C程序输出

Question

#include<stdio.h>
int main()
{

        FILE *fp,*fs;
        fs=fopen("c.dat","w");
        fp=fopen("c.dat","w");
        putc('a',fp);
        putc('1',fs);
        close(fs);
        close(fp);
        return 0;
}

Irrespective of the order execution

 putc('a',fp);putc('1',fs);   


 putc('1',fs);putc('a',fp);

contents of c.dat is '1' only. Can somebody explain the reason ?

Answer 1

For a start, you don't close() a file that you opened with fopen() , you have to fclose() it. You're mixing two levels of abstraction here, file handles and file pointers.

On top of that, I think you'll find that the two file pointers are maintaining independent file offsets and buffers. Whichever one closes the file last will write their data last, overwriting the other.

Change the order of your fclose() statements and you should see what I mean:

#include<stdio.h>
int main (void) {
    FILE *fp,*fs;
    fs=fopen("c.dat","w");
    fp=fopen("c.dat","w");
    fputc('a',fp);
    fputc('1',fs);
    fclose(fp);    // Try changing these
    fclose(fs);    //    two around.
    return 0;
}

I did that and consistently got the contents based on the last file closed.

And, before you say that you should have gotten a because fp was the last one you closed, you did not close fp (with an fclose() ).

What you did do was try to close() a file handle that was almost certainly invalid, because file handles are generally relatively small values (eg, 0 , 1 , 2 ) while file pointers are generally relatively large values (eg, 0xdeadbeef , 0xcaf3f00d ).

So the closure of both those files was left up to the C runtime environment when you exited (and that would most likely have done it in a deterministic way, which is why you always got the same value in the file).

Answer 2

You are opening the same file for writing twice. All bets are off at that point.

Answer 3

You didnt specify the unerlying OS, but on win and *nix systems file buffers are shared between processes, so :

    putc('a',fp); 
    putc('1',fs);

are writing to exactly the same address in the same buffer if you reversed the order of the putc statements you will end up with an 'a' in the file. *

NOT

Just goes to show how tricky this all is. I have tried this and what affects the output is the order in which the files are opened -- the first open always takes precedence.

Answer 4

我认为这是因为您首先通过fs = fopen打开了c.dat，然后某些锁不允许写入“ a”，因为这将要求再次打开文件。