weird fstream output

schneidz · 09-06-2005, 02:01 PM

Code:

schneidz@lq> cat schneidz-09.03.2005.c
#include "stdio.h"
#include "string.h"

main(int argc, char *argv[])
{
        char col1[255], temp[255], s[] = "0"  ;
        int c;
        FILE * fstream;

        fstream = fopen(argv[1], "r+");

        c = fscanf(fstream, "%s", &col1);
        while(c == 1)
        {
         if(strcmp(col1,"55555") == 0)
         {
          fprintf(fstream, "hello-world");
         }
         c = fscanf(fstream, "%s", &col1);
        }
        fclose(fstream);
}
schneidz@lq> cat schneidz.txt
hello
world
l33t
h4x0r
55555
chun-li
akuma
schneidz@lq> schneidz.x schneidz.txt
schneidz@lq> cat schneidz.txt
hello
world
l33t
h4x0r
55555
chun-li
akuma
hello
world
l33t
h4x0r
55555hello-worlduschneidz@lq>

what i would want it to look like is:

Code:

schneidz@lq> cat schneidz.txt
hello
world
l33t
h4x0r
55555hello-world
chun-li
akuma

can someone help me figure out how to get the correct output from 'fprintf'.
(why is it printing everything up to the 'strcmp' twice?)
'man fprintf' isn't explaining what i am seeing.

thanks,

perry · 09-06-2005, 02:15 PM

one thing you might consider is using fputs instead of fprintf as by default, fprintf expects the 2nd parameter to be it's formatting string. if anything in that parameter looks like a control sequence (aka "%s" or "%x") it's essentially going to look for a 3rd parameter which you have not specified. however, being c/c++, it will look for a 3rd parameter irreguardless if that is what you meant or not, getting the info off the stack. it's a wonder your app didn't bomb with stack violations or anything else... there again, this is Linux so you are a bit safer than the world of winblows.

hope this helps

- perry

schneidz · 09-06-2005, 02:42 PM

thanks perry but the same thing happened (without the last charecter being 'u').

does anyone have any other ideas?

i added this:

Code:

fflush(fstream);

after the second fscanf.

and it yeilded this:

Code:

schneidz@lq> cat  schneidz.txt
hello
world
l33t
h4x0r
55555
chun-li
akuma

55555hello-worldschneidz@lq>

but it's still not edited in place.

exvor · 09-06-2005, 03:48 PM

hehe looks like your trying to do a cat like program. I have done one before. You may
want to try outputing each charecter from the stream rather then obtain string information.

at least thats how i got mine to work.

Code:

 
#include "stdio.h"
#include "string.h"

main(int argc, char *argv[])
{
        char col1[255], temp[255], s[] = "0"  ;
        int c;
        FILE * fstream;

        fstream = fopen(argv[1], "r+");

        c = fscanf(fstream, "%s", &col1);
        while(c == 1)
        {
         if(strcmp(col1,"55555") == 0)
         {
          fprintf(fstream, "hello-world");
         }
         c = fscanf(fstream, "%s", &col1);
        }
        fclose(fstream);
}

also you have decalared a pointer here but your code does nothing with it

s[] = "0"

this one is also never used
temp[255]