Does fflush increment file pointer??

[juice]

I am writing a code, and its giving unexpected output, and after some debugging I have found out that a FILE pointer is getting incremented after a call to fflush. Now, fflush is not supposed to change the position of the file pointer, so why could this be happening, any ideas??

[laserlight]

Post the smallest and simplest compilable program that demonstrates the problem. Also, post the input provided, the expected output and the actual output.

[juice]

Appears to be a bug in MSVC 2010, the same code runs perfect with Pelles C.

[laserlight]

Appears to be a bug in MSVC 2010, the same code runs perfect with Pelles C.

Maybe, but I am more inclined to suspect a bug in your code first.

[juice]

He..he. Hold on. I'll give you the source code.

[juice]

Input:

Enter elements for acc
1
a
15
2
b
10

Enter elements for trans
1
d
15
2
w
5

Expected output

1 a 30.000000
2 b 5.000000

Actual output

1 a 30.000000
2 b 10.000000

#include<stdio.h>

  
int main(void)
{
	struct
	{
		int accno;
		char transtype;
		float amt;
	}trans;
    
	struct
	{
		int accno;
		char name[30];
		float bal;
	}acc;
	FILE *fa, *ft;
	long pos1, pos2;

	if((fa=fopen("acc.txt","w+"))==NULL)
		puts("Unable to open");

	if((ft=fopen("trans.txt","w+"))==NULL)
		puts("Unable to open");

	puts("Enter elements for acc");
	while((scanf("%d %s %f",&acc.accno,acc.name,&acc.bal)) == 3)
		fwrite(&acc,sizeof(acc),1,fa);

	puts("Enter elements for trans");
	while(scanf("%d %c %f",&trans.accno,&trans.transtype,&trans.amt)==3)
		fwrite(&trans,sizeof(trans),1,ft);

	fclose(fa);
	fclose(ft);

	if((fa=fopen("acc.txt","r+"))==NULL)
		puts("Unable to open");

	if((ft=fopen("trans.txt","r"))==NULL)
		puts("Unable to open");

	while(fread(&trans,sizeof(trans),1,ft) == 1)
	{
		rewind(fa);
		pos2 = ftell(fa);
		while(fread(&acc,sizeof(acc),1,fa) == 1)
		{
			if(acc.accno == trans.accno)
			{
				fseek(fa,pos2,0);
				if(trans.transtype == 'd')
					acc.bal+=trans.amt;
				else
					acc.bal-=trans.amt;
				
				fwrite(&acc,sizeof(acc),1,fa);
			}
			
			pos2=ftell(fa);
			fflush(fa);
			/*fseek(fa,pos2,0);*/
		}
		
	}

	rewind(fa);
	printf("\nOutput\n");
	while(fread(&acc,sizeof(acc),1,fa)==1)
		printf("%d %s %f\n",acc.accno,acc.name,acc.bal);

	fclose(fa);
	fclose(ft);
}

Run this under MSVC 2010, then under Pelles C, OR otherwise, run it under MSVC 2010 first with fseek commented out, and then with fseek uncommented.

[stahta01]

IMHO, fwrite and fread should only be used on Binary files; not on Text files like you are doing.

Tim S.

[laserlight]

The bug is in your code.

If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

Observe this loop carefully:

while (fread(&acc, sizeof(acc), 1, fa) == 1)
{
    if (acc.accno == trans.accno)
    {
        fseek(fa, pos2, 0);
        if (trans.transtype == 'd')
            acc.bal += trans.amt;
        else
            acc.bal -= trans.amt;

        fwrite(&acc, sizeof(acc), 1, fa);
    }

    pos2 = ftell(fa);
    fflush(fa);
}

Now, fa is an update stream. Notice that there is a call to fwrite. This satisfies the requirement for "an update stream in which the most recent operation was not input". However, the call to fwrite happens if acc.accno == trans.accno. If not, the most recent operation on fa would be the call to fread, i.e., an input operation, leading to undefined behaviour when control reaches fflush(fa).

Looking at what you commented out, perhaps you intended to write this instead:

while (fread(&acc, sizeof(acc), 1, fa) == 1)
{
    if (acc.accno == trans.accno)
    {
        fseek(fa, pos2, 0);
        if (trans.transtype == 'd')
            acc.bal += trans.amt;
        else
            acc.bal -= trans.amt;

        fwrite(&acc, sizeof(acc), 1, fa);
        fflush(fa);
    }

    pos2 = ftell(fa);
    fseek(fa, pos2, 0);
}

[juice]

Is it necessary that fread and fwrite be used only with binary files?

[CommonTater]

Is it necessary that fread and fwrite be used only with binary files?

You need to spend some time reading the details on these functions and perhaps studying a decent tutorial on C...

Look in your compiler's library documentation, it should give you the full skinny on what each function does.
(IIRC... you're using Pelles C... so this is simple, put your text cursor (the flashing bar) on one of he keyworkds and press F1 on your keyboard)

Basically...
Functions such as fprintf(), fscanf(), fputs(), fgets() work with human readable text. There is a translation from binary to text when writing then from text back to binary when reading...

Functions such as fwrite() and fread() do no translations. They simply copy memory to a disk file or a disk file to memory. So the binary content of memory ends up in your disk file. You might be able to read some of it... but it's not text.


Thing is, I've been watching you launch thread after thread about this EOF boggle you seem to have going on... No offense, but you really do need to do some reading up on this stuff. It's not difficult... unless you don't know it.