string copy function 4

[manichandra]

what is wrong with this code? its not working . can anybody explain in detail please ???
____________________________

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

void newStrCpy(char* q, char* p);

int main (void)
{

char *q = "MANICHANDRA";

char *p ;

newStrCpy(q,p);

//printf("%d",(int)p);

printf("%s\n",p);

return 0;
}

void newStrCpy(char* q, char* p)
{

//printf("%d",(int)p);

char *temp =(char*) malloc(sizeof(*q));

int i=0;

do{

*(temp+i) = *(q+i);

i++;

}

while(*(q+i)!='\0');

//printf("%s",temp);

p = temp;

}

 

[xwb]

 char *temp =(char*) malloc(sizeof(*q));

This allocates 1 byte. You need

char *temp = (char*) malloc (strlen(q) + 1);

 

[manichandra]

Thanks xwb , i tried by
char *temp = (char*) malloc (strlen(q) + 1);

but still the output is "null"..

in my code i checked the value of "p" before going out of the function by debugging.
at this point "p" is pointing to "temp".

But once the compiler goes out of the function p is pointing to the initial value.

 

[xwb]

If you wish to change the value of p, then you need to pass it as a pointer to the char*

void newStrCpy(char* q, char** p)
{
...

   *p = temp;
}

 

[ArkM]

Some additions:

1. There is a wonderful operator in C. It's defined as

a[expr] <=> *(a+(expr))

Now you may write much more clear codes with

p[i] /*instead of correct but cumbersome *(p+i) */

2. String literal "..." is a special kind of array of chars in C. The content of "abc" is exactly the same as

{ 'a', 'b', 'c', '\0' } /* constant array of four chars */

By historical reasons it's possible to get non-constant pointer to this array in C but any correct program can't assign values via such a pointer.
Now let's remember that an array is implicitly converted to the pointer to the 1st element of this array (everywhere except sizeof and address of (unary &) operators argument). That's why you can write this declaration:

char* q = "MANICHANDRA";
/* Better write more correctly as below: */
const char* q = "MANICHANDRA";

Now q is a pointer to the 1st character of (constant!) array of chars (to 'M' letter in this case).
Compare with the different construct:

char s[] = "MAN";

This is a special kind of an array declaration with initialization. After that s is a name of array of 4 characters. It's the same as

char s[4] = { 'M', 'A', 'N', '\0' }; /* see zero byte terminator */

Feel free to change all four elements of s:

s[0], s[1], s[2], s[3]

3. Function arguments are passed "by value" in C. In other words, a function receives copies of argument values. Let's consider

void f(char* p)
{
   char s[] = "MAN";
   p = s;
}
...
char ss[] = "WOMAN";
f(ss);
printf("%s\n",ss); /* prints WOMAN */

We have passed a copy of a pointer to ss. Now (in the function body) p is a name of this copy - local pointer variable. After that the function assigns another value (a pointer to the local array s) to this copy (parameter) variable. It has no relation to ss argument. The function f does nothing visible outside its scope!

May be it helps you to understand your snippets.

The last advice: try to get the best book about C - The C Programming Language by K&R.

Good luck!

 

[manichandra]

Thank you ArkM,

I got the solution as follows
________________________________
#include <stdio.h>
#include<stdlib.h>
#include<string.h>

void newStrCpy(char* q, char* p);

int main (void)
{

char *src;
printf("\n----------------------------------------");
printf("\nPROGRAM TO COPY THE CONTENTS OF ONE STRING TO ANOTHER ");
printf("\n----------------------------------------");
printf("\n\n\t ENTER A STRING...: ");
scanf("%s",src);
char *dst =(char*) malloc(sizeof(*src));
newStrCpy(src,dst);
printf("\n\t copied string is..: %s\n",dst);

return 0;
}

void newStrCpy(char* q, char* p)
{

int i=0;

do{
*(p+i) = *(q+i);
i++;
}while(*(q+i)!='\0');

}

__________________________

I tried to use **p, but i couldn't figure it out . can you tell me how to do with using **p ,
what is the most efficient solution for this ?

 

[ArkM]

Alas, there are tons of defects in every line of your code.
Please, read this excellent article written by Julienne Walker (All About Pointers):

http://eternallyconfuzzled.com/tuts/languages/jsw_tut_pointers.aspx

then try to rewrite your code from the scratch...

The key point: try to understand the difference between arrays (real memory objects) and pointers (addresses of real memory objects).