C / C++ / MFC Discussion Boards

Re: How to read and write to a BINARY file?

Member 419459329-Oct-09 13:05

Member 4194593

29-Oct-09 13:05

Just CreateFile with binary access, write it, seek to the front, and read it back record by record. If you want random access, then the records must be the same size. If you want variable length records, just precede each binary record with a DWORD with the record size when you write it, then when you read it back, read the DWORD to get the record size, then read the data block, - this also implies that the file must be sequentially accessed both for writing and reading.

Dave.

Re: How to read and write to a BINARY file?

enhzflep29-Oct-09 13:06

enhzflep

29-Oct-09 13:06

#include <stdio.h>

typedef struct _myStruct
{
    int x;
    int y;
    int z;
} myStruct;

// use this main() to create the file
int main()
{
    FILE *fp;
    char *fileName = "output.dat";
    char *mode = "wb";
    myStruct theArray[10];
    int i;
    for (i=0; i<10; i++)
    {
            theArray[i].x = i;
            theArray[i].y = 10-i;
            theArray[i].z = i * (10-i);
    }
    fp = fopen(fileName, mode);
    fwrite(theArray, sizeof(struct myStruct), 10, fp);
    fclose(fp);
}


// use this main() to read the file
/*
int main()
{
    FILE *fp;
    char *fileName = "output.dat";
    char *mode = "rb";
    myStruct theArray[10]={};
    int i;
    fp = fopen(fileName, mode);
    fread(theArray, sizeof(myStruct), 10, fp);
    fclose(fp);
    for (i=0; i<10; i++)
    {
        printf("Item %d---------\n", i+1);
        printf("<%d, %d, %d>\n\n", theArray[i].x, theArray[i].y, theArray[i].z);
    }
    system("pause");
    return 0;
}
*/

Re: How to read and write to a BINARY file?

dipuks29-Oct-09 15:27

dipuks

29-Oct-09 15:27

Thanks for the sample code.

I am working on C++ using WIN32 API's in Visual Studio 2003.

In the function here for binart read/write,

i am using the functions, _write and _ read.

Does that make any difference in writing/reading? compared to using fwrite and fread?

Re: How to read and write to a BINARY file?

enhzflep29-Oct-09 15:53

enhzflep

29-Oct-09 15:53

Please, allow me to refer you to this short discussion: fread vs _read[^]

Re: How to read and write to a BINARY file?

David Crow29-Oct-09 17:01

David Crow

29-Oct-09 17:01

dipuks wrote:
i am using the functions, _write and _ read.

See here and here.

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Re: How to read and write to a BINARY file?

Adam Roderick J29-Oct-09 17:58

Adam Roderick J

29-Oct-09 17:58

For opening binary file use CreateFile [^] with OPEN_EXISTING flag.

For reading the file use ReadFile [^]

For writing the file use WriteFile [^]

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A General Problem

WindowsVsLinux29-Oct-09 8:49

WindowsVsLinux

29-Oct-09 8:49

Hi,
I have the following program.
I want to use char array at the end of while loop so that after while loop if i use printf() statement it will be printed.I have already copy the directory in an char array.Actually i am something confusing at this time and cann't solve the problem.

#include<windows.h>
#include<string.h>
#include<stdio.h>

int main()
{
int length1,length2;
WIN32_FIND_DATA FindFileData;
HANDLE hFind = INVALID_HANDLE_VALUE;
char DirSpec[MAX_PATH + 1];
DWORD dwError;
char dirpath[MAX_PATH];

printf("Enter the path of directory: ");
gets(dirpath);

printf ("Target directory is %s\n",dirpath);
strncpy (DirSpec, dirpath, strlen(dirpath)+1);
strncat (DirSpec, "\\*", 3);

hFind = FindFirstFile(DirSpec, &FindFileData);

if (hFind == INVALID_HANDLE_VALUE)
{
printf ("Invalid file handle. Error is %u\n", GetLastError());
return (-1);
}
else
{

length1=sizeof(FindFileData.cFileName);
char firstdir[length1];
strcpy(firstdir,FindFileData.cFileName);
printf("First file name is %s\n",firstdir);
while (FindNextFile(hFind, &FindFileData) != 0)
{
length2=sizeof(FindFileData.cFileName);
char otherdir[length2];
strcpy(otherdir,FindFileData.cFileName);
printf("Other dir is:%s\n",otherdir);
}
/* ****************************************************
* here i want a char array *
* printf("All dir except first is:%s",array); *
* *
****************************************************/

dwError = GetLastError();
FindClose(hFind);
if (dwError != ERROR_NO_MORE_FILES)
{
printf ("FindNextFile error. Error is %u\n", dwError);
return (-1);
}
}
return (0);
}
I have commented the line where i want the char array.Actually i want this array outside while loop.

