«

#include "stdio.h"
#include "malloc.h"
struct student{
int age;
char *nms;
struct student *next;
};
struct student *create(){
int ags=0,size=sizeof(struct student);
char *nms=" ";
struct student *head=NULL,*tail=NULL,*p=NULL;
scanf("%d%s",&ags,nms); //-------------------here!!!!!!
while(ags!=0){
p=(struct student * )malloc(size);
nms=(char *)malloc(20);
p->age=ags;
p->nms=nms;
p->next=NULL;
if(head==NULL){
head=p;
}else{
tail->next=p;
}
tail=p;
scanf("%d%s",&ags,nms);
}
return head;
}
int main(void) {
struct student *ptr=create();
while(ptr){
printf("%d====%s\n",ptr->age,ptr->nms);
free(ptr->nms);
free(ptr);
ptr=ptr->next;
}
}
************************************
i think that is no problem with it.but it doesn't work!
why?

On Feb 27, 9:25 am, "softwindow" <softwin...@gmail.com> wrote:
[...]
> free(ptr);
> ptr=ptr->next;
> }}
>
> ************************************
> i think that is no problem with it.but it doesn't work!
> why?

You didn't give us much of a hint as to what you think "doesn't work"
means. But one obvious problem is the code above. Once you pass a
pointer to free, you can't then try to dereference it. Try something
like

tptr = ptr->next;
free(ptr);
ptr = tptr;

(with an appropriate declaration of tptr, of course.)

Regards,
-=Dave

On Feb 27, 10:25 am, "softwindow" <softwin...@gmail.com> wrote:
> #include "stdio.h"
> #include "malloc.h"
> struct student{
> int age;
> char *nms;
> struct student *next;};
>
> struct student *create(){
> int ags=0,size=sizeof(struct student);
> char *nms=" ";
> struct student *head=NULL,*tail=NULL,*p=NULL;
> scanf("%d%s",&ags,nms); //-------------------here!!!!!!
> while(ags!=0){
> p=(struct student * )malloc(size);
> nms=(char *)malloc(20);
> p->age=ags;
> p->nms=nms;
> p->next=NULL;
> if(head==NULL){
> head=p;
> }else{
> tail->next=p;
> }
> tail=p;
> scanf("%d%s",&ags,nms);
> }
> return head;}
>
> int main(void) {
> struct student *ptr=create();
> while(ptr){
> printf("%d====%s\n",ptr->age,ptr->nms);
> free(ptr->nms);
> free(ptr);
> ptr=ptr->next;
> }}
>
> ************************************
> i think that is no problem with it.but it doesn't work!
If there is no problem, then why do you say it doesn't work.

Sorry, I am not a mind reader, you will have to spell out what exactly
your issue is.

> why?

On Feb 27, 10:25 am, "softwindow" <softwin...@gmail.com> wrote:
> #include "stdio.h"
> #include "malloc.h"
> struct student{
> int age;
> char *nms;
> struct student *next;};
>
> struct student *create(){
> int ags=0,size=sizeof(struct student);
> char *nms=" ";
> struct student *head=NULL,*tail=NULL,*p=NULL;
> scanf("%d%s",&ags,nms); //-------------------here!!!!!!

What is your input, do you have enough memory allocated for 'nms'?

> while(ags!=0){
> p=(struct student * )malloc(size);
> nms=(char *)malloc(20);
> p->age=ags;
> p->nms=nms;
> p->next=NULL;
> if(head==NULL){
> head=p;
> }else{
> tail->next=p;
> }
> tail=p;
> scanf("%d%s",&ags,nms);
> }
> return head;}
>
> int main(void) {
> struct student *ptr=create();
> while(ptr){
> printf("%d====%s\n",ptr->age,ptr->nms);
> free(ptr->nms);
> free(ptr);
> ptr=ptr->next;
> }}
>
> ************************************
> i think that is no problem with it.but it doesn't work!
> why?

