398)
>     at actions.c:412
> #5  0x804d55f in yyparse () at parser.y:216
> #6  0x804ddd5 in translator (params=0xbffffb80) at parser.y:272
> #7  0x804b08f in main (argc=1, argv=0xbffffc98) at main.c:195
> #8  0x4005b2db in __libc_start_main () from /lib/libc.so.6
> (gdb) 
> 
> I believe that malloc is called twice here due to line 198:
> 
>     if (!((node = (struct node *)malloc(sizeof(struct node)))  &&
>           (node->data = malloc(sizeof(struct symbol))))) {
> 
> and sizeof(struct symbol) is 52.
> 
> 
> 
> > On [Thu, 26 Jul 2001 03:15:46 +0300], Shaul Karl <shaulka@bezeqint.net> wrote:
> > > Breakpoint 2, insert_symbol (sym=0xbfffe25c) at symbols.c:197
> > > 197         if (!((node = (struct node *)malloc(sizeof(struct node)))  &&
> > > (gdb) l 197
> > > 192
> > > 193     enum flag insert_symbol(struct symbol *sym)
> > > 194     {
> > > 195         struct node  *node;
> > > 196
> > > 197         if (!((node = (struct node *)malloc(sizeof(struct node)))  &&
> > > 198               (node->data = (struct symbol *)malloc(sizeof(struct 
> > > symbol))))) {
> > > 199             fprintf(stderr, sym_tab_msg[MEMORY_ALLOCATION_FAILURE]);
> > > 200             return FALSE;
> > > 201         }
> > > (gdb) n
> > > 
> > > Program received signal SIGSEGV, Segmentation fault.
> > > 0x400af19e in malloc () from /lib/libc.so.6
> > > (gdb) 
> > > 
> > > 
> > > How can it be? If malloc can not allocate memory it should return a NULL 
> > > pointer. How can it Seg fault?
> > > 
> > > [03:09:45 16]$ free
> > >              total       used       free     shared    buffers     cached
> > > Mem:         63584      60936       2648      31452       1344      20472
> > > -/+ buffers/cache:      39120      24464
> > > Swap:       116924      52580      64344
> > > [03:09:49 16]$ 
> > > 
> > > Since all the memory is used and the machine is running for some time now, 
> > > doesn't that precludes hardware problems?
> > > 
> > > Obviously I am missing something.
> > > 
> > > 
> > > -- 
> > > 
> > > 	Shaul Karl <shaulka@bezeqint.net>
> > > 
> > > 
> > > 
> > > -- 
> > > To UNSUBSCRIBE, email to debian-user-request@lists.debian.org 
> > > with a subject of "unsubscribe". Trouble? Contact listmaster@lists.debian.org
> > > 
> > 
> > -- 
> > Shao Zhang                          Tel:  (02) 9209 4838
> > Software Engineer                   Fax:  (02) 9209 4992
> > Redfern Broadband Networks (RBN)    Mail: szhang@rbni.com
> > 
> > 
> > -- 
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> > with a subject of "unsubscribe". Trouble? Contact listmaster@lists.debian.org
> > 
> 
> -- 
> 
> 	Shaul Karl <shaulka@bezeqint.net>
> 
> 
> 
> -- 
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> 
> 

...RickM...