why does this pointer definition crashes...

[sanddune008]

main()
{

    char const *p = "hello";
    char *anp ="google";

	//p = anp;

    printf("value of p:%s\n",*p);        Line K
    printf("value of anp:%u\n",anp);
    printf("value of p:%u\n",p);

	return 0;

}

Why does this program crashes?
Commenting the Line K work fine?
Is this an seg fault?

Thanks in advance

[quzah]

%s prints a string. You are dereferencing the pointer, giving you a constant character, not a string. You should have: %c if you want just one character, or %s and just the name of p, not *p.

You really should be listening to your compiler. It will tell you what you're doing wrong most of the time.


Quzah.

[msh]

%s specifier expects a pointer to the first element of a null-terminated array, i.e. string, which a string literal (e.g. "hello") or array name happen to be equivalent to. In this case, you're feeding it 'h' which is 0x68 hex, a memory address you're program is unlikely to have access to. Hence the segfault.

(This is to the best of my understanding. Please correct me if I'm totally off.)

[sanddune008]

Thanks all for your time..........

From this arises another problem:

void mycpy(char *d,char *s);

int main()
{
    char *dest,*src ;//= "google";
    *src = "google" ;
    mycpy(dest,src);
	printf("content of dest:%s\n",dest);
	printf("content of src:%s\n",src);
	return 0;
}

void mycpy(char *d,char *s)
{

	while((*d++ = *s++),*s!='\0')  line L
	//while((*d++ = *s++))                       line J
        *d = '\0';                                      // add null char at the last
	printf("In mycpy content of dest:%s\n",d);
	printf("In mycpy content of src:%s\n",s);
}

Another crash at line L........same with line J uncommented
Looks like i need a more understanding on this.....

What may be the reason for this crash?

[quzah]

dest has no memory allocated for it, and isn't actually set to point any place specific, so you're just writing over top of some random spot in memory.


Quzah.

[Elysia]

Multiple problems.
Using pointers without allocating memory: SourceForge.net: Common mistakes and errors - cpwiki
Trying to assign a string to a char, dereferencing said pointer in the process.

%s prints a string. You are dereferencing the pointer, giving you a constant character, not a string. You should have: %c if you want just one character, or %s and just the name of p, not *p.

You really should be listening to your compiler. It will tell you what you're doing wrong most of the time.


Quzah.

Out of the two popular compilers, GCC and Visual C++, only GCC complains about this.
Problems like these are not picked up by most compilers.

[sanddune008]

dest has no memory allocated for it, and isn't actually set to point any place specific, so you're just writing over top of some random spot in memory.


Quzah.

then how do i go abt this without using dynamic memory allocation.......

[tabstop]

then how do i go abt this without using dynamic memory allocation.......

If you need to use a pointer, you don't. If you can use an array, you would do

char dest[SOME_LARGE_NUMBER];

[sanddune008]

Thanks all....and for the document...

void mycpy(char *d,char *s)
{
	while
		
		
		
		((*d++ = *s++));
}

how come following while declaration still remains valid?
How will the lexical parser interpret the following source file?
As the complete statement is across the 4 line?

[tabstop]

Thanks all....and for the document...

void mycpy(char *d,char *s)
{
	while
		
		
		
		((*d++ = *s++));
}

how come following while declaration still remains valid?
How will the lexical parser interpret the following source file?
As the complete statement is across the 4 line?

As C tokenizes a source file, different types of whitespace are not considered significant (a space versus a newline versus a tab all work the same). So the fact that there are three lines between "while" and "(" is just the same to the compiler as if there was just one space between.

[iMalc]

Can you



            still read this?

So can your compiler!

[sanddune008]

I have yet another code that crashes:

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



typedef struct node 
{
	int val;
	struct node *next;
 
}sNodeDetails;

sNodeDetails *head;


void vAdd(int valu);
void vDisplay(sNodeDetails *NDetails);

int main()
{
    
	head = (sNodeDetails*)(0);
    vAdd(1);
	vAdd(2);
	vAdd(3);
	vAdd(4);
	vAdd(5);

	vDisplay(head);
	//free(head);
	return 0;
}


void vAdd(int valu)
{
	sNodeDetails *temp,*NodeDetails;
    temp = head;

	NodeDetails = (sNodeDetails*)(malloc(sizeof(sNodeDetails)));  Problem Line
	//if(NodeDetails)
		//printf("no memory:\n");

    
	NodeDetails->val = valu;


    if(head == NULL)
	{
		head = NodeDetails;
		(*head).next = NULL;
	
	}
	else
	{
		while((*temp).next == NULL)
        {
			temp->next = NodeDetails;
			temp = (*head).next;			
			break;
		}

	}
}

void vDisplay(sNodeDetails *NDetails)
{
	while(NDetails)
	{
	
		printf("Display val:\n");
		printf("%d\n",(*NDetails).val);
		NDetails = NDetails->next;
	}

}

Looks like In Problem line:

Twice malloc is assiging memory from the heap........but further it doesn't

What may be the cause?


Thanks in advance

[sanddune008]

I was at fault......dumb mistake

void vAdd(int valu)
{
	sNodeDetails *temp,*NodeDetails;
    temp = head;

	NodeDetails = (sNodeDetails*)(malloc(sizeof(sNodeDetails)));
	if(NodeDetails == 0)
		printf("no memory:\n");

    
	NodeDetails->val = valu;


    if(head == NULL)
	{
		head = NodeDetails;
		(*head).next = NULL;
	
	}
	else
	{
		while((*temp).next != NULL)
			temp = (*temp).next; 
		if((*temp).next == NULL)
        {
			temp->next = NodeDetails;
			temp->next->next = NULL;
		
		}

	}
}

[sanddune008]

Hi,

I am trying to do a string comparison

int iStrCmp(const char *str1,const char *str2)
{
	while(*str1++ == *str2++,(str1!='\0'));
	return (*str1 - *str2);

}

I am unable to determine why the following function fails?

Okay following line is at fault

while(*str1++ == *str2++,(str1!='\0'));

Replace it with


while(*str1++ == *str2++,(*str1!='\0'));

Thanks in advance

[tabstop]

Are you using comma operator for a reason? Do you know what that reason is?

str1 is a pointer and cannot/should not be compared to a character.