Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>You want to use a pointer-to-pointer:</p> <pre><code>void someFunction (int **data) { *data = malloc (sizeof (int)); } void useData (int *data) { printf ("%p", data); } int main () { int *data = NULL; someFunction (&amp;data); useData (data); return 0; } </code></pre> <p>Why? Well, you want to change your pointer <code>data</code> in the main function. In C, if you want to change something that's passed in as a parameter (and have that change show up in the caller's version), you have to pass in a pointer to whatever you want to change. In this case, that "something you want to change" is a pointer -- so to be able to change that pointer, you have to use a pointer-to-pointer...</p> <p>Note that on top of your main problem, there was another bug in the code: <code>sizeof(data)</code> gives you the number of bytes required to store the pointer (4 bytes on a 32-bit OS or 8 bytes on a 64-bit OS), whereas you really want the number of bytes required to store <em>what the pointer points to</em> (an <code>int</code>, i.e. 4 bytes on most OSes). Because typically <code>sizeof(int *)&gt;=sizeof(int)</code>, this probably wouldn't have caused a problem, but it's something to be aware of. I've corrected this in the code above.</p> <p>Here are some useful questions on pointers-to-pointers:</p> <p><a href="https://stackoverflow.com/questions/897366/how-do-pointer-to-pointers-work-in-c">How do pointer to pointers work in C?</a></p> <p><a href="https://stackoverflow.com/questions/758673/uses-for-multiple-levels-of-pointer-dereferences">Uses for multiple levels of pointer dereferences?</a></p>
    singulars
    1. This table or related slice is empty.
    1. POHow can I allocate memory and return it (via a pointer-parameter) to the calling function?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USCommunity
    1. USMartin B
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POHow can I allocate memory and return it (via a pointer-parameter) to the calling 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. COI suggest not calling it a "double pointer" - too easily confused with a "pointer to double".
      singulars
      1. PO
      1. USNefrubyr
    2. CO"double pointer" and "triple pointer" are actually quite common terms
      singulars
      1. PO
      1. USa_m0d
    3. COThanks for the comments -- even though I agree "double pointer" is quite common, I've eliminated it to avoid confusion.
      singulars
      1. PO
      1. USMartin B
 

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