a problem we couldnt fix...

This is a discussion on a problem we couldnt fix... within the C Programming forums, part of the General Programming Boards category; Code: #include<stdio.h> #include<conio.h> #include<stdlib.h> char* saisit_chaine_sans_tampon() { int i,s; char *q; printf("Ecrivez une chaine de caractere-Pour sortir touche '1'"); (char*)realloc(q,(1*sizeof(char))); ...

  1. #1
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    a problem we couldnt fix...

    Code:
    #include<stdio.h>
    #include<conio.h>
    #include<stdlib.h>
    char* saisit_chaine_sans_tampon()
    {
           int i,s;
           char *q;
           printf("Ecrivez une chaine de caractere-Pour sortir touche '1'");
           (char*)realloc(q,(1*sizeof(char)));
           *q='a';//je donne une valeur initialement,sinon il y une errruer
           for(i=0;q[i-1]!='1';i++)
           {
                                  q[i]=getch();
                                  s=i+1;
                                  (char*)realloc(q,(s*sizeof(char)));
    
                                  }
                                  q[i-1]='\0';
                                  return q;
                                  free(q);
    }
    
    int main()
    {
        char *p;
        p=saisit_chaine_sans_tampon();
        printf("%s",p);
        system("PAUSE");
     return 0;
    }
    for compiler everything is allright but its not working.so couldnt get whats wrong please help me
    Attached Files Attached Files
    • File Type: c ex05.c (977 Bytes, 67 views)
    Last edited by Salem; 02-24-2009 at 12:52 PM. Reason: [CODE][/CODE] go around the code!!!

  2. #2
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    You are coding obviously with C. Why are you including one of the C++ header file?
    More information: http://en.wikipedia.org/wiki/Conio.h

    return q;
    free(q);

    You cant free after return.
    Always check return value of realloc to be sure that memory has been occupied for your use....
    Last edited by Skvr; 02-24-2009 at 01:01 PM. Reason: Link

  3. #3
    Registered User carrotcake1029's Avatar
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    You can't realloc without ever having allocated room for that pointer in the first place.

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    Quote Originally Posted by carrotcake1029 View Post
    You can't realloc without ever having allocated room for that pointer in the first place.
    You can:
    Quote Originally Posted by man realloc
    If ptr is NULL, realloc() behaves like malloc() for the specified size. If size is 0 and ptr is not a null pointer, the space pointed to is freed.
    However, if ptr is just whatever it happens to be on the stack, like in this case, it's undefied.


    For i = 0:
    Code:
    q[i-1]!='1'
    is undefined.

    --
    Mats
    Last edited by matsp; 02-24-2009 at 01:37 PM.
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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    guys all right i got u thanks anyway hao can i create a function which cover getch() as like c++ or is there any kind of function in c cover it?

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by ibrahim630
    guys all right i got u thanks anyway hao can i create a function which cover getch() as like c++ or is there any kind of function in c cover it?
    Skvr is right to say that <conio.h> is not part of standard C, but wrong to say that it part of standard C++. As far as I know, the library provided by <conio.h>, if it is available, is available as a C library, as Skvr's own link indicates.

    However, as carrotcake1029 and matsp have pointed out, you have more important problems to worry about than the conio library.
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    Oh, and if realloc moves the block, this will fail horribly!
    Code:
                                 (char*)realloc(q,(s*sizeof(char)));
    You loose the value returned by realloc.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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    Yes Laserlight is right, just havent never used <conio.h> myself because i have thought it is Windows platform & C++ thing.I should read the links i post. Im little bit lost anyway what you trying to achieve.
    Is your target to do following:

    Get character one by one as long user enters 1
    when 1 is entered return entered string of entered characters and print them.

    If you want to use getch() include <ctype.h>
    please write your comments in english its easier to understand the code right away, even for the beginner like me.
    Last edited by Skvr; 02-24-2009 at 01:59 PM.

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    '
    ill care next time about french comments anyway thank you for explanation about getch()

  10. #10
    Registered User ssharish2005's Avatar
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    >If you want to use getch() include <ctype.h>
    Where did the getch() come under ctype.h. The getch is an non-standard function which is defines under conio.h. The alternative for it could be the getchar function.

    >(char*)realloc(q,(s*sizeof(char)));
    Don’t cast the return type realloc. And you get the return address of the realloc to be assigned to something which I don’t anything like. I would suggest creating temp point and assigning the return of realloc to temp pointer and then assign it to the original pointer. Just to be on the safer side. Other you will loss your whole data if the realloc was suppose to fail.

    -ssharish
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

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    This is not ment to be copy paste code. I put it on this forum so you could see and examine how things are done. And i think anyway that this aint easy for understand without real example. I dont know btw is this rightway to do this but it compiles with no errors and works. A friend showed for me how to play with those pointer indexes they were pretty hard for me.

    Code:
    #include<stdlib.h>
    #include<string.h>
    #include<stdio_ext.h>/*Maybe in Linux only*/
    
    char* user_input(){
      char *ptr;
      char *tmp_ptr;
      int i=0;
      char ch;
      
      printf("Enter letters: (to quit enter 1)\n");
      
      ptr=malloc(1*sizeof(char));
       if(ptr==NULL){
          printf("malloc() failed.\n");
          return 0;
        }
    
       ptr[i]=0;
    
      while(1){
        scanf("%1c",&ch);
        __fpurge(stdin); /*This is not standard, if this dont work find away to get rid of '\n'*/
          if(ch=='1')
    	break;
    
          tmp_ptr=realloc(ptr,i*sizeof(char)+2);
          
          if(tmp_ptr==NULL){ /*Check if space has been allocated succesfully*/
    	printf("realloc() failed.\n");
    	return 0;
          }
          else 
    	ptr=tmp_ptr; /*Enlarge the ptr memory space with allocated space if allocation ok*/
    
         ptr[i]=ch;
         ptr[i+1]='\0';
         i++;
    
         printf("Letters:%s size:%i\n",ptr,i+1);
      }
    
    return ptr;
    }
    
    int main()
    {  
      char *p;
    
      p=user_input();
      printf("String: %s\n",p);
    
      free(p);
    return 0;
    }
    Tell if something is done wrong way or is there a better way to do some parts of the code.
    Last edited by Skvr; 02-26-2009 at 08:12 AM. Reason: English is sometimes hard.

  12. #12
    CSharpener vart's Avatar
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    there is a better way
    do not realloc on each char - increase buffer in big enough steps at once

    read FAQ how to clear the stdin from the unwanted characters
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    the last 10% takes the other 90% of the time.

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    Is there any other reason why not to realloc every single char than optimization? I can imagine it would be resource consuming if this would be done in some RealWorld application, but because this is an example of using realloc to enlarge previously malloced space i dont see any reason why not to do that in this case.

  14. #14
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    Quote Originally Posted by Skvr View Post
    Is there any other reason why not to realloc every single char than optimization? I can imagine it would be resource consuming if this would be done in some RealWorld application, but because this is an example of using realloc to enlarge previously malloced space i dont see any reason why not to do that in this case.
    It is certainly only an optimization, but it is also a good idea to AT LEAST comment that this needs to be done if the difference between initial size and final size is large - a former colleague of mine worked on a problem with a graphics driver, which would allocate another 4KB every time it ran out of buffer. With a 16MB buffer, that was a lot of copying before it was large enough (4096 times if I don't get the math wrong, with an average of 8MB of memory being copies - that's a few gigabytes).

    Of course, for this type of case, the string is likely to be a few hundred bytes, which is not a big issue.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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