On 2ÔÂ27ÈÕ, ÏÂÎç11ʱ35·Ö, "Dave Hansen" <i...@hotmail.com> wrote:
> On Feb 27, 9:25 am, "softwindow" <softwin...@gmail.com> wrote:
> [...]
>
> > free(ptr);
> > ptr=ptr->next;
> > }}
>
> > ************************************
> > i think that is no problem with it.but it doesn't work!
> > why?
>
> You didn't give us much of a hint as to what you think "doesn't work"
> means. But one obvious problem is the code above. Once you pass a
> pointer to free, you can't then try to dereference it. Try something
> like
>
> tptr = ptr->next;
> free(ptr);
> ptr = tptr;
>
> (with an appropriate declaration of tptr, of course.)
>
> Regards,
> -=Dave

******************************************
i have change code as your code,it is my fault;
now i find it throw error at the first " scanf("%d
%s",&ags,nms); //-------------------here
"
when i input "10 finy", it throw a error.

On Feb 27, 10:49Â*am, "softwindow" <softwin...@gmail.com> wrote:
> On 2月27日, 下�11时35分, "Dave Hansen" <i...@hotmail.com> wrote:
>
>
>
>
>
> > On Feb 27, 9:25 am, "softwindow" <softwin...@gmail.com> wrote:
> > [...]
>
> > > Â* Â* Â* Â* Â* Â* Â* Â* free(ptr);
> > > Â* Â* Â* Â* Â* Â* Â* Â* ptr=ptr->next;
> > > Â* Â* Â* Â* }}
>
> > > ************************************
> > > i think that is no problem with it.but it doesn't work!
> > > why?
>
> > You didn't give us much of a hint as to what you think "doesn't work"
> > means. Â*But one obvious problem is the code above. Â*Once you pass a
> > pointer to free, you can't then try to dereference it. Â*Try something
> > like
>
> > Â* Â*tptr = ptr->next;
> > Â* Â*free(ptr);
> > Â* Â*ptr = tptr;
>
> > (with an appropriate declaration of tptr, of course.)
>
> > Regards,
> > Â* Â*-=Dave
>
> ******************************************
> i have change code as your code,it is my fault;
> now i find it throw error at the first " scanf("%d
> %s",&ags,nms); //-------------------here
> "
> when i input "10 finy", it throw a error.- Hide quoted text -
>
> - Show quoted text -

okay now let's see the '10' gets stored in ags and 'finy' which needs
5 (4 bytes for "finy" and 1 extra byte for '\0') bytes of storage
space, you try to store it in 'nms' which has how much space ... ?
(Ans. 1 byte)

On 2ÔÂ28ÈÕ, ÉÏÎç12ʱ13·Ö, "Manish" <mar...@gmail.com> wrote:
> On Feb 27, 10:49 am, "softwindow" <softwin...@gmail.com> wrote:
>
>
>
>
>
> > On 2ÔÂ27ÈÕ, ÏÂÎç11ʱ35·Ö, "Dave Hansen" <i...@hotmail.com> wrote:
>
> > > On Feb 27, 9:25 am, "softwindow" <softwin...@gmail.com> wrote:
> > > [...]
>
> > > > free(ptr);
> > > > ptr=ptr->next;
> > > > }}
>
> > > > ************************************
> > > > i think that is no problem with it.but it doesn't work!
> > > > why?
>
> > > You didn't give us much of a hint as to what you think "doesn't work"
> > > means. But one obvious problem is the code above. Once you pass a
> > > pointer to free, you can't then try to dereference it. Try something
> > > like
>
> > > tptr = ptr->next;
> > > free(ptr);
> > > ptr = tptr;
>
> > > (with an appropriate declaration of tptr, of course.)
>
> > > Regards,
> > > -=Dave
>
> > ******************************************
> > i have change code as your code,it is my fault;
> > now i find it throw error at the first " scanf("%d
> > %s",&ags,nms); //-------------------here
> > "
> > when i input "10 finy", it throw a error.- Hide quoted text -
>
> > - Show quoted text -
>
> okay now let's see the '10' gets stored in ags and 'finy' which needs
> 5 (4 bytes for "finy" and 1 extra byte for '\0') bytes of storage
> space, you try to store it in 'nms' which has how much space ... ?
> (Ans. 1 byte)- Òþ²Ø±»ÒýÓÃÎÄ×Ö -
>
> - ÏÔʾÒýÓõÄÎÄ×Ö -

oh!my god! yes,you are right!
thanks!

