C - Declare a file in function parameters

Question

so here is my problem :

int isopen()
{
    int fd;

    fd = open("myfile", O_RDONLY);
    if (fd == 0)
        printf("file opening error");
    if (fd > 0)
       printf("file opening success");
    return(0);
}

int main(void)
{
   isopen();
    return(0);
}

Is use this code to check if this the open command worked, as i'm just starting to lurn how to use it.

Basically this code is working just fine, but I would like to declare the file I would like to open directly in the parameters of my function isopen.

I saw some other posts using main's argc and argv, but I really need to declare my file in the parameters of my function isopen, not using argc & argv.

Is it even possible ?

Thank you for your help, I'm quite lost here.

I suggest you start by reading the man page for `open()` and get right the return value, before making the file name more general. — Weather Vane, Nov 29 '16 at 09:30
In `main` you don't call `isopen` but you call `size` whicht you didn't show BTW. — Jabberwocky, Nov 29 '16 at 09:32
@WeatherVane I already red the man, and I know that the open function returns the file descriptor, I don't how knowing this piece of information is giving me any help wth using a file as a parameter ? I'm sorry.. — Marilou Cassar, Nov 29 '16 at 09:34
"Return Value ... returns a file descriptor for the opened file. A return value of -1 indicates an error;" — Weather Vane, Nov 29 '16 at 09:35
@MichaelWalz Sorry, I just modified my original function name for this post, and forgot to change it in the main function, editing that right away ! — Marilou Cassar, Nov 29 '16 at 09:36
@MarilouCassar still wrong, try harder. Now you call the inextstant `myfile`. — Jabberwocky, Nov 29 '16 at 09:37
@WeatherVane Yup, I know that, I mean this isn't answering my question, I just preferred to print a string so I have more visual.. — Marilou Cassar, Nov 29 '16 at 09:38
Sorry to be picky, but if you don't start with the correct use of the function, there is no point going further. It **returns -1** on error, and you have not trapped it. — Weather Vane, Nov 29 '16 at 09:39
@WeatherVane You would prefer to see something like " if function open returns -1, then return that there was error" ? — Marilou Cassar, Nov 29 '16 at 09:42
I would prefer to see `if (fd < 0) printf("file opening error"); else printf("file opening success"); return fd;` — Weather Vane, Nov 29 '16 at 09:44
@WeatherVane Sorry, but I won't do that as I'm not allowed to do it. I lurn at school 42 and we have a very strict norm, forbidding us to use a function directly in 'if' parameters. Sorry ! — Marilou Cassar, Nov 29 '16 at 09:46
I have not the faintest idea what you are talking about, my code is almost exactly like yours except for the correct test, and returning a useful value from the function. Good luck with the lurning. — Weather Vane, Nov 29 '16 at 09:48
@WeatherVane It's just that I'm turning at a particular school that wants us to write our codes in a very strict manner, and what you told me to do is something forbidden ! Thanks for trying to help ! — Marilou Cassar, Nov 29 '16 at 09:54
In that case why did you accept an answer almost identical to my comment? — Weather Vane, Nov 29 '16 at 09:55
@WeatherVane It don't understand, I don't feel like it's identical ? Well sorry, anyway, this question is answered, thanks for you advices ! — Marilou Cassar, Nov 29 '16 at 09:59

score 2 · Accepted Answer · answered Nov 29 '16 at 09:45

2

Your question is unclear, but maybe you want this:

int isopen(const char *filename)
{
    int fd;

    fd = open(filename, O_RDONLY);
    if (fd < 0)                           //BTW <<<<<<<<<<<<  fd < 0 here !!
        printf("file opening error"); 
    else                                  // else here
       printf("file opening success");

    return(0);
}


int main(void)
{
   isopen("myfile");
    return(0);
}

BTW, the isopen function as it stands here is still pretty useless as it just opens the file and throwing away fd.

answered Nov 29 '16 at 09:45

Jabberwocky

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Thank you so much ! This is exactly what I needed. Sorry for unclear instructions, I struggle a bit with english as it's not my mother language. And thanks again for the little help with my if conditions ! – Marilou Cassar Nov 29 '16 at 09:51
It's actually worse than useless - it leaks the file descriptor, leaving it open, and a process only has a limited number of available descriptors. – Andrew Henle Nov 29 '16 at 10:47

score 0 · Answer 2 · answered Nov 29 '16 at 09:47

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int isOpen(char *filename)
{
   return open(filename, O_RDONLY);
}

int main() 
{
    printf("%d\n", isOpen("/home/viswesn/file1.txt"));
    printf("%d\n", isOpen("file2.txt"));
    return 0;
}

Output

    viswesn@viswesn:~$ cat /home/viswesn/file1.txt
    hello
    viswesn@viswesn:~$
    viswesn@viswesn:~$ cat /home/viswesn/file2.txt
    cat: /home/viswesn/file2.txt: No such file or directory
    viswesn@viswesn:~$
    viswesn@viswesn:~$ ./a.out
    3     <---------- File exist and it give file descriptor number '3'
                      STDIN-0, STDOUT-1, STDERR-2 are reserved and 
                      next file opened will start with 3 and it keeps going
    -1    <---------  File not found; so open gives -1 as error

@MarilouCassar :-) don't write "his answer is useful" but rather upvote it ! — Jabberwocky, Nov 29 '16 at 10:02

C - Declare a file in function parameters

2 Answers2