[SOLVED] Open a file from a c program using system call.

abhishekgit · 01-26-2012, 01:09 PM

Hi everyone,
I am trying to open a simple file using system call open(). The code I am using is

Code:

#include<sys/types.h>
#include<sys/types.h>
#include<fcntl.h>
#include<stdio.h>
int open(const char *name, int O_WRONLY);
int main(void)
{
	int fd;
	fd=open("/usr/bin/file", O_WRONLY);
	if(fd==-1)
		fprintf(stdout, "Error\n");
	return(0);
}

I am compiling using

Code:

$ gcc filename.c -o filename

The error is

Code:

open.c:5: error: expected ‘;’, ‘,’ or ‘)’ before numeric constant

Kindly help. Thanks.
Regards...

jhwilliams · 01-26-2012, 01:18 PM

open is declared in fcntl.h, you shouldn't be re-declaring it; just use it directly.

Valery Reznic · 01-27-2012, 05:49 AM

Quote:

Originally Posted by jhwilliams

open is declared in fcntl.h, you shouldn't be re-declaring it; just use it directly.

You can just remove redundant declaration.

The problem is O_WRONLY is define so 'int O_WRONLY' expanded by preprocessor to something like 'int 2'

abhishekgit · 01-27-2012, 05:51 AM

I got rid of the error but replaced by another annoying one which i have encountered a lot many times. I modified the code like you said

Code:

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

int main(void)
{
	int fd;
	fd=open("/home/abhishek/downloads/linux.pdf", O_RDONLY);
	if(fd==-1)
		/*error*/
		
	return(0);
}

The error again

Code:

open2.c: In function ‘main’:
open2.c:14: warning: control reaches end of non-void function

I am grateful for your help. Thanks...

abhishekgit · 01-27-2012, 06:30 AM

@jhwilliams
I got rid of the last error too! Now the code is error free, but the pathname i mentioned in open() isn't opening in a new window. The value of the variable fd(file descriptor) is now 3. Am i correct so far? Thanks.

jhwilliams · 01-27-2012, 02:57 PM

Quote:

Originally Posted by abhishekgit

@jhwilliams
I got rid of the last error too! Now the code is error free, but the pathname i mentioned in open() isn't opening in a new window. The value of the variable fd(file descriptor) is now 3. Am i correct so far? Thanks.

open() is used to start access to a file, for use with read() or write().

If you want to display the file in a graphical user interface, you could try just using evince, from the command line, instead of writing a C program.

Code:

 evince file.pdf

theNbomr · 01-27-2012, 07:22 PM

To further clarify what jhwilliams said, the open() function simply creates a file descriptor (which your code assigns to fd). Subsequent to that, you can perform various operations on the file, such as reading, writing (but in this case not: O_RDONLY ), seeking, closing, etc. In each case, the file descriptor is used to identify the open file upon which your code is to operate. You will not visibly see anything to indicate that a file is open.

There is no windowing associated with the open() call on a file. Indeed, windowing is not part of the C programming language, per se. Windowing and other GUI oriented actions are done using functions compiled into special libraries for that purpose. There are numerous categories of such libraries, depending on such things as the underlying graphical subsystem (such as X), the OS (Windows vs Linux/X), etc. Most windowing systems are therefore quite non-portable across hardware and OS architectures.

Opening of files using the open() system call in C is, by contrast, quite portable. It is a fairly sure bet that any architecture that has some kind of filesystem and a C compiler will use the open() call to open files for reading or writing.

--- rod.

abhishekgit · 01-27-2012, 10:07 PM

@jhwilliams
Thank you. I got that. I was looking for the file to be opened in gui. evince works like a charm though. :-)

abhishekgit · 01-27-2012, 10:10 PM

@theNbomr Thank you. Got it! :-)