Programming Forums > Application Development > C

xavier · Dec 9th, 2007, 11:57 AM

 c Syntax (Toggle Plain Text) 
int * add(int x,int y)
 {
 int z;
 z = x+y;
 return(&z);
 }
int * add(int x,int y)
 {
 int z;
 z = x+y;
 return(&z);
 }

Ok, that works. How about when i have something like :

char ** thing(){
return (???);
}

and : char *(??) = thing().

How exactly should it look ?

I'm quite sure this is a no brainer .. but ..

Help.

Jessehk · Dec 9th, 2007, 2:05 PM

In your first snippet, z is local to the function add(). What your doing is returning the address of a variable that doesn't exist outside the function -- it's an error.

In your second example, If thing() returned a pointer to memory on the heap (allocated by malloc()), then to return it, you'd just return the pointer from the function -- there's no need to return a pointer to a pointer to a char (char **).

If I've made any errors, any of the numerous C-gods present will hopefully correct them.

EDIT: I ran his code involving add() and although valgrind reports an error, the correct answer is computed. I'm 99.99% sure that it is wrong. Why is 5 being computed at all?

 c Syntax (Toggle Plain Text) 
#include <stdio.h>
 
int *add( int x, int y ) {
    int z = x + y;
 
    return &z;
}
 
int main() {
    int *ans = add( 2, 3 );
    printf( "%d\n", *ans );
 
    return 0;
}
#include <stdio.h>

int *add( int x, int y ) {
    int z = x + y;
    
    return &z;
}

int main() {
    int *ans = add( 2, 3 );
    printf( "%d\n", *ans );
    
    return 0;
}

(Toggle Plain Text)

==7676== Memcheck, a memory error detector.
==7676== Copyright (C) 2002-2007, and GNU GPL'd, by Julian Seward et al.
==7676== Using LibVEX rev 1732, a library for dynamic binary translation.
==7676== Copyright (C) 2004-2007, and GNU GPL'd, by OpenWorks LLP.
==7676== Using valgrind-3.2.3-Debian, a dynamic binary instrumentation framework.
==7676== Copyright (C) 2000-2007, and GNU GPL'd, by Julian Seward et al.
==7676== For more details, rerun with: -v
==7676== 
==7676== Invalid read of size 4
==7676==    at 0x80483B3: main (in /home/jesse/example)
==7676==  Address 0xBEB40904 is just below the stack ptr.  To suppress, use: --workaround-gcc296-bugs=yes
5
==7676== 
==7676== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 11 from 1)
==7676== malloc/free: in use at exit: 0 bytes in 0 blocks.
==7676== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==7676== For counts of detected errors, rerun with: -v
==7676== All heap blocks were freed -- no leaks are possible.

Dameon · Dec 9th, 2007, 2:34 PM

Yes, the code is wrong, valgrind is right. As usual

When you make a function call, the stack pointer will be adjusted by some number of bytes.
This effectively reserves space on the stack for local variables, the return address, parameters, etc.

The integer z is a local variable, so it's located on the stack. Probably somewhere around the address to return to, etc. &z gives the address of z as you expect.

When add() returns, the stack pointer is changed back to where it was before add() was called. The local variables, etc are still there. You can still access that address and get the correct answer. However, the next time you call a function, z will get overwritten by something else, because its parameters and local variables may occupy the same space. It works right now, but will surely break later.

Jessehk · Dec 9th, 2007, 3:03 PM

Thanks for the explanation Dameon.

I realized that I didn't write a correct definition for int main( void ). I will make a sacrificial offering to the C gods when time permits.

grumpy · Dec 9th, 2007, 5:58 PM

Quote:

Originally Posted by Jessehk

Thanks for the explanation Dameon.

Dameon told some minor fibs. His explanation is just one of many possibilities for what happens. The formal result of the code is undefined: any set of observable or unobservable effects are allowed to happen.

Quote:

Originally Posted by Jessehk

I realized that I didn't write a correct definition for int main( void ). I will make a sacrificial offering to the C gods when time permits.

Definitions of main() as "int main()" and "int main(void)" are equivalent. Even the latest C standard uses both forms on occasion.