Manish wrote:
> "softwindow" <softwin...@gmail.com> wrote:
>> "Dave Hansen" <i...@hotmail.com> wrote:
>> i have change code as your code,it is my fault;
>> now i find it throw error at the first " scanf("%d
>> %s",&ags,nms); //-------------------here
>> "
>> when i input "10 finy", it throw a error.
>>
>
> okay now let's see the '10' gets stored in ags and 'finy' which needs
> 5 (4 bytes for "finy" and 1 extra byte for '\0') bytes of storage
> space, you try to store it in 'nms' which has how much space ... ?
> (Ans. 1 byte)
>


> char *nms=" ";

nms doesn't have any storage that he's allowed to write to. It points
to a string literal.

See: http://c-faq.com/decl/strlitinit.html

John

"Manish" <> writes:
[...]
> okay now let's see the '10' gets stored in ags and 'finy' which needs
> 5 (4 bytes for "finy" and 1 extra byte for '\0') bytes of storage
> space, you try to store it in 'nms' which has how much space ... ?
> (Ans. 1 byte)

The declaration of nms was

char *nms=" ";

so nms points to *two* bytes (' ' and '\0'). But, as someone else
already pointed out, you're not allowed to write to those bytes,
because they're part of a string literal. (You *might* get away with
it, but it's undefined behavior.)

Another comment: white space can make your code easier to read.
Rather than

char *nms=" ";

try:

char *nms = " ";

And so on.

--
Keith Thompson (The_Other_Keith) kst- <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

softwindow wrote:
>
> #include "stdio.h"
> #include "malloc.h"
> struct student{
> int age;
> char *nms;
> struct student *next;
> };
> struct student *create(){
> int ags=0,size=sizeof(struct student);
> char *nms=" ";
> struct student *head=NULL,*tail=NULL,*p=NULL;
> scanf("%d%s",&ags,nms); //-------------------here!!!!!!
> while(ags!=0){
> p=(struct student * )malloc(size);
> nms=(char *)malloc(20);
> p->age=ags;
> p->nms=nms;
> p->next=NULL;
> if(head==NULL){
> head=p;
> }else{
> tail->next=p;
> }
> tail=p;
> scanf("%d%s",&ags,nms);
> }
> return head;
> }
> int main(void) {
> struct student *ptr=create();
> while(ptr){
> printf("%d====%s\n",ptr->age,ptr->nms);
> free(ptr->nms);
> free(ptr);
> ptr=ptr->next;
> }
> }

/* BEGIN new.c */

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

#define LENGTH 19

struct student {
int age;
char *nms;
struct student *next;
};

struct student *create(void)
{
struct student *head = NULL;
struct student *tail = NULL;
struct student *p = NULL;
int ags;
char *nms;

do {
p = malloc(sizeof *p);
if (p == NULL) {
puts("p == NULL");
exit(EXIT_FAILURE);
}
p -> next = NULL;
nms = malloc(LENGTH + 1);
if (nms == NULL) {
puts("nms == NULL");
exit(EXIT_FAILURE);
}
if (scanf("%d%s", &ags, nms) != 2) {
puts("scanf(\"%d%s\", &ags, nms) != 2");
exit(EXIT_FAILURE);
}
p -> age = ags;
p -> nms = nms;
if (head == NULL) {
head = p;
} else {
tail -> next = p;
}
tail = p;
} while (ags != 0);
return head;
}

int main(void)
{
struct student *next;
struct student *ptr = create();

while (ptr != NULL) {
next = ptr -> next;
printf("%d====%s\n", ptr -> age, ptr -> nms);
free(ptr -> nms);
free(ptr);
ptr = next;
}
return 0;
}

/* END new.c */

--
pete