Thread: sysmalloc: Assertion SIGABRT crash

Simple study aid.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
  int arr[10];
} foo_t;

void show ( foo_t *p ) {
  for ( int i = 0 ; i < 10 ; i++ ) {
    printf("%d ", p->arr[i]);
  }
  printf("\n");
}
int main ( ) {
  foo_t original = { {
    1,2,3,4,5,6,7,8,9,10
  } };
  show(&original);
  foo_t *p = malloc(sizeof(*p));
  memcpy(p,&original,sizeof(p));  //!! oops, size of pointer
  show(p);  // garbage
  memcpy(p,&original,sizeof(*p)); //!! better, size of thing being pointed to
  show(p);  // better
  *p = original;  // yes, struct assignment works too
  show(p);
  return 0;
}


$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:21:28: warning: argument to ‘sizeof’ in ‘memcpy’ call is the same expression as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
   memcpy(p,&original,sizeof(p));
                            ^
$ ./a.out 
1 2 3 4 5 6 7 8 9 10 
1 2 0 0 0 0 0 0 0 0 
1 2 3 4 5 6 7 8 9 10 
1 2 3 4 5 6 7 8 9 10 
$

Yes, fix this valgrind error (which repeats ad nauseam)

Code:

$ gcc -Wall main.c
$ gcc -Wall -g main.c
$ valgrind ./a.out 
==10664== Memcheck, a memory error detector
==10664== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==10664== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==10664== Command: ./a.out
==10664== 
==10664== Invalid write of size 1
==10664==    at 0x4C3106F: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10664==    by 0x401185: ffnode (main.c:306)
==10664==    by 0x4019C4: syntax (main.c:443)
==10664==    by 0x401AF1: main (main.c:478)
==10664==  Address 0x5203165 is 0 bytes after a block of size 5 alloc'd
==10664==    at 0x4C2FB55: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10664==    by 0x40198C: syntax (main.c:440)
==10664==    by 0x401AF1: main (main.c:478)

Code:

  pt->value = calloc(strlen(S)+1, sizeof(char));
  strcpy(pt->value, S);
  
  err = ffnode(pt, lex);

ffnode also (ab)uses pt->value as well - should it?

Can you see it yet?

Code:

$ gcc -Wall -g main.c
$ gdb ./a.out
Reading symbols from ./a.out...done.
(gdb) b 306
Breakpoint 1 at 0x40115c: file main.c, line 306.
(gdb) run
Starting program: /home/sc/Documents/a.out 

Breakpoint 1, ffnode (inode=0x604080, lex=0x604010 "D AO IW D SO IW SO D OW OW") at main.c:306
306			 strcpy(inode->value, tokens[i]);
(gdb) print inode->value
$1 = 0x6040b0 "MOPN"
(gdb) print tokens[i]
$2 = 0x604480 "OPNP1"
(gdb) up
#1  0x00000000004019c5 in syntax (pt=0x604080, lex=0x604010 "D AO IW D SO IW SO D OW OW") at main.c:443
443	  err = ffnode(pt, lex);
(gdb) print S
$3 = 0x401cf5 "MOPN"
(gdb) print strlen(S)+1
$4 = 5

You allocated 5 bytes.
OPNP1 takes 6

Well I've shown you how to use gdb, so it's time to see what you can do.