char** in struct amd malloc()

This is a discussion on char** in struct amd malloc() within the C Programming forums, part of the General Programming Boards category; Hi, I am trying to pass data to a .DLL I want to use. DLL accepts this structure for its ...

  1. #1
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    char** in struct and malloc()

    Hi, I am trying to pass data to a .DLL I want to use. DLL accepts this structure for its data.

    Code:
    PTR 01FB317C    -> 01EEB208 -> ASCII "DATA0"
                       018947A0 ->          01F72f30 -> ASCII "DATA1"
                                     +8     01F74558 -> ASCII "DATA2"
                                     +12    01F742E8 -> ASCII "DATA3"
               			 +16    01F73FF8 -> ASCII "DATA4"
                                     +20    01FD3CF8 -> ASCII "DATA5"
              		         +24    01FD4EC8 -> ASCII "DATA6"
                			 +28    01F724D8 -> ASCII "DATA7"
                     		 +32             -> ASCII "DATA8"
                			 +36             -> ASCII "DATA9"
                			 +40             -> ASCII "DATA10"
    	    			 +44             -> ASCII "DATA11"
                			 +48             -> ASCII "DATA12"
    Pointers are from a call from real application.ASCII strings are of size 256.


    So i created this structure:


    Code:
    typedef struct
    	{
    	     char *s1;
                 char ** ss1;
    	} Rec1;

    Now I am trying to malloc before I put data on it


    Code:
          Rec1 *a;
    	
            a = (Rec1 *)malloc(sizeof(Rec1));
    	
    	a->s1 = (char*)malloc(16*sizeof(char));
    	
    	strcpy(a->s1, "DATA0"); <--- This works OK as seen on debugger
    	
    	a->ss1 = (char**) malloc(48);  <--- !CRASH!!! ; Trying to allocate space for pointers
    	
    	for( i=0; i < 12; i++)
    	a->ss1[i] = (char *)malloc(256);
    	
    	
    	strcpy(a->ss1[0], "DATA1");
    
            .......
    
    
            free(a);
    	for( i=0; i < 12; i++)
                 free( a->ss1[i] );
    	free( a->ss1 );
    Problem is the above malloc() crashes with C0000005

    Can you help me how is the correct way to do it?


    PS: On 1st declaration of structure it is char *........1; I keep editing the message and it keeps ignoring my edit always showing ......1; !!!! @#!
    PS2: I managed to do it
    Last edited by alexopth1512; 10-27-2010 at 03:56 AM.

  2. #2
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    It might help if you defined ss1 in your struct.

  3. #3
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    OK I solved it.
    Problem created by

    Code:
    strcpy(a->s1, "RI");
    that preceded the malloc()

    Now works like a charm, I ll check the strcpy calls, maybe change them with strncpy

  4. #4
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    In C there is no typecast for malloc needed. For constants you should use defines.
    Code:
    #define MAXSTRSIZE 257
    #define NUMPOINTER 12
    
          Rec1 *a;
    	
            a = malloc(sizeof*a);
    	
    	a->s1 = malloc(MAXSTRSIZE);
    	
    	strcpy(a->s1, "DATA0"); <--- This works OK as seen on debugger
    	
    	a->ss1 = malloc(NUMPOINTER * sizeof *a->ss1); 
    	
    	for( i=0; i < NUMPOINTER ; i++)
    	  a->ss1[i] = malloc(MAXSTRSIZE);
    	
    	
    	strcpy(a->ss1[0], "DATA1");
    
            .......
    
    
    	for( i=0; i < NUMPOINTER ; i++)
                 free( a->ss1[i] );
    	free( a->ss1 );
            free(a->s1);
            free(a);

  5. #5
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    > a->ss1 = (char**) malloc(48); <--- !CRASH!!! ; Trying to allocate space for pointers
    Assuming pointers have a particular size, and not using sizeof is the biggest problem here.

    BillyTKid's answer looks about right.
    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.
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  6. #6
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    Thanx guys for your help.
    malloc(48) was changed for test, i use now sizeof.
    everything works fine now, i changed strcpy with strncpy.
    Concerning typecast i dont seem to be able to get rid of it, compiler throws error, i use MS Visual C++ 2010 express

  7. #7
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    On VStudio you must work with *.c files for C programming, otherwise your code will compiled as C++!

  8. #8
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    Quote Originally Posted by BillyTKid View Post
    On VStudio you must work with *.c files for C programming, otherwise your code will compiled as C++!
    You are very correct!

    But you see, i am not a professional C programmer, i just make some utilities to make my work life easier. However I need it more and more.
    This little project is finished now and I am so happy
    On first i tried .c files but compiling failed, i guess there must be some options i have to set. I also dont know how to get rid of debugging library it needs all the time. It is very useful for debugging purposes but later I guess there is no need for debugging info.
    Anyway,
    I have searched for the topic I asked and couldn't find really useful info, I hope that this thread will help others.
    I wish I had more time to watch the forum but I have so little free time.

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