使用指针在C中追加字符串

Question

For the life of my I cannot figure out why this program is not working. I'm trying to concatenate two strings using pointers and keep receiving this error:

a.out(28095) malloc: *** error 
for object 0x101d36e9c: pointer being realloc'd was not allocated
*** set a breakpoint in malloc_error_break to debug

My str_append.c:

#include <stdio.h>
#include <stdlib.h>
#include "stringlibrary.h"  /* Include the header (not strictly necessary here) */

//appends s to d
void str_append(char *d, char *s){
  int i=0, j=0;

  d = realloc(d, strlength(d)+strlength(s)+1);
  //find the end of d
  while(*(d+i)!='\0'){
    i++;
  }


  //append s to d
  while(*(s+j)!='\0'){
    *(d+i)=*(s+j);
    i++;
    j++;
  }
  *(d+i)='\0';


}

I have my own strlength function which I'm 100% sure works.

My main.c:

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

int main(int argc, char **argv)
{
 char* str = (char*)malloc(1000*sizeof(char));
 str = "Hello";
 char* str2 = (char*)malloc(1000*sizeof(char)); 
str2 = " World";

str_append(str, str2);


 printf("Original String: %d\n", strlength(str));
 printf("Appended String: %d\n", strlength(str));


return 0;
}

I've tried reallocating to a temporary variable and receive the same error. Any help is appreciated.

EDIT: Thanks for all the answers. This site is awesome. Not only do I know where I went wrong (simple mistake I guess), but I found a pretty big hole in what I didn't know about strings. Since I can't use the strcpy function I've implemented my own. It's the source code for strcpy basically.

char *string_copy(char *dest, const char *src)
{
 char *result = dest;
 while (*dest++ = *src++);
 return result;
}

Answer 1

Your problem is here

char* str = (char*)malloc(1000*sizeof(char));
str = "Hello";

First you allocate space for 1000 chars and point your pointer to the beginning of that memory.
Then in the second line you point your pointer to a string literal causing a memory leak.
You pointer no longer points to the allocated memory.
And later in your function you try to change the string literal which is read-only.

Answer 2

You're attempting to realloc a pointer to a static variable. When you set

str = "Hello";

You're statically assigning that variable (ie it will be assigned at compile time). It is then not a valid pointer to use for other purposes, including realloc . You're also wasting all of the space you retrieved with malloc on the line above by throwing away your only pointer to that memory.

What you need to do instead is to use strcpy to assign the value:

strcpy(str, "Hello");

Then you still have a dynamically allocated pointer, which you can use for realloc .

Answer 3

char* str = (char*)malloc(1000*sizeof(char));
str = "Hello";

should be:

char* str = malloc (1000);
strcpy (str, "Hello");

The former allocates some memory and stores the address of that memory into the str pointer, then changes the str pointer to point to different (unmalloced) memory.

That's why you're seeing pointer being realloc'd was not allocated .

The latter code segment leaves str pointing at the malloced memory and just copies the string into that memory.

---

And, as an aside, you should never cast the return value from malloc in C - it can hide certain subtle errors and it's unnecessary since C is perfectly capable of implicitly casting the void* returned into any other pointer type.

Also, since sizeof(char) is always one, you never need to multiply by it. It usually clutters the code unnecessarily.

Lastly, though probably not too important in this case, the C standard reserves identifiers beginning with str , mem and wcs (each followed by a lowercase letter) for future library directions, so you may want to reconsider your use of things like strlength() if you want future portability.

Answer 4

strings literals (ex : "MAMA" "MEME" ) are immutable. you cant reallocate them, however, if you use pointer of chars (ex. char * s = (char*)malloc(sizeof(char) * LEN ) and allocate them then they are mutable.

here :

char* str = (char*)malloc(1000*sizeof(char));
str = "Hello"; //no error BUT wasted memory and cant be reallocated anymore (literal)

you should use built-in functions in string manipulation...