Interestingly enough declarations (eg function prototypes) of such functions are different: the first form declares a function that takes an unspecified set of arguments, and the second declares a function that takes no arguments.

This is one of the minor anomalies introduced into the latest C standard: and some of the reasons are, I suspect, non-technical. The "gods of C" have not earned any sacrificial offerings with this one.

xavier · Dec 9th, 2007, 10:12 PM

ok, thanks for the replies .
I didn't really check that code, int * .. I was using it as an example for a function returning a pointer.

the

char ** thing(){
return (???);
}

and : char *(??) = thing().

is actually a function generated by rpc. So I'm stuck with it. The problem is that i don't know how to return the value, and how to use the function in another part of the program.

grumpy · Dec 10th, 2007, 3:42 AM

You'll have to be a bit more precise. What is the caller expecting to receive when thing() returns?

And don't tell me a char **. I mean what operations does it expect to do with that pointer, and what does it expect the pointer to point at?

xavier · Dec 10th, 2007, 3:55 AM

This is the original :
Server :

 c Syntax (Toggle Plain Text) 
static double found;
double * average_1(input_data *input,CLIENT *client) 
{
  found = 1;
  return (&found);
}
 
double * average_1_svc(input_data *input,struct svc_req *svc)
{
  CLIENT *client;
  return(average_1(input,client));
}
static double found;
double * average_1(input_data *input,CLIENT *client) 
{
  found = 1;
  return (&found);
}

double * average_1_svc(input_data *input,struct svc_req *svc)
{
  CLIENT *client;
  return(average_1(input,client));
}

average_1_svc will returne to the client;

Client:

 c Syntax (Toggle Plain Text) 
 double  *result_1;
 result_1 = average_1(&average_1_arg.input_data, clnt);
 printf("%e\n",*result_1);
 double  *result_1;
 result_1 = average_1(&average_1_arg.input_data, clnt);
 printf("%e\n",*result_1);

Now I modified the rpc .x file to return a string. In turn it generated functions like:

 c Syntax (Toggle Plain Text) 
char ** average_1(input_data *input,CLIENT *client) {
}
char ** average_1(input_data *input,CLIENT *client) {
}

The problem is that in the function i don't know how to return a string
like :

char * string = malloc(sizeof(char)*100);
return string;

and if i return it ... how do I use the function on the client side?

grumpy · Dec 10th, 2007, 5:51 AM

So much for a question being asked in a clear form.

Quote:

Originally Posted by xavier

This is the original :
Server :
c Syntax (Toggle Plain Text)

static double found;
double * average_1(input_data *input,CLIENT *client)
{
found = 1;
return (&found);
}
double * average_1_svc(input_data *input,struct svc_req *svc)
{
CLIENT *client;
return(average_1(input,client));
}

static double found; double * average_1(input_data *input,CLIENT *client) { found = 1; return (&found); } double * average_1_svc(input_data *input,struct svc_req *svc) { CLIENT *client; return(average_1(input,client)); }

Well, as described previously, anything the caller does with these functions will yield undefined behaviour. A local variable is not guaranteed to exist when a function returns, so returning it's address gives a pointer to something (potentially) non-existent. If the caller dereferences (eg writes to or reads from) that address, the net result is undefined behaviour.

Quote:

Originally Posted by xavier

c Syntax (Toggle Plain Text)

char ** average_1(input_data *input,CLIENT *client) {
}

char ** average_1(input_data *input,CLIENT *client) { }

The problem is that in the function i don't know how to return a string
like :

char * string = malloc(sizeof(char)*100);
return string;

and if i return it ... how do I use the function on the client side?

You don't need a function to return a pointer to a pointer if you simply want to give the caller a string.

For example;

(Toggle Plain Text)

#include <stdio.h>
#include <stdlib.h>

char *function()
{
     char *x = malloc(100);
     strcpy(x, "Hello");
     return x;
}

int main()
{
     char *str = function();
     fprintf(stdout, "%s\n", str);
     free(str);    /* necessary to release the data malloc()'ed by function */
}

The reason this works is that function() returns the value of x, not it's address. That value is the address of memory that has been obtained using malloc(), which is guaranteed to exist until the corresponding call of free().

