Construct your own strcpy and strcat/implementation of strcpy, strcat function in C

strcpy

String copy or strcpy is a library function. We are discussing how this function works. It takes two buffers- first one the the destination buffer where string should be copied. This can be an uninitialized buffer. Source buffer is the second argument. This should be a NULL terminated string. Strcpy has no way to validate destination buffer length thus programmer should provide a buffer which is at least the size of source string (destination buffer size >= source string). If destination buffer is smaller than source then strcpy can corrupt data or heap segment depending on the location of destination buffer.

strcpy - pictorial view

strcpy implementation code

 
 /***************************************************
* FUNCTION: strcpy()
* __________________________________________________
*
* DESCRIPTION:
*   Copies source string to destination string.
*
* INPUTS:
*
* dest_ptr - Destination pointer of this operation
* src_ptr  - Source string of this operation.
*
* OUTPUTS:
*   None.
*
* RETURNS:
*   Returns the destination string.
*
* NOTES:
*
***************************************************/

 char* strcpy(char * dest_ptr, const char * src_ptr)

 {

   char* strresult = dest_ptr;

   if((NULL != dest_ptr) && (NULL != src_ptr))

   {

     /* Start copy src to dest */

     while (NULL != *src_ptr)

     {

       *dest_ptr++ = *src_ptr++;

     }

     /* put NULL termination */

     *dest_ptr = NULL;

   }

   return strresult;

 }/* End: strcpy() */

strlen

String length or strlen is a library function. It takes a string buffer and this should also be a NULL terminated string. We iterate till the NULL character is reached. There a variable to note down the length and it is initialized to zero. Each character iteration we are doing in the loop, we are incrementing the length value by one. Thus this will give us the length when NULL is reached. This logic is again used in the strcat function to reach at the end of the string.

strlen implementation code

 /***************************************************

* FUNCTION: strlen()
          
* __________________________________________________

*

* DESCRIPTION:

*   Concatenates source string to destination.

*

* INPUTS:

*  src_ptr - Source string for this operation

*

* OUTPUTS:

*   None.

*

* RETURNS:

*   Returns the length of string.

*

* NOTES:

*

***************************************************/

 char* strlen(const char * src_ptr)

 {

   int len = 0;

   if(NULL != src_ptr)

   {

     /* Iterate till end of  string */

     while(NULL != *src_ptr)

     {

       len++;

       src_ptr++;

     }

   return len;

 }/* End : strlen() */

strcat

String Concatenation or strcat is again a library function. It also take two buffer, first one the the destination buffer where string should be combined. This should be a null terminated string. Source buffer is the second argument. This should also be a NULL terminated string. Strcat has no way to validate destination buffer length thus programmer should provide a buffer which is at least the combined size of source string and destination string (destination buffer size >= source string + destination string). If destination buffer is smaller than it can corrupt data or heap segment depending on the location of destination buffer.

strcat - pictorial view

strcat implementation code

 /***************************************************
* FUNCTION: strcat()
* __________________________________________________
*
* DESCRIPTION:
*   Concatenates source string to destination.
*
* INPUTS:
*  dest_ptr - Destination pointer of this operation
*  value    - Source string for this operation
*
* OUTPUTS:
*   None.
*
* RETURNS:
*   Returns the destination string.
*
* NOTES:
*
***************************************************/

 char* strcat(char* dest_ptr,const char * src_ptr)

 {

   char* strret = dest_ptr;

   if((NULL != dest_ptr) && (NULL != src_ptr))

   {

     /* Iterate till end of dest string */

     while(NULL != *dest_ptr)

     {

       dest_ptr++;

     }

     /* Copy src string starting from the end NULL of dest */

     while(NULL != *src_ptr)

     {

       *dest_ptr++ = *src_ptr++;

     }

     /* put NULL termination */

     *dest_ptr = NULL;

   }

   return strret;

 }/* End : strcat() */

Note: Length of a buffer is calculated by (string length + 1). One extra character is for taking NULL termination into consideration. While allocating memory correct allocation should be malloc(strlen(src)+1);.

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