FILE *f = fopen("c:\\V530b18\\system.par","r+");//Open File for read
	if(f==NULL)
	{
		SayError("Error open file <system.par>...");
		return  0 ;
	}
	FILE *fn = fopen("c:\\V530b18\\system_tmp.par","w");

	long curpos = ftell(f);
	Debug("OK open file\n");
	const char* charCompare = "Startup macro:";
	const char* charInsertDll = " c:\\v530b18\\lithostar.dll\n";
	char charBuf[ 80 ];
	
	int n =  0 ; 
	int fl = - 1 ;
	
	Debug("Start find string\n");
	while (fgets(charBuf,  80 , f) != NULL)
	{
		Debug("While Ok!!!\n");
		fputs(charBuf, fn);
		if (strstr(charBuf, charCompare) != NULL)
		{
			if (strstr(charBuf, charInsertDll) != NULL)
			{
				Debug("On string!!!\n");
			}
			else
			{
				Debug("Insert string!!!\n");
				fseek(fn,-2L, SEEK_CUR);
				fputs(charInsertDll, fn);
			}
			
			Debug("Step WHILE string!!!\n");
		}
		Debug("End WHILE string!!!\n");
		n++;
	}

	fclose(f);
	//fclose(fn);
	//rename("c:\\v530b18\\system.par","c:\\v530b18\\kasdh.txt");
	remove("c:\\v530b18\\system.par");
	cout << "errno == " << errno <<endl;
	rename("c:\\v530b18\\system_tmp.par","c:\\v530b18\\system.par");
	return  0 ;

#include <iostream.h>
#include <stdio.h>
#include <string.h>  
#include <stdlib.h>
#include <io.h>

 
int main()
{
	FILE *f = fopen("c:\\V530b18\\system.par","r+");//Open File for read
	if(f==NULL)
	{
		return  0 ;
	}
	FILE *fn = fopen("c:\\V530b18\\system_tmp.par","w");

	long curpos = ftell(f);
	const char* charCompare = "Startup macro:";
	const char* charInsertDll = " c:\\v530b18\\lithostar.dll\n";
	char charBuf[ 80 ];
	
	int n =  0 ; 
	int fl = - 1 ;
	
	while (fgets(charBuf,  80 , f) != NULL)
	{
		fputs(charBuf, fn);
		if (strstr(charBuf, charCompare) != NULL)
		{
			if (strstr(charBuf, charInsertDll) != NULL)
			{
			}
			else
			{
				fseek(fn,-2L, SEEK_CUR);
				fputs(charInsertDll, fn);
			}
		}
		n++;
	}

	fclose(f);
	fclose(fn);
	remove("c:\\v530b18\\system.par");
	rename("c:\\v530b18\\system_tmp.par","c:\\v530b18\\system.par");
	return  0 ;
}

#include <iostream.h>
#include <stdio.h>
#include <string.h>  
#include <stdlib.h>
#include <io.h>

 
int main()
{
	FILE *f = fopen("file.txt","r+");
	if(f==NULL)
	{
		return  0 ;
	}
	FILE *fn = fopen("file_tmp.txt","w");

	const char* charCompare = "string1";
	const char* charInsertDll = " string2";
	char charBuf[ 80 ];
	
	while (fgets(charBuf,  80 , f) != NULL)
	{
		fputs(charBuf, fn);
		if (strstr(charBuf, charCompare) != NULL)
		{
			if (strstr(charBuf, charInsertDll) != NULL)
			{
			}
			else
			{
				fseek(fn,-2L, SEEK_CUR);
				fputs(charInsertDll, fn);
			}
		}
	}

	fclose(f);
	fclose(fn);
	remove("file.txt");
	rename("file_tmp.txt","file.txt");
	return  0 ;
}