create a file with open.

[conradin]

Hi everyone!
I want to write a simple program in C that creates a file with 775 permissions, using the open command. Im not sure which mode arguments I need to make that happen with open.
Can someone help me out?
heres what I have:



#include <sys/types.h> //Specified in man 2 open
#include <stdlib.h>
#include <errno.h> //Allows use of error numbers
#include <fcntl.h> //Specified in man 2 open
#include <stdio.h>

int main(int argc, char ** argv){
FILE *fp;
fp = open("argv[1]", "O_CREAT", "MODE???");

close(fp); /* Close the file */
return 0;
}

[gateway67]

You should run this command:


man 2 open
One thing you might notice is that open() returns an int value, which is meant to represent a file descriptor. If you want to work with a FILE pointer, then you should use fopen(), fclose(), etc.

Typically open() is used for low-level I/O. For normal stuff, like reading or creating a file, fopen() is used.


P.S. If you choose to use fopen() to create the file, you can subsequently use chmod() to set its permission mode. See "man 2 chmod" for details on this particular function.

[conradin]

yeah, Im on man open(2), Im confused about what to do with multiple modes for open.

#include <sys/types.h> //Specified in man 2 open
#include <stdlib.h>
#include <errno.h> //Allows use of error numbers
#include <fcntl.h> //Specified in man 2 open
#include <stdio.h>

int main(int argc, char *argv[]){
int fd;
fd = open(argv[1], O_CREAT, S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);

return 0;
}
gives me errors. open is what I want, eventually, I will develop device drivers. That really doesn't matter though.
heres my errors, this thing is acting like its never seen the open modes before...

gcc -Wall -o "createFile775" "createFile775.c" (in directory: /home/h/Desktop/cpro)
createFile775.c: In function ‘main’:
createFile775.c:12: error: ‘S_IRWXU’ undeclared (first use in this function)
createFile775.c:12: error: (Each undeclared identifier is reported only once
createFile775.c:12: error: for each function it appears in.)
createFile775.c:12: error: ‘S_IRWXG’ undeclared (first use in this function)
createFile775.c:12: error: ‘S_IROTH’ undeclared (first use in this function)
createFile775.c:12: error: ‘S_IXOTH’ undeclared (first use in this function)
Compilation failed.

[gateway67]

You need to include <sys/stat.h> to obtain those macro definitions (re-read the man page for open() a little more carefully).

[conradin]

$ ll balls3
----r-S--- 1 h h 0 2012-02-13 06:29 balls3
$ ./createFile775 balls4
$ ll balls4
--wxr-S--- 1 h h 0 2012-02-13 06:30 balls4*
When I omit the mode flags, I get a file with the permissions of balls3.
When I put anything in quotes for the mode I get permissions consistent with balls4

[gateway67]

Check the 'umask' setting in your working environment.


umask

In my working environment it is set to 0002.

Btw, from the code you posted earlier, I got the following to work:


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

int main(int argc, char *argv[])
{
int fd = open(argv[1], O_CREAT, S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);

close(fd);

return 0;
}

The file that is created has permissions 775.

[conradin]

How do i set my umasq? is that in the bashrc?

[conradin]

Thank you everyone! I got it!

#include <sys/types.h> //Specified in man 2 open
#include <sys/stat.h>
#include <stdlib.h>
#include <errno.h> //Allows use of error numbers
#include <fcntl.h> //Specified in man 2 open
#include <stdio.h>

int main(int argc, char *argv[]){
int fd;
fd = open(argv[1], O_CREAT, S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);

return 0;
}