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Memory allocation problem

This is a discussion on Memory allocation problem within the C Programming forums, part of the General Programming Boards category; Hi all, I am new to C and am having a memory allocation problem. I am creating a char* and ...

  1. #1
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    Memory allocation problem

    Hi all, I am new to C and am having a memory allocation problem. I am creating a char* and initializing to NULL, then passing it to another function which allocates some memory for it, and copies some characters. When the second function returns, my char* returns to NULL. I'm sure I am missing something simple here but cannot figure it out. Any help would be greatly appreciated. See the example code below:

    Code:
    void functionA() {
      char *text = NULL;
      functionB(text);
      //variable text is still NULL
    }
    
    void functionB(char *text){
      char *tempText = "12345";
      size_t len = strlen(tempText);
      text = (char*)malloc(len * sizeof(char) + 1);
      strcpy(text, tempText );
    }
    Last edited by rtlm; 08-26-2011 at 01:47 AM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Because C only has pass by value.
    So if you want functionB() to change a pointer declared in functionA, then you need to pass a pointer to that pointer.

    Code:
    void functionA() {
      char *text = NULL;
      functionB(&text);
    }
    
    void functionB(char **text){
      char *tempText = "12345";
      size_t len = strlen(tempText);
      *text = malloc(len * sizeof(char) + 1);  // cast not needed in C
      strcpy(*text, tempText );
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Thanks for the reply, that has clarified a lot for me however I do have one more question. If the function that allocates the memory for the char* is another level deep (say functionC() called from functionB()), I am assuming that I will need to pass the char** in functionB() as a reference, and functionC() will pick it up as a char***. I was wondering if there was any way to simply pass the memory address from start to finish? I'm asking as the software I am working on is quite badly written, and may require the allocation to happen in perhaps 3 or 4 nested functions (unless I do a significant re-write). I'd rather not be working with char******...etc

  4. #4
    Registered User manasij7479's Avatar
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    If the function that allocates the memory for the char* is another level deep (say functionC() called from functionB()), I am assuming that I will need to pass the char** in functionB() as a reference, and functionC() will pick it up as a char***
    No..(AFAIK), just passing the same pointer should suffice ..(As long as all the allocation take place on the heap...and you don't return local pointers .)
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
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    2.The fast and unsteady suddenly falls asleep while running !



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    Quote Originally Posted by rtlm View Post
    Thanks for the reply, that has clarified a lot for me however I do have one more question. If the function that allocates the memory for the char* is another level deep (say functionC() called from functionB()), I am assuming that I will need to pass the char** in functionB() as a reference, and functionC() will pick it up as a char***. I was wondering if there was any way to simply pass the memory address from start to finish? I'm asking as the software I am working on is quite badly written, and may require the allocation to happen in perhaps 3 or 4 nested functions (unless I do a significant re-write). I'd rather not be working with char******...etc
    Quote Originally Posted by manasij7479 View Post
    No..(AFAIK), just passing the same pointer should suffice ..(As long as all the allocation take place on the heap...and you don't return local pointers .)
    Manasij7479 is correct, you are not going to keep adding layers of redirection. Just keeping passing the pointer the same way you did in the first function, where the original pointer is referred to as *text.
    Code:
    void foo(void){
         char *text=NULL;
         fooA(&text);
    }
    void fooA(char **textA){
         *textA = <whatever>
         fooB(&*textA);
    }
    void fooB(char **textB){
         *textB = <whatever>
    }
    ect...
    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

  6. #6
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    thanks for all the help guys!

  7. #7
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    Code:
    fooB(&*textA);
    A little redundant, no?
    Salem likes this.

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