Filling a linked list

This is a discussion on Filling a linked list within the C Programming forums, part of the General Programming Boards category; Code: datafile=fopen("data.abd","r"); if(datafile!=NULL){ firstp=malloc(sizeof(struct person)); currentp=firstp; while(1){ newp=malloc(sizeof(struct person)); fread(currentp,sizeof(currentp),1,datafile); if(currentp->next==NULL){ break; } currentp->next=newp; currentp=newp; } fclose(datafile); } This should ...

  1. #1
    Registered User 00Sven's Avatar
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    Filling a linked list

    Code:
        datafile=fopen("data.abd","r");
        if(datafile!=NULL){
                     firstp=malloc(sizeof(struct person));
                     currentp=firstp;
                     while(1){
                              newp=malloc(sizeof(struct person));
                              fread(currentp,sizeof(currentp),1,datafile);
                              if(currentp->next==NULL){
                                                       break;
                              }
                              currentp->next=newp;
                              currentp=newp;
                     }
                     fclose(datafile);
        }
    This should fill the linked list from a file called "data.txt". For some reason it only fills in the first one. Why?
    ~Sven
    Windows XP Home Edition - Dev-C++ 4.9.9.2
    Quote Originally Posted by "The C Programming Language" by Brian W. Kernignhan and Dennis M. Ritchie
    int fflush(FILE *stream)
    On an output stream, fflush causes any buffered but unwritten data to be written; On an input stream, the effect is undefined. It returns EOF for a write error, and zero otherwise. fflush(NULL) flushes all output streams.
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  2. #2
    ATH0 quzah's Avatar
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    malloc does not null its values when called. All it does is give you a pointer to some place in memory. Just like local non-static variables, they're unintialized upon creation, and contain some random data. That said, you're doing this pretty much ass backwards.

    You should be looping based on the return value of fread, not on if what you've happened to read has somehow magically set the 'next' pointer to NULL or not.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
    Registered User 00Sven's Avatar
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    This is from a file that was already printed from a linked list. I make the linked list in my program and then fwrite it into "data.txt" and read it back next time the program is run. So if currentp->next was NULL when it was printed into the file shouldn't it be NULL when it comes back out?
    ~Sven
    Windows XP Home Edition - Dev-C++ 4.9.9.2
    Quote Originally Posted by "The C Programming Language" by Brian W. Kernignhan and Dennis M. Ritchie
    int fflush(FILE *stream)
    On an output stream, fflush causes any buffered but unwritten data to be written; On an input stream, the effect is undefined. It returns EOF for a write error, and zero otherwise. fflush(NULL) flushes all output streams.
    board.theprogrammingsite.com

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    [edit]
    Change your if() to test the return value of fread(), as suggested by Quzah.
    [/edit]
    Last edited by dwks; 04-21-2006 at 03:53 PM.
    dwk

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  5. #5
    Registered User 00Sven's Avatar
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    But I read the structure from the file into currentp. So doesn't it have that stuff in it?
    ~Sven
    Windows XP Home Edition - Dev-C++ 4.9.9.2
    Quote Originally Posted by "The C Programming Language" by Brian W. Kernignhan and Dennis M. Ritchie
    int fflush(FILE *stream)
    On an output stream, fflush causes any buffered but unwritten data to be written; On an input stream, the effect is undefined. It returns EOF for a write error, and zero otherwise. fflush(NULL) flushes all output streams.
    board.theprogrammingsite.com

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    Code:
    fread(currentp,sizeof(currentp),1,datafile);
    You might want sizeof(*currentp).
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


    Other boards: DaniWeb, TPS
    Unofficial Wiki FAQ: cpwiki.sf.net

    My website: http://dwks.theprogrammingsite.com/
    Projects: codeform, xuni, atlantis, nort, etc.

  7. #7
    Registered User 00Sven's Avatar
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    Oh, I had that in my program I must have mistyped it into here. How do I make this work though?
    ~Sven
    Last edited by 00Sven; 04-21-2006 at 04:00 PM.
    Windows XP Home Edition - Dev-C++ 4.9.9.2
    Quote Originally Posted by "The C Programming Language" by Brian W. Kernignhan and Dennis M. Ritchie
    int fflush(FILE *stream)
    On an output stream, fflush causes any buffered but unwritten data to be written; On an input stream, the effect is undefined. It returns EOF for a write error, and zero otherwise. fflush(NULL) flushes all output streams.
    board.theprogrammingsite.com

  8. #8
    Registered User 00Sven's Avatar
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    OK I got it. I read through my program a few times and realized that I was writing the entries to the file as soon as they were made. So they were the last entries causing their pointer to be NULL when in the file so it stopped at the first one everytime. Stupid mistake.
    ~Sven
    Windows XP Home Edition - Dev-C++ 4.9.9.2
    Quote Originally Posted by "The C Programming Language" by Brian W. Kernignhan and Dennis M. Ritchie
    int fflush(FILE *stream)
    On an output stream, fflush causes any buffered but unwritten data to be written; On an input stream, the effect is undefined. It returns EOF for a write error, and zero otherwise. fflush(NULL) flushes all output streams.
    board.theprogrammingsite.com

  9. #9
    and the hat of wrongness Salem's Avatar
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    > Oh, I had that in my program I must have mistyped it into here. How do I make this work though?
    Here's a tip - use copy/paste of your actual code.

    Simply typing here what you think you have isn't good enough. You only need to be off by a very small number of characters before the advice goes off in a completely different direction to your actual problem.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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