realloc segfaults.

This is a discussion on realloc segfaults. within the C Programming forums, part of the General Programming Boards category; Are you sure you are needing to use realloc, is this some realloc exercise? As calling this over and over ...

  1. #16
    Registered User slingerland3g's Avatar
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    Are you sure you are needing to use realloc, is this some realloc exercise? As calling this over and over for each new student roll to be entered is costly.

    I would prefer you change over to strictly using an array for this and is much faster to insert.

    Code:
     StdInfo students[10];
    
    ...
    
     do
        {
          printf ("Enter Roll of Student %d :\t", i);
          scanf ("%d", &(students[i++].roll));
          printf ("\n\nEnter another ? (1/0):\t");
          scanf ("%d", &s);
        }
      while (s == 1);
    Remove your realloc and any malloc calls, you do no need them then!


    Then do double check that they have been inserted try something like this:

    Code:
      int z = 0;
    
      for (; z < i; z++)
        {
          printf ("Students rolls are:  student %d, roll = %d\n", z,
                  students[z].roll);
        }

  2. #17
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    Quote Originally Posted by slingerland3g View Post
    Are you sure you are needing to use realloc, is this some realloc exercise?
    You can assume something like this.I was showing some of my friends about when we might need to use realloc. suddenly I got stuck and more than 8 hours past still trying to find the answer. I must solve it by using malloc and realloc only.

  3. #18
    Algorithm Dissector iMalc's Avatar
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    The OP has already been told that he can't scanf into a short, here: http://www.gamedev.net/community/for...opic_id=515880
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  4. #19
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    Quote Originally Posted by iMalc View Post
    The OP has already been told that he can't scanf into a short, here: http://www.gamedev.net/community/for...opic_id=515880
    Currently there is no short in my code I've changed all of them to normal int

  5. #20
    Jack of many languages Dino's Avatar
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    %d pulls in an integer.
    s is a short.

    Can you spell O-V-E-R-L-A-Y?
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  6. #21
    Jack of many languages Dino's Avatar
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    Quote Originally Posted by noobcpp View Post
    Currently there is no short in my code I've changed all of them to normal int
    Then what's the problem? (and none of the code you've posted has s as an int.)
    Mac and Windows cross platform programmer. Ruby lover.

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    Amen brother!

  7. #22
    Registered User slingerland3g's Avatar
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    What Dino said. Your variable assignment for
    Code:
     unsigned short int s = 1
    should be

    Code:
    int s = 1;

  8. #23
    Captain Crash brewbuck's Avatar
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    Doubling the size of the array on each entry will make it exceed 4 gigabytes within 32 iterations...
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  9. #24
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    Quote Originally Posted by slingerland3g View Post
    What Dino said. Your variable assignment for
    Code:
     unsigned short int s = 1
    should be

    Code:
    int s = 1;
    Ya I've already done it and it gives the same result no change.
    Well this is my current code.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    struct StudentInfo{
    	//char* name;
    	unsigned int roll;
    };
    
    typedef struct StudentInfo StdInfo;
    
    int main(int argc, char** argv){
    	StdInfo* students;
    	unsigned int size = 3, i=0, j;
    	int s = 1;
    	char* dump_name;
    
    	students = malloc( size*sizeof(StdInfo) );
    
    	do{
    		//StudentInfo.name removed
    
    		printf("Enter Roll of Student %d :\t", i);
    		//fflush(stdin);
    		int dmp_roll;
    		scanf("%d\n", &dmp_roll);
    		students[i].roll = dmp_roll;
    		printf("\n\nEnter another ? (1/0):\t");
    		//fflush(stdin);
    		scanf("%d\n", &s);
    		//fflush(stdin);
    		if(s == 1){
    			size += 1;
    			void* dump = realloc( students, size*sizeof(StdInfo) );
    			if(dump != 0x0){
    				students = dump;
    			}else{
    				printf("realloc Failed\n");
    				return 2;
    			}
    		}
    		i++;
    	}while(s == 1);
    
    	free(students);
    	students = 0x0;
    
    	return 0;
    }
    It also makes scanf problem.

    Quote Originally Posted by brewbuck View Post
    Doubling the size of the array on each entry will make it exceed 4 gigabytes within 32 iterations...
    I've already changed it to size += 1 So its not doubling the size.

  10. #25
    Algorithm Dissector iMalc's Avatar
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    That code does not crash, but it's question asking does get out of sync with my input. Being one who has practically never used scanf (I program real-world apps ), I'll refrain from trying to help you there.
    It is a bit odd that you'd allocate space for two more items than you are actually using though.
    Those commented out fflush's of stdin in better stay that way. Putting them in would be a bad thing to do as it's UB. Best remove those lines.
    I could also comment about many style issues that bug me. For example dump != NULL would be more appropriate than dump != 0x0.
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