create a file with open.

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:

Code:

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#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;
}

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You should run this command:

Code:

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man 2 open

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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.

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

Code:

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#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;
}

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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...

Code:

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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.

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

Code:

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$ 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*

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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

Check the 'umask' setting in your working environment.

Code:

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umask

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In my working environment it is set to 0002.

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

Code:

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#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;
}

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The file that is created has permissions 775.

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

Thank you everyone! I got it!

Code:

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#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;
}

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