Please explain why the program seg fault?

[sathishc58]

Hi All,

I tried to understand the reason for core dump but I could not get the reason. Could anyone help me to figure out the issue.

     1  #include <stdio.h> 
     2  #include <string.h> 
     3  #include <malloc.h> 
     4 
     5  int main() 
     6  { 
     7      int   i; 
     8      char *ptr[];   --> If dimension is given then the program 
is working fine. 
     9 
    10      for ( i = 0; i < 2; i++ ) 
    11      { 
    12          ptr[i] = malloc(6); 
    13 
    14          memcpy(ptr[i], "Hello", 6); 
    15      } 
    16 
    17      for ( i = 0; i < 2; i++ ) 
    18      { 
    19          printf("The value of i is %d\n", i); 
    20          printf("The string is %s\n", ptr[i]); 
    21          free(ptr[i]); 
    22      } 
    23 
    24      printf("out %d\n", i); 
    25 
    26      return 0; 
    27  }

[08:49:22]/home/e025400/sathish <-> cc -o arrofptr arrofptr.c
"arrofptr.c ", line 8: warning: null dimension: ptr


[08:49:28]/home/e025400/sathish <-> arrofptr
The value of i is 0
The string is Hello
The value of i is 1
The string is Hello
out 2
Segmentation Fault(coredump)


If I give dimension at "line no 8" then the program works fine else it dumps core after printing out all the values correctly. Please let me know the reason.

And I have a basic doubt, who argv[] in main() works.


Thanks & Regards
Sathish Kumar

[Banfa]

[08:49:22]/home/e025400/sathish <-> cc -o arrofptr arrofptr.c
"arrofptr.c ", line 8: warning: null dimension: ptr

This says it all really, you have created an array with a 0 dimension, then later in the program you use the array resulting in an out of bounds memory access.

If you don't want to give a dimension at compilation time declare it as char ** and allocate data for the array of pointers at run time.

[sathishc58]

Dear Banfa,

Thanks for your reply. Could you please let me know the suggestion/answer as how argv[] in main() behaves.

Because argv[] would be declared with NULL dimension always if I am not wrong. I wrote the program by keeping argv[] in mind.


Thanks & Regards
Sathish Kumar

[Dheeraj Joshi]

Dimension of argv is not NULL. Please read this.

Regards
Dheeraj Joshi

[Banfa]

In any function prototype, definition or declaration using the syntax int parameter[] is equivalent to int* parameter, that is it equates to the declaration of a pointer because you can not pass arrays as parameters or return them from functions.

In a block of code using [] declares an array. Using [] can be useful because then the compiler declares the array with the correct size to hold its initialisation data, for example

char text[] = "Hello World";

the compiler can tell that "Hello World" requires 12 char to contain it so automatically makes the array that size. If there is no initialiser list

char text[];

then there is nothing to hold and this becomes the equivalent of

char text[0];

An array with no or null size, which is not at all useful which is why your compiler produced a warning.