However, if you change function to;

(Toggle Plain Text)

char *function()   /*  This function makes the caller exhibit undefined behaviour */
{
     char a[100];
     strcpy(a, "Hello");
     return a;
}

the caller will exhibit undefined behaviour because the array a does not exist when the function returns. And the return statement returns the address of the first element in a. But a will no longer exist when the function returns.....

xavier · Dec 10th, 2007, 8:32 AM

Ok, the method signatures are generated with rpcgen. So i'm not modifying them.
Now , it's all interesting and stuff , and I do appreciate you taking the time to answer Grumpy.

But the question is quite simple . How can I work with " char ** " method signatures ?

Dec 9th, 2007, 11:57 AM	#1
xavier Professional Programmer Join Date: Oct 2004 Location: .ro Posts: 383 Rep Power: 4	Function returning double pointer ? c Syntax (Toggle Plain Text) int * add(int x,int y) { int z; z = x+y; return(&z); } int * add(int x,int y) { int z; z = x+y; return(&z); } Ok, that works. How about when i have something like : char ** thing(){ return (???); } and : char *(??) = thing(). How exactly should it look ? I'm quite sure this is a no brainer .. but .. Help. __________________ Don't take life too seriously, it's not permanent !

Dec 9th, 2007, 2:05 PM	#2
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 644 Rep Power: 4	Re: Function returning double pointer ? In your first snippet, z is local to the function add(). What your doing is returning the address of a variable that doesn't exist outside the function -- it's an error. In your second example, If thing() returned a pointer to memory on the heap (allocated by malloc()), then to return it, you'd just return the pointer from the function -- there's no need to return a pointer to a pointer to a char (char ). If I've made any errors, any of the numerous C-gods present will hopefully correct them. EDIT: I ran his code involving add() and although valgrind reports an error, the correct answer is computed. I'm 99.99% sure that it is wrong. Why is 5 being computed at all? c Syntax** (Toggle Plain Text) #include <stdio.h> int add( int x, int y ) { int z = x + y; return &z; } int main() { int ans = add( 2, 3 ); printf( "%d\n", ans ); return 0; } #include <stdio.h> int add( int x, int y ) { int z = x + y; return &z; } int main() { int ans = add( 2, 3 ); printf( "%d\n", ans ); return 0; } (Toggle Plain Text) ==7676== Memcheck, a memory error detector. ==7676== Copyright (C) 2002-2007, and GNU GPL'd, by Julian Seward et al. ==7676== Using LibVEX rev 1732, a library for dynamic binary translation. ==7676== Copyright (C) 2004-2007, and GNU GPL'd, by OpenWorks LLP. ==7676== Using valgrind-3.2.3-Debian, a dynamic binary instrumentation framework. ==7676== Copyright (C) 2000-2007, and GNU GPL'd, by Julian Seward et al. ==7676== For more details, rerun with: -v ==7676== ==7676== Invalid read of size 4 ==7676== at 0x80483B3: main (in /home/jesse/example) ==7676== Address 0xBEB40904 is just below the stack ptr. To suppress, use: --workaround-gcc296-bugs=yes 5 ==7676== ==7676== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 11 from 1) ==7676== malloc/free: in use at exit: 0 bytes in 0 blocks. ==7676== malloc/free: 0 allocs, 0 frees, 0 bytes allocated. ==7676== For counts of detected errors, rerun with: -v ==7676== All heap blocks were freed -- no leaks are possible. ==7676== Memcheck, a memory error detector. ==7676== Copyright (C) 2002-2007, and GNU GPL'd, by Julian Seward et al. ==7676== Using LibVEX rev 1732, a library for dynamic binary translation. ==7676== Copyright (C) 2004-2007, and GNU GPL'd, by OpenWorks LLP. ==7676== Using valgrind-3.2.3-Debian, a dynamic binary instrumentation framework. ==7676== Copyright (C) 2000-2007, and GNU GPL'd, by Julian Seward et al. ==7676== For more details, rerun with: -v ==7676== ==7676== Invalid read of size 4 ==7676== at 0x80483B3: main (in /home/jesse/example) ==7676== Address 0xBEB40904 is just below the stack ptr. To suppress, use: --workaround-gcc296-bugs=yes 5 ==7676== ==7676== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 11 from 1) ==7676== malloc/free: in use at exit: 0 bytes in 0 blocks. ==7676== malloc/free: 0 allocs, 0 frees, 0 bytes allocated. ==7676== For counts of detected errors, rerun with: -v ==7676== All heap blocks were freed -- no leaks are possible. __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