Re: A General Problem

«_Superman_»29-Oct-09 9:04

«_Superman_»

29-Oct-09 9:04

Please post your code withing the code block.
You are more likely to get and answer if you do that.

ravi 12 wrote:
char otherdir[length2]

You cannot create an array with a variable (length2) as its size.
You need to have a constant instead.

«_Superman_»
I love work. It gives me something to do between weekends.

Microsoft MVP (Visual C++)

Re: A General Problem

WindowsVsLinux29-Oct-09 18:03

WindowsVsLinux

29-Oct-09 18:03

Hi «_Superman_»,
thank you very much for your suggestion.You are right that i must create a constant size array.thanks

Re: A General Problem

David Crow29-Oct-09 9:30

David Crow

29-Oct-09 9:30

Are you wanting a char array of all files found in the while() loop by the FindFirstFile()/FindNextFile() pair? Unless you have an aversion against it, use std::vector or std::list.

Re: A General Problem

WindowsVsLinux29-Oct-09 18:05

WindowsVsLinux

29-Oct-09 18:05

Hi DavidCrow,
Thanks for your suggestion.Yes i want a char array of all files found in the while() loop by the FindFirstFile()/FindNextFile() pair.

Re: A General Problem

enhzflep29-Oct-09 9:36

enhzflep

29-Oct-09 9:36

Perhaps «_Superman_» overlooked the difference between c and c++ when it came to dynamically sized array declarations - they work just fine here too.

Um, this is really very easy.
You need to declare somewhere to hold a string.
After each new filename has been retrieved it's tacked onto the end of the existing string
When the loop's over the string is printed.

I can think of at least 3 ways around this mountain, though (surprise, surprise) went for the easiest/quickest/dodgiest (is that even a word?) way.

1. Declare a new var in main and initialize to 4096 times "\0" (since I'm testing in C:\ and I know there to be under 4kb worth of file/folder names there)

char finalString[4096];

2. Add to it each time through the loop

while (FindNextFile(hFind, &FindFileData) != 0)
{
    strcat(finalString, FindFileData.cFileName);
    strcat(finalString, "\n");
}

3. Print it out

printf("%s\n", finalString);

The other two ways that come to mind are:

1. Dynamically resizing the memory used to hold the final string.
2. Getting a count of the number of dir-entries, then creating an array of char* with that many entries. You then alloctae some memory, point dirEntryArray[curEntryNum] at this memory, and make sure you copy the current filename into said mem.

You can probably see why I went for the 'coding-horrors' way. Sigh | :sigh:

EDIT: ah crap! See, that's why David is an MVP, and I'm not. Sniff | :^)

Re: A General Problem

WindowsVsLinux29-Oct-09 18:08

WindowsVsLinux

29-Oct-09 18:08

hi enhzflep,
very very thank of you for your suggestion.Now it works fine.Actually i was useing strcpy() function instead of strcat() function.I don't know why i was unable to do that.Thanks again..........

Re: A General Problem

Adam Roderick J29-Oct-09 18:21

Adam Roderick J

29-Oct-09 18:21

I think it will better if we can use heap allocation instead of stack for 4k, because, these 4k allocations finally leads to stack overflow. Sniff | :^)

.

Ravi you can follow which ever method according to the need.
If you follow David's method then it will be less error prone and more maintainable Smile | :)

but enhzflep's method is much faster Smile | :)

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modified on Friday, October 30, 2009 4:38 AM

pointer and switch

kbury29-Oct-09 8:18

kbury

29-Oct-09 8:18

I am trying to write a program as a math tutor. Can someone look at this code. I keep getting errors on int and pointer as well as telling me the switch case has illegal pointer.

Thanks, any advice would be greatful.

#include <stdio.h>
#include <stdlib.h>


#define SENT 4 //"Quit" menu choice


/*  Function Prototypes */


	void DisplayMenu (void);
	void GetMenuChoice (void);
	void Gen2Rand (int*r1, int*r2);
	void DrillOneProb (int*c, int*r1, int*r2);






/*============Mainline Procedures===============*/

int main (void)
{
	int	   c;   //Menu Choice (1-4)
	int   r1,  //First Random Integer:  2-12 inclusive
	      r2;  //Second Randon Integer:  2-12 inclusive


}

/*===========CHILD FUNCTIONS===============*/

/*  Display Title and Menu */

void DisplayMenu (void)

	{
		printf("MENU OF OPERATIONS\n\n");
		printf("1. Addition.\n");
		printf("2. Subtraction.\n");
		printf("3. Multiplication.\n");
		printf("4. Quit.\n\n");

    }


/* Get Menu Choice */

void GetMenuChoice (void)

	{
		int c;
		printf ("Enter the number of the operation to try (1-4):\n");
		scanf ("%d", &c);
		while
			 (c<1 || c>SENT)
				printf("\aInput value is out of range.\n");


