how to append in a existing file descriptor in linux and also insert a new line.

mot12 · 08-29-2011, 06:34 AM

hello,

In my code i opened a file and write in it now i want each time i reopen it,data must be written from next line.
my code snippet is:

unsigned char buffer[8]={0};
int cmd=1;
int tt=100;
tt<<16;
tt|cmd;
sprintf((char*)buffer,"%d",tt);
FILE *fp;
fp=fopen("root/test.txt","w");
if(fp==NULL)
perror("file pointer:");
fwrite(buffer 8,1,fp);
fclose(fp);

I also searched but did'nt got any right answer.
Thanks in advance

tronayne · 08-29-2011, 07:14 AM

What you want to do is fopen to append; e.g.,

Code:

fp = fopen ("root/test/txt","a");

"a" will open for appending (writing at end of file). The file is created if it does not exist. The stream is positioned at the end of the file.

"a+", on the other hand, will open for reading and appending (writing at end of file). The file is created if it does not exist. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

Hope this helps some.

tronayne · 08-29-2011, 07:15 AM

What you want to do is fopen to append; e.g.,

Code:

fp = fopen ("root/test/txt","a");

"a" will open for appending (writing at end of file). The file is created if it does not exist. The stream is positioned at the end of the file.

"a+", on the other hand, will open for reading and appending (writing at end of file). The file is created if it does not exist. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

Hope this helps some.

mot12 · 08-29-2011, 11:43 PM

Quote:

Originally Posted by tronayne

What you want to do is fopen to append; e.g.,

Code:

fp = fopen ("root/test/txt","a");

"a" will open for appending (writing at end of file). The file is created if it does not exist. The stream is positioned at the end of the file.

"a+", on the other hand, will open for reading and appending (writing at end of file). The file is created if it does not exist. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

Hope this helps some.

thanks for reply but i want that my next stream will come at the beginning of the next line and so on.........please help me out.

tronayne · 08-30-2011, 07:48 AM

Quote:

Originally Posted by mot12

thanks for reply but i want that my next stream will come at the beginning of the next line and so on.........please help me out.

That's precisely where it will be -- the "beginning of the next line" is right after the last line feed (line feed is the terminator of a line of text). If you write to the file, that's where you'll write.

If you were to write a little program something like this

Code:

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

int     main    (void)
{
        FILE    *outfile;

        if ((outfile = fopen ("junk", "a")) == (FILE *) NULL) {
                (void) fprintf (stderr, "Can't write names\n");
                exit (EXIT_FAILURE);
        }
        /*      write the time on to the end of "junk"  */
        (void) fprintf (outfile, "%ld\n", time (0));
        exit (EXIT_SUCCESS);
}

compile it and execute it, wait a while, execute it again, wait a while, execute it again, then cat junk. From what you're saying, this will do exactly that.

Hope this helps some.