Dec 9th, 2007, 2:34 PM	#3
Dameon Troll Join Date: Apr 2005 Location: Texas Posts: 732 Rep Power: 4	Re: Function returning double pointer ? Yes, the code is wrong, valgrind is right. As usual When you make a function call, the stack pointer will be adjusted by some number of bytes. This effectively reserves space on the stack for local variables, the return address, parameters, etc. The integer z is a local variable, so it's located on the stack. Probably somewhere around the address to return to, etc. &z gives the address of z as you expect. When add() returns, the stack pointer is changed back to where it was before add() was called. The local variables, etc are still there. You can still access that address and get the correct answer. However, the next time you call a function, z will get overwritten by something else, because its parameters and local variables may occupy the same space. It works right now, but will surely break later. __________________ MD5(sig) = bcef75433db02e9ad9bf81d6f7c5c270

Dec 9th, 2007, 3:03 PM	#4
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 644 Rep Power: 4	Re: Function returning double pointer ? Thanks for the explanation Dameon. I realized that I didn't write a correct definition for int main( void ). I will make a sacrificial offering to the C gods when time permits. __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS! Last edited by Jessehk; Dec 9th, 2007 at 3:15 PM.

Dec 9th, 2007, 10:12 PM	#6
xavier Professional Programmer Join Date: Oct 2004 Location: .ro Posts: 383 Rep Power: 4	Re: Function returning double pointer ? ok, thanks for the replies . I didn't really check that code, int * .. I was using it as an example for a function returning a pointer. the char ** thing(){ return (???); } and : char *(??) = thing(). is actually a function generated by rpc. So I'm stuck with it. The problem is that i don't know how to return the value, and how to use the function in another part of the program. __________________ Don't take life too seriously, it's not permanent !

Dec 10th, 2007, 3:42 AM	#7
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	Re: Function returning double pointer ? You'll have to be a bit more precise. What is the caller expecting to receive when thing() returns? And don't tell me a char **. I mean what operations does it expect to do with that pointer, and what does it expect the pointer to point at?

Dec 10th, 2007, 3:55 AM	#8
xavier Professional Programmer Join Date: Oct 2004 Location: .ro Posts: 383 Rep Power: 4	Re: Function returning double pointer ? This is the original : Server : c Syntax (Toggle Plain Text) static double found; double * average_1(input_data input,CLIENT client) { found = 1; return (&found); } double * average_1_svc(input_data input,struct svc_req svc) { CLIENT client; return(average_1(input,client)); } static double found; double average_1(input_data input,CLIENT client) { found = 1; return (&found); } double * average_1_svc(input_data input,struct svc_req svc) { CLIENT client; return(average_1(input,client)); } average_1_svc will returne to the client; Client: c Syntax* (Toggle Plain Text) double result_1; result_1 = average_1(&average_1_arg.input_data, clnt); printf("%e\n",result_1); double result_1; result_1 = average_1(&average_1_arg.input_data, clnt); printf("%e\n",result_1); Now I modified the rpc .x file to return a string. In turn it generated functions like: c Syntax (Toggle Plain Text) char ** average_1(input_data input,CLIENT client) { } char ** average_1(input_data input,CLIENT client) { } The problem is that in the function i don't know how to return a string like : char * string = malloc(sizeof(char)*100); return string; and if i return it ... how do I use the function on the client side? __________________ Don't take life too seriously, it's not permanent !

Dec 10th, 2007, 8:32 AM	#10
xavier Professional Programmer Join Date: Oct 2004 Location: .ro Posts: 383 Rep Power: 4	Re: Function returning double pointer ? Ok, the method signatures are generated with rpcgen. So i'm not modifying them. Now , it's all interesting and stuff , and I do appreciate you taking the time to answer Grumpy. But the question is quite simple . How can I work with " char ** " method signatures ? __________________ Don't take life too seriously, it's not permanent !

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Programming Forums > Application Development > C

Function returning double pointer ?