	}

/* Generate and return 2 integers between 2-12 inclusive */

void Gen2Rand (int *r1p, int *r2p)

	{
		int c;
		c=0;

		if (c>=1 && c<SENT)

		int r1; // First random number
		int r2; //Second random number

		r1 = 2 + rand() %11;
		r2 = 2 + rand() %11;

	   *r1p = r1;
	   *r2p = r2;

	   printf("Program complete\n");

	}

/* Display two random numbers and ask user what the answer would be after the chosen operation*/

void DrillOneProb (int *c, int *r1, int *r2)

{
		int CorAns, //Correct Answer
			Reply;  // Users Reply

			{
			printf("\nWhat is %d", & r1);

			r1 = 2 + rand() % 11;
			}


	switch (c);
	{
		case '1':  1 = +; break;
		CorAns = r1 + r2;

		case '2':  2 = -; break;
		CorAns = r1 - r2;

		case '3':  3 = x; break;
		CorAns = r1 x r2;

	}

	printf("%d ?  %d\n", r2, Reply);

   if
  		Reply = CorAns
	   		{
			   printf("Yes, that is correct. Good Job!");
			}

		else
			printf("No, the correct answer is: %d", CorAns);
			printf("\n\n");

return (c);




    print_menu(); //Print the menu option, and configure some needed things.


    return (0); //The end of our simple game.

Re: pointer and switch

«_Superman_»29-Oct-09 8:21

«_Superman_»

29-Oct-09 8:21

kbury wrote:
switch (c);

This must be switch (c) without the ;

I also believe that it must actually be switch (*c)

«_Superman_»
I love work. It gives me something to do between weekends.

Microsoft MVP (Visual C++)

Re: pointer and switch

kbury29-Oct-09 8:24

kbury

29-Oct-09 8:24

Thanks, I just changed and it fixed that part.

Re: pointer and switch

CPallini29-Oct-09 10:58

CPallini

29-Oct-09 10:58

kbury wrote:
1 = +;

kbury wrote:
2 = -;

kbury wrote:
3 = x;

kbury wrote:
CorAns = r1 x r2;

kbury wrote:
if
Reply = CorAns

I think you should read very carefully a syntax reference of the C programming language.
Smile | :)

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-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

[My articles]

Re: pointer and switch

kbury30-Oct-09 2:31

kbury

30-Oct-09 2:31

I am only trying to follow a flow chart given and it is very confusing. Thank you

Re: pointer and switch

CPallini30-Oct-09 2:52

CPallini

30-Oct-09 2:52

While following the flow, you shouldn't forget there is also the syntax... Roll eyes | :rolleyes:

Re: pointer and switch

kbury30-Oct-09 9:37

kbury

30-Oct-09 9:37

I am stuck with this section of my code so far: error codes: expected int and got pointer to int and operands of = illegaltype pointer to int and int

/* Display two random numbers and ask user what the answer would be after the chosen operation*/

void DrillOneProb (int*c, int*r1, int*r2)

{
		int CorAns, //Correct Answer
			Reply;  // Users Reply

			{
			printf("\nWhat is %d",r1);	1st error		                scanf("%d", r1);

			r1 = 2 + rand() % 11;           2nd error
			}

Re: pointer and switch

CPallini30-Oct-09 10:43

CPallini

30-Oct-09 10:43

kbury wrote:
printf("\nWhat is %d",r1);

change to

printf("\nWhat is %d", *r1);

kbury wrote:
r1 = 2 + rand() % 11;

change to

*r1 = 2 + rand() % 11;

The above steps, however just fix compiler errors...I don't know if the semantic will be correct.

BTW what about a good C language programming book?
Smile | :)

Re: pointer and switch

kbury30-Oct-09 10:57

kbury

30-Oct-09 10:57

change to

printf("\nWhat is %d", *r1); (I already changed this)I figured that out.

kbury wrote:
r1 = 2 + rand() % 11;

change to

*r1 = 2 + rand() % 11; (This defineately fixed this.

The above steps, however just fix compiler errors...I don't know if the semantic will be correct.

switch (c)

	{     (states that this is illegal type pointer to int in       switch expression.)

		case '1':
		  	printf("+");
		   	CorAns = r1 + r2;  (operands of + have illegal of 'pointer to int' and 'pointer to int')

		  break;

Re: pointer and switch

CPallini30-Oct-09 11:00

CPallini

30-Oct-09 11:00

kbury wrote:
CorAns = r1 + r2;

change to

CorAns = *r1 + *r2;

The real solution, however, would be reading the f*#?n' book. Big Grin | :-D

Re: pointer and switch

kbury30-Oct-09 11:05

kbury

30-Oct-09 11:05

I do not believe that has been in any of the chapters that we have read so far, but I will look again at the latest chapter because each assignment is associated with that chapter.

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