Thread: Help with file pointer

I have the code below to open a file, append some text and then display the contents of that file on the screen. I am new to programming and trying to learn on my own for the company I work for. Any help is greatly appreciated. I was expecting to have this statement - if (fptr == NULL);
be false but when I execute the program I see the message - We are having problems. Any advice? I am also having some trouble understanding how the functions pass paramaters so any links would be helpful on that subject or array of structures which I was told I will need to understand well by the end of the month. Thanks.

-Carl

Code:

#include<stdio.h>

#define MAXLEN 10

main()
{

FILE *fptr;

char myline[MAXLEN];
char filename [] = "data.txt";

fptr = fopen(filename, "a");

if (fptr == NULL);
{
printf("\nWe are having problems\n");
}

fprintf (fptr, "This is working  - testing append!\n");
fprintf (fptr, "This is ridiclous!\n");


fptr = fopen(filename, "r");

while (fgets(myline, MAXLEN, fptr) != NULL)
{
 printf("%s", myline);
}
fclose (fptr);

return 0;
}

Ugh.. I wrote a long reply and then my internet connection decided to act up.

Anyway - You can open the file in both read and write modes. Try "a+", for example.

However, if you do want to reopen the file with different modes, use freopen() ( Find it on www.cppreference.com under Standard I/O)

As to the fopen() call failing - the only possible reason is that the file doesn't exist(Names are case-sensitive, mind you) or that you don't have permissions to the file.

By the way - it would be optimal to exit after you find out you can't open the file. Trying to write to a NULL file pointer will cause a segfault.

Thanks for your reply. I did add exit(-1) and the program exits before taking any action. What is bothering to me is that the filename is correct becuse every time I run it I get new lines appended to the file data.txt. It just thinks fptr is null for some reason and displays the printf below it which I think should not happen based on what little I have learned.

-Carl

Ok I corrected the code as was suggested and it is below. Thanks for the advice. It exits properly BUT my confusion is if I comment out the exit(-1) the file is being appended to properly so does anyone know why fptr == NULL? Thanks again.

Code:

#include<stdio.h>

#define MAXLEN 10

main()
{

FILE *fptr;

char myline[MAXLEN];
char filename [] = "data.txt";

fptr = fopen(filename, "a");

if (fptr == NULL);
{
printf("\nWe are having problems\n");
exit(-1);
}

fprintf (fptr, "This is working  - testing append!\n");
fprintf (fptr, "This is ridiclous!\n");

fclose (fptr);

fptr = fopen(filename, "r");

while (fgets(myline, MAXLEN, fptr) != NULL)
{
 printf("%s", myline);
}
fclose (fptr);

return 0;
}

Thanks everyone for the help! This site is great for us newbies.

-Carl

Originally Posted by Elysia

http://cpwiki.sf.net/Implicit_main

Elysia, I'm picturing you like a mechanic at a car shop... Guy pulls up with a tow truck and offloads his car, which is steaming and on fire, rods thrown through the hood, windows shattered out, tranny dangling on the ground...

You point and say, "Your tire is flat."