Resize the character array in c

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Hi guys,
A MFC coder want to learn some basic about charecter array intialisation and deletion of element.Take following examples campare with MFC (there is CString so no need of memory allocation or de allocation but same needed in c.)
(dont have std::string interface :-()

Example 1:-
To construct string we us following code in MFC.

C++

CString constructString;
constructString = "";
constructString = "ABC";
constructString = constructString + "PQR";
constructString = constructString + "LMN";

whatever size of string we have this will work.

for C i used following code

C++

#define DEFAULT_ARRAY_SIZE			20000
char* constructString  = new char[DEFAULT_ARRAY_SIZE];
strcpy(constructString ,"");
strcat(constructString ,"ABC");
strcat(constructString ,"PQR");
strcat(constructString ,"LMN");

Problem :-
1)Code will work fine till my char* constructString size is less than 20000 but when it exceed i dont have solution,how to resize my array so it will take more charecters.
2)I intialize char* constructString with 20000 but when my string is very small of size 10 then my remaining 18990 charecters are wasted or not i dont know,will this effect my executable perfomance.If yes then how to delete my remaining dummy charecters.

Example 2:-
To read content from file we use following code in MFC.

C++

CStdioFile ReadFile;
ReadFile.Open("Sample.txt",CFile::typeText|CFile::Read);
CString CurrentString;
CStringArray WholeFile;
while(ReadFile.ReadString(CurrentString))
{
    WholeFile.Add(CurrentString);
}

Whitever size of File it will work fine.

For C i use following code

C++

#define MAX_FILE_SIZE				65534
FILE *ptr_file;
const char* list[MAX_FILE_SIZE];
wchar_t CurrentString[1000];
ptr_file =fopen("Sample.txt","rb");
int __index = 0;
while(fgetws (CurrentString , 1000 , ptr_file) != NULL)
{
	char* errorDes;
	errorDes = new char[1000];
	wcstombs(errorDes, CurrentString, 1000);
	list[__index] = errorDes;
	__index++;
}

Problem :-
1)Same as above if my one line charecters exceed 1000 then more than 1000 charecters are not consider and vise versa.
2)If my file size exceed 65534 then char* list array will not fill properly and vise versa.

________________________________________
Please provide me any link,block of code,suggestion that help me to solve all problem in pure c.

Thanks.

Posted 7-May-13 1:52am

Coder Block

Updated 7-May-13 2:12am

CPallini

v4

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CPallini · Accepted Answer · 2013-05-07T02:34:00

All your problems are solved using dynamic memory allocation: you shouldn't allocate a static array but a bit more complex data structure (emulating CString or std::string, behaviour). e.g.

C

typedef struct _MyString
{
  char * str;
  int len;
  int capacity;
} MyString;

Then you should provide some functions for manipulating such a string, for instance:

C

int mystring_init(MyString * pms, const char * src);
int mystring_cat(MyString *pms, const char * src);
int mystring_destroy(MyString * pms);
// ...and so on....

A rough mystring_init implementation could be:

C

// error checking omitted for sake of brevity
int mystring_init(MyString * pms, const char * src)
{
  pms->len = strlen(src+1);
  pms->capacity = pms->len;
  pms->str = (char*) malloc(pms->capacity+1);
  strcpy(pms->str, src);
  return 0;
}

in mystring_cat implementation you need to reallocate the internal memory, that's accomplished using the realloc[^] function:

C

// error checking omitted for sake of brevity
int mystring_cat(MyString * pms, const char * src)
{
  pms->len += strlen(src+1);
  pms->capacity = pms->len;
  pms->str = (char*) realloc(pms->str, pms->capacity+1);
  strcat(pms->str, src);
  return 0;
}

Please note the above are (untested) naive implementation, written only to give you an idea.