c - what is causing SIGSEV?

itemprop="text">

/*

learning
from all the post - please correct me if i am
wrong..

now it makes sense- if i remember it
right, the stack is a fixed memory segment- allocated on program start up... while the
virtual memory can be sized/resized programmatically using malloc, realloc, free...


the struct pointer array -

long size = 10000000;
struct foo
*bar[size];

should have been allocated from heap
- using malloc()... instead of just a fixed size stack (program text)

*/

This
one SIGSEV's:

#include
            
#include 

int main(void)
            {

 struct foo {
 int x;
 char s[5];

            };


 long size = 10000000;
 struct foo
            *bar[size];

 long i = 0;
 while (i < size)
            {
 printf("%ld \n", i);
 i++;

            }
}

This
one works - commenting out the struct foo pointer
array:

#include
            
#include 

int main(void)
            {

 struct foo {

 int x;
 char
            s[5];
 };

 long size = 10000000;
 //struct foo
            *bar[size];

 long i = 0;
 while (i < size)
            {
 printf("%ld \n", i);

 i++;

            }
}

This one
works - commenting our the while
loop:

#include
            
#include 


int
            main(void) {

 struct foo {
 int x;
 char
            s[5];
 };

 long size = 10000000;
 struct foo
            *bar[size];


 long i = 0;

 while (i
            < size) {
 //printf("%ld \n", i);
 i++;

            }

}

/*
what i really am trying to achieve is this... which SIGSEVS -
ok thanks for
all your replies i really appreciate it...
- will look int and use explore
using heap memory-- thanks guys
*/

int
main(void) {

struct foo
            {
 int x;
 char
            s[5];

};

long size =
            10000000;
struct foo *bar[size];

long i =
            0;
while (i < size) {
 bar[i] = (struct foo *)
            malloc(sizeof(struct foo));
 free(bar[i]);

            i++;

}
return
            EXIT_SUCCESS;

}

Answer

long size =
            10000000;
struct foo
            *bar[size];

will
create a very big array, which may cause , and therefore your program receive the
SIGSEV.

You should create this array
dynamically:

struct foo *bar =
            malloc(size * sizeof(struct foo
            *));

/>

Why does the program work normally if these is not
any function call in
main()?

The
definition of foo will cause main() to
have a large stack frame at runtime. If you does not call any function in
main(), this large stack frame will not be actually allocated
or accessed (the entrance code of main() only make sure that
amounts of memory be reserved by manipulating some registers and memory cells); but if
you call a function in main(), the calling itself will try to
access some addresses in that main() stack frame, because of ,
those addresses may not be valid, this will cause SIGSEV be
sent.

If you disassemble and compare the working
and not-working versions of this program, this would be obvious. You could also find it
out by stepping through the instructions of not-working main()
one by one.

/>

Without function call in
main():

0x00001ff0
            : push %ebp

0x00001ff1 : mov
            %esp,%eax
0x00001ff3 : mov %esp,%ebp
0x00001ff5
            : sub $0x2625a10,%esp
0x00001ffb : mov
            %eax,%esp
0x00001ffd : leave 
0x00001ffe
            : ret
            

Call
exit() in
main():

0x00001fe0
            : push %ebp
0x00001fe1 : mov
            %esp,%ebp
0x00001fe3 : sub $0x2625a28,%esp
0x00001fe9
            : movl $0x0,(%esp) <==== This causes segfault.
0x00001ff0
            : call 0x3000