Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>In C, <strong>everything is passed by value</strong>.</p> <p>What you are passing to fun() is a <strong>copy</strong> of the pointer you have in main().</p> <p>That means the <strong>copy</strong> of ptr is aimed at the allocated memory, and that memory set to 115.</p> <p>The ptr in main() still points at an undefined location because it has never been assigned.</p> <p>Try passing <strong>a pointer to the pointer</strong>, so that within <code>fun()</code> you have access to the pointer itself:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; int* fun(int**); // &lt;&lt;-- CHANGE int main() { int a=5; int* ptr; // ptr=(int*)malloc(sizeof(int)); fun(&amp;ptr); // &lt;&lt;-- CHANGE a=*ptr; printf("\n the val of a is:%d",a); return 0; } int* fun(int** another_ptr) // &lt;&lt;-- CHANGE { *another_ptr = (int*)malloc(sizeof(int)); // &lt;&lt;-- CHANGE **another_ptr = 115; // &lt;&lt;-- CHANGE return *another_ptr; } </code></pre> <p>The other option would be to make <code>fun()</code> actually return the updated pointer (as advertised), and assign this to ptr:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; int* fun(int*); int main() { int a=5; int* ptr; // ptr=(int*)malloc(sizeof(int)); ptr = fun(ptr); // &lt;&lt;-- CHANGE a=*ptr; printf("\n the val of a is:%d",a); return 0; } int* fun(int* another_ptr) { another_ptr = (int*)malloc(sizeof(int)); *another_ptr = 115; return another_ptr; // &lt;&lt;-- CHANGE } </code></pre> <p><strong>Edit:</strong> I renamed the variable in <code>fun()</code> to make it clear that it is <strong>different</strong> from the one you use in <code>main()</code>. Same name doesn't mean anything here.</p>
    singulars
    1. This table or related slice is empty.
    1. POdynamic memory created inside a function
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USDevSolar
    1. USDevSolar
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POdynamic memory created inside a function
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. CO@Devsolar: Yes it will be a pass-by-value in the case where malloc is inside the fun(). But it would still be a pass-by-value if the malloc is made in main(). Could you please tell me why one would work and one wont ?
      singulars
      1. PO
      1. UStomkaith13
    2. CO***Everything*** is pass by value! Re-read the first line.
      singulars
      1. PO
      1. USAlex Budovski
    3. COIn your code: Let's say you would initialize `ptr` in main(), to 0x12345678 or whatever. Your commented-out line calls `malloc()` and assign its return value to `ptr`, making it point to the memory location allocated by `malloc().` -- Your function `fun()`, in contrast, receives a **copy** of `ptr` (pointing to 0x12345678), calls `malloc()`, assigns its return value to the copy of `ptr`, and returns. (Erroneously without returning the `int*` as advertised.) After the function returns, you are again working on the *real* `ptr` - which still points to 0x12345678...
      singulars
      1. PO
      1. USDevSolar
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload