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:

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void build_index_url(const char *domain, char *index_url);

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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:

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#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
}

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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?

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

Code:

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char url[50];

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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:

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char preBuild[50]="http://www.";
char postBuild[50]="/index.html";

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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.

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

Sorry, I get it now.

Code:

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char preBuild[50]="http://www.";
char postBuild[50]="/index.html";

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THANK YOU!

I'd also check that stack variable:

Code:

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char url[50];

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and make sure you don't have to initialize it to:

Code:

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char url[50] = "\0";

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You shouldn't need to modify your fixed strings at all.

Code:

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    const char preBuild[]="http://www.";
    const char postBuild[]="/index.html";
    strcpy(index_url, preBuild); 
    strcat(index_url, domain);
    strcat(index_url, postBuild);

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