Question about strcpy

This is a discussion on Question about strcpy within the C Programming forums, part of the General Programming Boards category; Alright, well I would imagine the anwer to this question is quite obvious to anyone who programs in c often. ...

  1. #1
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    Red face Question about strcpy

    Alright, well I would imagine the anwer to this question is quite obvious to anyone who programs in c often. Basically I am trying to use the standard library function strcpy inside of a function I am writing. For some reason when I use strcpy inside of main() it works fine. When I use strcpy outside of main() it compiles fine, but causes a runtime error. I wrote a simple program to demonstrate the problem. The programs runs fine the way it is, but if you uncomment the line in main() that calls testFunction, then it causes a run time error.

    Code:
    #include<stdio.h>
    #include<string.h>
    
    void testFunction(char *string);
    
    int main()
    {
    	char *a;
    	char *b = "test string";
    	strcpy(a,b);
    	puts(a);
    
    	/*testFunction("test string2"); */	
    	return 0;
    }
    
    void testFunction(char *string){
    	char *a;
    	strcpy(a, string);
    	puts(a);
    	
    }

  2. #2
    Gawking at stupidity
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    Actually, you're only getting lucky that the strcpy() call in main() is working. The problem you're having is that you're not allocating any space for the destination string. char *a creates a pointer to type char. The problem is that that pointer doesn't point anywhere meaningful before you're trying to copy a string to it. You either need to use malloc() and friends to allocate some memory in which to store your string or you need to use char arrays to hold the strings.
    If you understand what you're doing, you're not learning anything.

  3. #3
    aoeuhtns
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    Your code is incorrect in either case. That it works in one situation but not the other is because you got lucky.

    Your pointer 'a' is pointing at an arbitrary memory location. strcpy writes characters to the memory address stored by the value 'a'. This means it writes characters somewhere, and this location could be some memory address that contains another variable, since you have not initialized 'a' at all. You need to assign to 'a' a valid memory address. This might be the address of an array of fixed size, or it might be a memory address of a region of memory that has been allocated by the procedure, malloc().

  4. #4
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    Thanks for taking the time to reply. I used a char array and it worked properly.

  5. #5
    Bond sunnypalsingh's Avatar
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    Quote Originally Posted by Kevinmun
    Thanks for taking the time to reply. I used a char array and it worked properly.
    Now it will work since you are statically allocating memory.....if you want to do it the previous way then follow what was mentioned in previous replies(use malloc).....

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