Thread: error while call function that returns a string to a structure

I am a beginner to C programming. I have done a few programs here and there and am also following the book 'The C programming language' by K&R.


The program consists of 2 parts: the main and 1 function.

Function is initialised as follows:

Code:

 

char genCallNR();

this function char genCallNr() puts into a string the current date e.g 0120609


the string that holds this date is defined:

Code:

char callnr[11];

return callnr[0]; //trying to return this string to the main function

the structure is defined as:

Code:

typedef struct call{
int d;
int qty;
char con[100], owner[100];
char* callnr;
} CALL;

I try to put the return value from the genCallNr function into char* callnr:

Code:

CALL c;
c.callnr = genCallNr();

when I try to execute the program I get a stack smashing error.

also, I get some compile warnings.

Anyyone can explain to me what I am doing wrong?

PS I couldnt copy the errors from VM ware to thisd computer, many appologies

Update: The only compile error I get is - "warning: assignment makes pointer from ineger without cast"

and the offending line is:

Code:

c.callnr = genCallNr();

If you want to assign to a pointer, return a pointer. The prototype should look like this:

Code:

char *genCallNR();

And -- important! -- the return value in the function should not include an array subscript[0].

Code:

return callnr;

However, there will be a problem because as is, callnr is a stack variable local to genCallNR(). Which means the memory allocated to it is no longer protected after the function call ends and returns a pointer.
So you must manually allocate "heap" memory to a variable if you wish to use it as a return value with an indefinite lifespan:

Code:

#include <stdlib.h>  /* this library has malloc() and free() */
char genCallNR() {
     char *callnr = malloc(11);  /* EDIT corrected silly mistake */

When you are done using the string (ie, after the last time it is referenced in the program), you should "deallocate" this heap memory (ie, release it for some other use) since your computer does not have an infinite memory:

Code:

free(c.callnr);

Thank you MK27

I corrected the things that you pointed out. However things don't run smoothly.
I managed to get the code from the VMware:

Code:

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

char *genCallNr();

typedef struct call
{

   int date1;
   int qty;
   char contract[100], owner[100], partnr[100];
   char* callnr;
} CALL;

int main(void)
{
 CALL c;
 strcpy(c.contract, "DELL");
 strcpy(c.owner, "EMEA");
 strcpy(c.partnr, "615TW");
 c.callnr = genCallNr();

printf("%s\t%s\t%s\t%s", c.contract, c.owner, c.partnr, c.callnr);
free (c.callnr);

}

char *genCallNr(){
char callnr = malloc(11);
time_t now;
 struct tm *ts;
 char buf[80];
 //char callnr[11];

 now = time(NULL);
 ts = localtime(&now);

 strftime(buf, sizeof(buf), "1%y%m%d%H%m", ts);
 printf("\n%s", buf);
 strncpy(callnr, buf,80);
 printf("\n%s", callnr);
return callnr;
}

I get segmentation fault. Any idea why ?

Code:

char *genCallNr(){
char callnr = malloc(11);
time_t now;
 struct tm *ts;
 char buf[80];
 //char callnr[11];

 now = time(NULL);
 ts = localtime(&now);

 strftime(buf, sizeof(buf), "1%y%m%d%H%m", ts);
 printf("\n%s", buf);
 strncpy(callnr, buf,80);
 printf("\n%s", callnr);
return callnr;
}

See anything wrong here?

The string you produce is like this:
"10905191405" - which is exactly 11 digits long, but you also need space for the zero to terminate string.

And I actually expect that you want %M at the end of your strftime string - or are you obfuscating it by putting the month in twice?

--
Mats

Originally Posted by Tom Bombadil

I get segmentation fault. Any idea why ?

Just as Tom Bombadil was likely to misplace the One Ring, you appear to have misplaced the One Asterisk that makes the difference between callnr being a char instead of a pointer to char.

Yes, if you malloc() 11 bytes, do not use more than 11 bytes! Or else malloc more to start with. And remember, a string in C *must* be null terminated (the sting functions such as strcpy do this automatically), which requires one "extra" byte at the end.

Originally Posted by laserlight

Just as Tom Bombadil was likely to misplace the One Ring, you appear to have misplaced the One Asterisk that makes the difference between callnr being a char instead of a pointer to char.

AND THAT WAS MY FAULT!

Sorry Tom, still brewing the coffee I will go back and correct my post but that should have been:

Code:

char *callnr = malloc(12)

I think your compiler will have complained right away...

Originally Posted by MK27

AND THAT WAS MY FAULT!

Will the real Tom Bombadil please stand up?

By the way, Tom Bombadil, please indent your code better. It makes it easier to read and spot errors.

Originally Posted by laserlight

Just as Tom Bombadil was likely to misplace the One Ring, you appear to have misplaced the One Asterisk that makes the difference between callnr being a char instead of a pointer to char.

Thanks everyone, got it working now. Didn't see all the other replies posted before my last post!

Much appreciated

And i will try to indent my C code better too. Enjoy the rest of the day!

I think you had better learn more about pointers, since it seems kind of obvious you don't know what you're doing...