Thread: help with reading from stream

Hello fellow C lovers. I have a questions concerning reading from a stream.

Im reading filenames out of a file like this text file:

file.txt
-------------------
filename1\n
filename2\n
etc.
-------------------

Im using this loop for now:

Code:

char filename[100];
int c, i = 0;
FILE *fp;

while ( (c = fgetc(fp)) != EOF) {
          if (c != '\n')
              filename[i++] = c;
          else
               break;

          filename[i] = '\0';

          /* here I open another file and use another while() loop
              reading chars with fgetc and fputc() those chars into
              a different fp                                                          */
}

My question is: It seems (going through gdb) that while in this loop
if I open another fp and do something and then come back to
the top of the loop to continue getting characters from the other
fp, the fp points back to the beginning of the stream and not to where
I left off in that stream. Is this normal, and is there a way to prevent
this?

Thanks

First of all, I hope you're initializing fp before you get to the loop. I'm assuming you do and you just took it out to shorten the code.

If you're using the same variable fp to point to a different file before you're done with the first, that's wrong. You need to have a second FILE * to point to reference the second file.

If I understand your question, look at fseek() and ftell()...
Also your going to get overflow if you read more than 100 characters (sizeof(filename)).

Also

Code:

filename[i] = '\0';

Will never get reached.

Why not just use fgets(), instead of brewing your own?

Consider:

Code:

/* ... */

FILE * fp;
char buff[100];

if((fp = fopen("file.txt", "rt")) == NULL)
{
    perror("Failed to open file.txt");
    exit(0);
}

while(fgets(buff, sizeof(buff), fp) != NULL)
{
    printf("Filename: %s\n", buff);
}

fclose(fp);
/* ... */

It's kinda difficult to explain.

1. I'm in a while() loop using fgetc() to read from one stream the name
of the filename my program will open. Say from this file:

----
filename1\n
filename2\n
EOF
----

I read 'filename1' into a char array and use fopen() to open that file
in my current directory.

I've also got a third stream open and am reading from filename1 stream
and writing to this 3rd stream (all within this while() loop)

(I'm writing from each stream I open with the filenames in the text file
to the 3rd stream (file) making one big joined file.)

Now I finish reading from filename1 and fclose() it and go back to the TOP
of my while loop() to read the next filename out of it to find the next file to
add to the 3rd stream.

When I go back to the TOP I am still getting the first filename (filename1)

Shouldn't this stream be at beginning of line 2?

Ahh!! maybe I'm doin this all wrong


P.S zacs7 thanks I forgot that fgets() reads up to and including newline. So i can use fgets() to read
a line at a time out of this file.

Yes, look at using a linked list of streams or an array. Just make sure you don't use the same file pointer for different files without closing it first

Oh no, I know I can't use the same fp.

Thanks

I think I'll use fgets()

Thanks brewbuck. I'll review the loop a little more and see what's going on, and then
I'll post the code if I'm still stuck. But yes, I'm using 3 different file pointers. Maybe I'm
just overlooking something. I'll see what I can do.