Thread: Segmentation fault (core dumped)

  1. #1
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    Segmentation fault (core dumped)

    Result is not as expected, line free(P); give output Segmentation fault (core dumped) what's the wrong with it
    Code:
    #include <stdio.h>#include <stdlib.h> 
    
    
     
    int main ()
    {
       int *P = NULL;
            P = malloc (1 * sizeof(int));
            
            if( P == NULL ){
              printf("Can't allocate memory!" );
            }
            else{
                  ( "Allocated memory!" );
                  P = 10;
                  printf(" P = %d", P);
            }
            
            printf(" P = %d", sizeof(P));
            free(P); // /*Release memory. */
            printf(" P = %d", sizeof(P));
            
        return 0;
    }

  2. #2
    and the hat of int overfl Salem's Avatar
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    Not compiling without a decent level of warnings enabled, to at least inform you when you're doing something dumb.
    Code:
    $ gcc -Wall foo.c
    foo.c: In function ‘main’:
    foo.c:13:15: warning: statement with no effect [-Wunused-value]
                   ( "Allocated memory!" );
                   ^
    foo.c:14:17: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
                   P = 10;
                     ^
    foo.c:15:22: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=]
                   printf(" P = %d", P);
                          ^
    foo.c:18:16: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long unsigned int’ [-Wformat=]
             printf(" P = %d", sizeof(P));
                    ^
    foo.c:20:16: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long unsigned int’ [-Wformat=]
             printf(" P = %d", sizeof(P));
                    ^
    Fix the errors, then try again.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    Quote Originally Posted by Salem View Post
    Not compiling without a decent level of warnings enabled, to at least inform you when you're doing something dumb.
    Fix the errors, then try again.
    Why the size of P is 4 after Release memory?

    P = 10
    P = 4
    P = 4

    Code:
    #include <stdio.h>#include <stdlib.h> 
     
    int main ()
    {
       int *P = NULL;
            P = malloc (1 * sizeof(int));
             
            if( P == NULL ){
              printf("Can't allocate memory!" );
            }
            else{
                  ( "Allocated memory!" );
                  *P = 10;
                  printf(" P = %d \n", *P);
            }
             
            printf(" P = %d \n", sizeof(*P));
            free(P); // /*Release memory. */
            printf(" P = %d", sizeof(*P));
             
        return 0;
    }

  4. #4
    and the hat of int overfl Salem's Avatar
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    > Why the size of P is 4 after Release memory?
    Because sizeof() tells you the size of objects.

    P = NULL;
    P = malloc (1 * sizeof(int));
    P = malloc (10000 * sizeof(int));

    Whether you do
    printf(" P = %d", sizeof(P));
    or
    printf(" P = %d", sizeof(*P));

    You'll get the same answer each time, no matter how (or even if) you initialise your pointer variable.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  5. #5
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    Quote Originally Posted by Salem View Post
    > Why the size of P is 4 after Release memory?
    Because sizeof() tells you the size of objects.
    What I think is

    P = NULL; memory is not allocated so size should be 0
    P = malloc (1 * sizeof(int)); memory allocated so size should be 4
    P = free(P); memory has been released so the size should be 0

  6. #6
    and the hat of int overfl Salem's Avatar
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    Well if you want those answers, then sizeof() is the wrong thing to use.

    sizeof() happens at compile time, so it can't possibly give you a run-time answer.

    In fact, there is no portable way to get the answers you want.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  7. #7
    C++ Witch laserlight's Avatar
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    I suppose there is a portable way to get the answers you want. It's called "keep track of the number of bytes allocated"
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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