How to get extended attributes of a file(UNIX/

问题:

When I type ls -l in the command line, sometimes an @ or + symbol comes up alongside the file permissions(btw, I am on OS X), as shown below:

-rw-r-----@  1 john  staff      6731 Sep 28 01:10 mutations.txt
drwxr-xr-x+ 71 john  staff      2414 Mar 25 18:16 ..

I know how to get the permission bits using the stat structure, but I don't think these extended permission values are there. Can someone point me in the right direction as to how to obtain these values via a C or POSIX API?

EDIT:

I attempted the following:

#include <sys/types.h>
#include <sys/xattr.h>
#include <sys/types.h>

int main () {
    char  l[1024];
    listxattr("/Users/john/desktop/mutations.txt", l, 1024,  XATTR_SHOWCOMPRESSION);

    printf("%s\n", l);
}

and got as output:

com.apple.metadata:kMDItemWhereFroms

Still trying to understand how to convert this to an @ or +?

回答1:

@ means the file has extended attributes. Use listxattr() to get a list of the names of all the extended attributes, and getxattr() to get the value of a particular attribute. If listxattr returns a non-zero result, you would display @ to indicate this.

Extended attributes are not in POSIX, but this API is available in Linux and OS X, at least.

You can find an example of how to use these functions here.

+ means the file has an access control list. In some filesystems, this is stored as a special extended attribute; in others it's stored separately. For access control lists, see acl(5) for a reference, and you can find an example program that displays it here.

回答2:

Following is some code I scraped off of the official implementation of ls given by Apple you will find here. The code is long so do CMD + F and search for "printlong".

#include <sys/types.h>
#include <sys/xattr.h>
#include <sys/types.h>
#include <sys/acl.h>
#include <stdio.h>

int main () {
    acl_t acl = NULL;
    acl_entry_t dummy;
    ssize_t xattr = 0;
    char str[10];
    char * filename = "/Users/john/desktop/mutations.txt";

    acl = acl_get_link_np(filename, ACL_TYPE_EXTENDED);
    if (acl && acl_get_entry(acl, ACL_FIRST_ENTRY, &dummy) == -1) {
        acl_free(acl);
        acl = NULL;
    }
    xattr = listxattr(filename, NULL, 0, XATTR_NOFOLLOW);
    if (xattr < 0)
        xattr = 0;

    str[1] = '\0';
    if (xattr > 0)
        str[0] = '@';
    else if (acl != NULL)
        str[0] = '+';
    else
        str[0] = ' ';

    printf("%s\n", str);
 }

Depending on the file used, the output will be a blank, @, or + in exactly the same manner ls -l displays it. Hope this helps !