Question

I recently started learning unix and I am trying out some simple programs related to files.
I am attempting to change the access permission of a file through code using the function F_SETFL.
I created the file with only write permission and now I am trying to update the permissions through the code.
But all the permissions are getting reset.

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

int main(void) {

int fd =0;
int fileAttrib=0;

/*Create a new file*/

fd = creat("/u01/TempClass/apue/file/tempfile.txt",S_IWUSR);

fileAttrib = fcntl(fd,F_GETFL,0);
if(fileAttrib < 0 ) {
    perror("An error has occurred");
}

printf("file Attribute is %d \n",fileAttrib);

switch(fileAttrib) {

case O_RDONLY:
    printf("Read only attribute\n");
    break;
case O_WRONLY:
    printf("Write only attribute\n");
    break;
case O_RDWR:
    printf("Read Write Attribute\n");
    break;
default:
    printf("Somethng has gone wrong\n");
}

int accmode = 0;

//accmode = fileAttrib & O_ACCMODE;
accmode = 777;

fileAttrib = fcntl(fd,F_SETFL,accmode);
if(fileAttrib < 0 ) {
    perror("An error has occurred while setting the flags");
}

printf("file Attribute is %d \n",fileAttrib);

/*Print the new access permissions*/

switch(fileAttrib) {

case O_RDONLY:
    printf("New Read only attribute\n");
    break;
case O_WRONLY:
    printf("New Write only attribute\n");
    break;
case O_RDWR:
    printf("New Read Write Attribute\n");
    break;
default:
    printf("New Somethng has gone wrong\n");
}

exit(0);
}

And this is my output

file Attribute is 1

Write only attribute

file Attribute is 0

New Read only attribute

Could someone tell me the right way to set the updated flags.? I referred the documentation but still not quite clear.

Answer 1

You should use chmod or fchmod to change files’ permitions. chmod with argu file’s path and fchmod with fd.

fcntl ‘s flag F_SETFL can only set O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.

F_SETFL (long)
Set the file status flags to the value specified by arg. File access mode (O_RDONLY, O_WRONLY, O_RDWR) and
file creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored. On Linux this command can
only change the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.