Thread: The pointer in my function is failing my assumption: Exercise 13 Chapter 13

  1. #1
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    The pointer in my function is failing my assumption: Exercise 13 Chapter 13

    Hello,

    I'm working on a problem and I'm stuck.
    <<< BEGIN QUESTION >>>
    Write the following function:
    Code:
    void build_index_url(const char *domain, char *index_url);
    domain points to a string containing an Internet domain, such as "knking.com". The function should add "http://www." to the beginning of this string and "/index_html" to the end of the string, storing the result in the string pointed to by index_url. (In this example, the result will be "http://www.knking.com/index.html".) You may assume that index_url points to a variable that is long enough to hold the resulting string. Keep the function as simple as possibleby having it use the strcat and strcpy functions.
    <<< END QUESTION >>>

    Code:
    #include <stdio.h>
    #include <string.h>
    // http://knking.com/books/c2/answers/c13.html
    
    
    void build_index_url(const char *domain, char *index_url);
    
    
    int main(){
        const char dom[]="knking.com";
        char url[50];
        build_index_url(dom, url);
        printf("url = %s\n", url);  // prints: url = 
        return (0);
    }
    
    
    void build_index_url(const char *domain, char *index_url){
        char preBuild[20]="http://www.";
        char postBuild[20]="/index.html";
        char newBuild[20];
        strcat(preBuild, domain);
        strcat(preBuild, postBuild);
        strcpy(index_url, preBuild);    // put data in location pointed to by index_url
        printf("function: index_url = %s\n", index_url);  // works fine
    }
    In the build_index_url function, I assumed that index_url was pointing to url as it was defined in main(). The problem: when the program returns to main(), I can't get url to print "http://www.knking.com/index.html". What an I not understanding?

  2. #2
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    I wonder if you are experiencing problems with your stack variable:

    Code:
    char url[50];
    What does the character array, url, contain? i.e. Is url a c string like strcpy requires?

    Actually I tried changing the sizes of these two arrays and everything works:

    Code:
    char preBuild[50]="http://www.";
    char postBuild[50]="/index.html";
    Last edited by G4143; 12-06-2022 at 08:03 PM.

  3. #3
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    Initially I 'think' char url[50] contains '\0' because I haven't put anything into it.
    At the end of build_index_url() I am trying give url a value of "http://www.knking.com/index.html" by using strcpy(index_url, preBuild);
    index_url is a pointer to url.

  4. #4
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    I think you find stack variables contain whatever is in memory(that the stack uses) unless you initialize that stack variable.

  5. #5
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    Sorry, I get it now.

    Code:
    char preBuild[50]="http://www.";
    char postBuild[50]="/index.html";
    THANK YOU!
    Last edited by mpatters; 12-06-2022 at 08:25 PM. Reason: oversight. I didn't see the [50] that was suggested.

  6. #6
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    I'd also check that stack variable:

    Code:
    char url[50];
    and make sure you don't have to initialize it to:
    Code:
    char url[50] = "\0";

  7. #7
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    You shouldn't need to modify your fixed strings at all.
    Code:
        const char preBuild[]="http://www.";
        const char postBuild[]="/index.html";
        strcpy(index_url, preBuild);   
        strcat(index_url, domain);
        strcat(index_url, postBuild);
    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.

  8. #8
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    I put it back to charurl[50]; and it works fine. Thank you.

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