string.......HARD!!Can any one help?tks

[hkNewbie]

Here is my source code,the problem is the string cannot hold the value after it left the function.Can any one give me some advise?

#include"tool.h"
void OpenAccountFile(FILE *uDB,int *index,int *AccNo);
void GetUserInfo(string Name,string ID,string* ptStr,int *slen);
string db = "userinfo.txt";
string pdb = "password.txt";

int main(void)
{
	FILE *userDB = NULL;
	string* ptStr = NULL;
	string Name = NULL , ID = NULL;
	//string Password = NULL;
	int index , AccNo , slen;

	srand(time(NULL));
	OpenAccountFile(userDB,&index,&AccNo);
	GetUserInfo(Name,ID,ptStr,&slen);
                //Variable Name and ID cannot pass out  after  function GetUserInfo was called
	puts(Name); <-----Error occured here
	puts(ID);  <------And here

	return 0;
}

void OpenAccountFile(FILE *uDB,int *index,int *AccNo)
{
	string tmpName = NULL , tmpId = NULL;

	if((uDB = fopen(db,"r")) == NULL)
	{
		uDB = fopen(db,"a");
		*index = 1;
		*AccNo = 1;
	}
	else
	{
		uDB = fopen(db,"r");
		while((fscanf(uDB,"%d %d %s %s",index,AccNo,tmpName,tmpId)) != EOF)
		{
			++*index;
			++*AccNo;
		}
	}
	fclose(uDB);

	return;
}

void GetUserInfo(string Name,string ID,string* ptStr,int *slen)
{
	char buf[20];

	ptStr = malloc(sizeof(string));
	Name = ptStr;
	printf("Please enter your apply name : ");
	scanf("%19[^\n]",buf);
	fflush(stdin);
	ptStr = malloc(strlen(buf) + 1);
	strcpy(Name,buf);
	*slen = strlen(Name);
	puts(Name);

	ptStr = malloc(sizeof(string));
	ID = ptStr;
	printf("Please enter your ID card number : ");
	GetIDCardNo(buf);
	fflush(stdin);
	ptStr = malloc(strlen(buf) + 1);
	strcpy(ID,buf);
	puts(ID);

	return;
}

[Salem]

If you want to return an updated pointer, you need to do it like this

void foo ( char **p ) {
  *p = malloc( 10 );
  strcpy( *p, "hello" );
}
int main ( void ) {
  char *str = NULL;
  foo ( &str );  // note, you now have a ptr to a ptr
  printf( "ptr is %p, string is %s\n", str, str );
  free( str );
  return 0;
}