 |  |  |  |  |  |  |  |
A puzzl about function
Please support our C advertiser: Programming Forums - DaniWeb Sister Site
 |
I don't how to description this puzzle, just like this:
//===============Result==============
4335768 // this is right to delete object "b"
123 // this is the return value
123 // why the object "b" isn't deleted? <---puzzle at here
123
C Syntax (Toggle Plain Text)
4335768 // this is right to delete object "b"
123 // this is the return value
123 // why the object "b" isn't deleted? <---puzzle at here
123
•
•
Join Date: Sep 2004
Posts: 185
Reputation: 
Solved Threads: 4
C Programmer
Greeting XianBin,
Pointers can be somewhat confusing, but let's see if we can go through this code one step at a time. It may help the understanding of how it works, and how useful these techniques can be.
Let's disect the functions first:
» What this does, is it creates an integer b, and returns the memory address of this variable. That's why the function's return-type is "int *".
» This does almost the same thing, as it creates a variable "char *" and returns it. Not the memory address since an array name is a pointer. If you were to send the memory address, then you would need to change the return-type to "char **". Oh, and the b variable would need a real block of memory, since right now it is only staticly created.
Now moving inside main, lets take a look at our routines:
» These are your local variables. One is a pointer to an integer, and the other, a char array [or pointer]. The "char *" array only needs data, though the integer pointer needs the memory address to look at the data we send.
» Ah, the messy stuff. Well, this is quite simple, so I'll explain it as simple as possible. This contains pfun. If you may know, pfun is not the actual data itself, its the address. Remember in fun_num() we returned the address of our local variable b, well that address will be sent directly to pfun. Now in order to read the data passed through the address we would be smart to use the unary operator "*".
The reason you need the unary (*) operator, is because we sent the memory address to our function. Let me clarify. If a variable contains data here (d) and its memory address is here (m), I would need to get to (m)->(d) to access the data from the address sent. Likewise, the * operator allows me to read the data from the address. Using the * will point to the data instead of looking or modifying the address. So overall, pfun is our address, and *pfun is our data from our address.
» This is somewhat the same, but different. Remember that this will only contain the data without touching the address. This syntax is correct since with the previous line of code we sent address to address, but now we are sending data to data.
» Ah, we are reading our data here.
» Same address, doesn't matter where we call it. Reading the same data as *pfun.
» Remember, pchar is now reading whatever fun_char() holds. They aren't exactly linked like pointers and addresses, but you sent the data from our function to our variable. As long as that occurs, the data will still be there. It can't be delete either, since b isn't exactly our property to touch in fun_char(). It's dynamically out there somewhere.
» Same data, but not same address. If this function were to have allocated a memory block we could possibly send the address anywhere we want.
Conclusion
Though it isn't recommended to do a statement like so:
I don't want to be harsh or anything telling you it isn't safe or anything like that. Using malloc() may save you some headache down the line, but I'll explain that when you're ready or have learned it and/or have further questions about it.
I hope this information has helped you in great detail. If you have any further questions, please feel free to ask.
- Stack Overflow
Pointers can be somewhat confusing, but let's see if we can go through this code one step at a time. It may help the understanding of how it works, and how useful these techniques can be.
Let's disect the functions first:
int *fun_num() { int b = 123; return &b; }
char *fun_char() { char *b = "123"; return b; }
Now moving inside main, lets take a look at our routines:
int *pfun; char *pchar;
C Syntax (Toggle Plain Text)
The reason you need the unary (*) operator, is because we sent the memory address to our function. Let me clarify. If a variable contains data here (d) and its memory address is here (m), I would need to get to (m)->(d) to access the data from the address sent. Likewise, the * operator allows me to read the data from the address. Using the * will point to the data instead of looking or modifying the address. So overall, pfun is our address, and *pfun is our data from our address.
C Syntax (Toggle Plain Text)
C Syntax (Toggle Plain Text)
C Syntax (Toggle Plain Text)
C Syntax (Toggle Plain Text)
C Syntax (Toggle Plain Text)
Conclusion
Though it isn't recommended to do a statement like so:
char *b = "123";I hope this information has helped you in great detail. If you have any further questions, please feel free to ask.
- Stack Overflow
Following the rules will ensure you get a prompt answer to your question. If posting code, please include BB [code][/code] tags. Your question may have been asked before, try the search facility.
IRC
Channel: irc.daniweb.com
Room: #c, #shell
IRC
Channel: irc.daniweb.com
Room: #c, #shell
•
•
Join Date: Apr 2004
Posts: 4,363
Reputation: 
Solved Threads: 242
•
•
•
•
Originally Posted by Stack Overflow
Now moving inside main, lets take a look at our routines:
» These are your local variables. One is a pointer to an integer, and the other, a char array [or pointer].int *pfun; char *pchar;
"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand
>Though it isn't recommended to do a statement like so
Right answer, wrong reason. Assigning a string literal to a pointer to char is a deprecated feature in C++. The correct way would be:
>why the object "b" isn't deleted? <---puzzle at here
Not to take away from Stack Overflow's answer, but I can summarize. The memory for b is released, but the memory for what b points to is not. The reason is that string literals have static storage duration. In other words, the memory exists and can be accessed (but never modified) for the duration of the program as long as you have a pointer to it.
Right answer, wrong reason. Assigning a string literal to a pointer to char is a deprecated feature in C++. The correct way would be:
C Syntax (Toggle Plain Text)
Not to take away from Stack Overflow's answer, but I can summarize. The memory for b is released, but the memory for what b points to is not. The reason is that string literals have static storage duration. In other words, the memory exists and can be accessed (but never modified) for the duration of the program as long as you have a pointer to it.
a net friend suggested me to try to make the assemble file and reading it,so I try to do it,then I konw clearly what the promgram does.
I try to introduce it, I hope everyone to understand what I said, because my English so poor.
==================The problem description==================
==============Writing By C Code==============
----------------Assemble Code----------------
I try to introduce it, I hope everyone to understand what I said, because my English so poor.
==================The problem description==================
C Syntax (Toggle Plain Text)
==============Writing By C Code==============
C Syntax (Toggle Plain Text)
C Syntax (Toggle Plain Text)
|
Similar Threads
- ASP Replace Function.... (ASP)
- missing function header (C++)
- Function that returns void (C++)
Other Threads in the C Forum
- Previous Thread: Using a for loop to sum an integer n and call function add_it
- Next Thread: Problem with working with math in C
| Thread Tools | Search this Thread |
You need to join our community in order to build your list of favorite forums. You can visit the forum index for a listing of all forums.
Latest C Posts
- Today's Posts
- Unanswered Threads
Related Forum Features
#include * ansi append array arrays asterisks bash binarysearch centimeter changingto char character convert copyimagefile cprogramme creafecopyofanytypeoffileinc createprocess() database dynamic execv fgets file floatingpointvalidation fork framework function getlogicaldrivestrin givemetehcodez grade gtkwinlinux hacking histogram ide inches include infiniteloop initialization input interest intmain() iso kernel keyboard kilometer km license linked linkedlist linux list lists looping lowest matrix meter microsoft number oddnumber open opendocumentformat openwebfoundation overwrite owf pdf pointer pointers posix power probleminc process program programming radix recursion recv recvblocked research reversing segmentationfault sequential single socket socketprogramming standard strchr string suggestions systemcall test testing threads turboc unix urboc user variable wab whythiscodecausesegmentationfault windowsapi
