how to implement a function

Hello

given the following code:

Code:

---

 
void func(){
char *var=NULL;
str_cpy(var,"text1");
str_cpy(var,var+2);
some print(var) // result: xt1
some_free(var)
}

---

i should implement the str_cpy function.
how would you implement it efficiently (without memory leaks)?

What have you tried?

Edit: With the improved formatting in your code, I can see that there's no real useful str_cpy() that can be written. You'd at the very least have to return a value from str_cpy() and assign it to var.

actually to solve this problem, i should call to allocate memory when i call to str_cpy.
but when i call it on second time, there will be memory leak if no free memory function will be called.
but if i ask to free memory before allocating a new one, then i lose data (i'm also free the input)

Strcpy() doesn't have memory leaks. It's actually one of the faster and most reliable string library functions. Of course, the way you're using it, it will crash, because you're trying to write data to address 0x0 in memory, which won't work no matter what string copy function you use. You have to allocate memory first, with malloc() or by using an array.

If you're looking to write a more efficient strcpy function (which will be very difficult, as the default is highly optimized), you could try:

• Copying the data in larger blocks of data, by using unsigned long long (8 bytes) or xmm registers that can hold up to sixteen bytes at a time.
• Writing the loop in assembly so you can access the said xmm registers, and bypass the stack access by only writing to and from registers and memory
• Prefetching data before you copy if your architecture's documentation specifies that this will improve performance
• Aligning data before and during the copy if your architecture can handle aligned i/o faster than unaligned i/o (most can)
• Unrolling the loop and copying multiple blocks of 16 bytes in each iteration
• Checking for zero (when to stop copying) with the mmx-specific "pcmpeq", "pmovmskb", and "bsf" instructions. You can find examples online on how to implement them in assembly.

Also, you might want to check out a quick strlen() rendition I scrapped together, as knowing the length beforehand will speed up the copy:

Code:

---

.globl _asm_strlen
_asm_strlen:
        movq    %rdi, %rax
        pxor    %xmm0, %xmm0
        andq    $-16, %rax
ln_loop:   
        addq    $16, %rax
        movdqa  (%rax), %xmm1
        pcmpeqb  %xmm0, %xmm1
        pmovmskb %xmm1, %edx
        bsf      %edx, %edx
        jz      ln_loop
        subq    %rdi, %rax
        addq    %rdx, %rax
        ret

---

Note: you can find something _similar_ by disassembling the default strlen() command in a debugger.

Quote:

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Originally Posted by BMWE

actually to solve this problem, i should call to allocate memory when i call to str_cpy.
but when i call it on second time, there will be memory leak if no free memory function will be called.
but if i ask to free memory before allocating a new one, then i lose data (i'm also free the input)

---

It can be avoided if you start by checking if the pointer you pass to the function is NULL. If it is, then you know it's the first time the pointer is used. If it isn't, then you concatenate to the existing string.

This requires that you initialize your pointer to NULL before you use it, which you have done in your example.

Quote:

---

Originally Posted by memcpy

Strcpy() doesn't have memory leaks. It's actually one of the faster and most reliable string library functions. Of course, the way you're using it, it will crash, because you're trying to write data to address 0x0 in memory, which won't work no matter what string copy function you use. You have to allocate memory first, with malloc() or by using an array.

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I may have missed something here but I think the objective is to create such string copy function called str_cpy(). It will allocate memory for the receiver if the pointer is NULL, other wise concatenate to the existing string.

@Subsonics, but then - how you are going to free the memory?

right now the only solution i see is

Code:

---

void str_cpy (char **dest, const char *source)
{   
        int len = strlen(source);
        if (char *tmp=(char *)malloc(sizeof(char)*(len+1)))
      {
              memmove(tmp,source,len);
              tmp[len]=0;
              str_free(dest);
              if (*dest=(char *)malloc(sizeof(char)*(len+1)))
              {
                      *dest=tmp;
              }
  }       
}

---

Quote:

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Originally Posted by BMWE

@Subsonics, but then - how you are going to free the memory?

---

I'm not sure what you mean, you pass the pointer to the function, so you can always free it after you are done with it. If you need to you realloc or create a new temporary string inside it in your implementation then you can free the old string inside the function and switch the pointer values in the function. Although as cas points out if you want this to have effect outside your function you need to either return the pointer or pass in the address of the pointer to the function.

Quote:

---

Originally Posted by BMWE

right now the only solution i see is

Code:

---

void str_cpy (char **dest, const char *source)
{   
        int len = strlen(source);
        if (char *tmp=(char *)malloc(sizeof(char)*(len+1)))
      {
              memmove(tmp,source,len);
              tmp[len]=0;
              str_free(dest);
              if (*dest=(char *)malloc(sizeof(char)*(len+1)))
              {
                      *dest=tmp;
              }
  }       
}

---

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Right, but you could start by checking if *dest == NULL. If it is you allocate strlen(source) +1 to *dest.

If *dest is not NULL, you reallocate *dest with strlen(*dest) + strlen(source) +1 and concatenate source to dest.