How to Set File Permissions (Cross Platform) in C++

How to set file permissions (cross platform) in C++?

Ironically, I have just run into this very same need earlier today.

In my case, the answer came down to what level of permission granularity I need on Windows, versus Linux. In my case, I only care about User, Group, and Other permission on Linux. On Windows, the basic Read/Write All permission leftover from DOS is good enough for me, i.e. I don't need to deal with ACL on Windows.

Generally speaking, Windows has two privilege models: the basic DOS model and the newer access control model. Under the DOS model there is one type of privilege: write privilege. All files can be read, so there is no way to turn off read permission (because it doesn't exist). There is also no concept of execute permission. If a file can be read (answer is yes) and it is binary, then it can be executed; otherwise it can't.

The basic DOS model is sufficient for most Windows environments, i.e. environments where the system is used by a single user in a physical location that can be considered relatively secure. The access control model is more complex by several orders of magnitude.

The access control model uses access control lists (ACL) to grant privileges. Privileges can only be granted by a process with the necessary privileges. This model not only allows the control of User, Group, and Other with Read, Write, and Execute permission, but it also allows control of files over the network and between Windows domains. (You can also get this level of insanity on Unix systems with PAM.)

Note: The Access Control model is only available on NTFS partitions, if you are using FAT partitions you are SOL.

Using ACL is a big pain in the ass. It is not a trivial undertaking and it will require you to learn not just ACL but also all about Security Descriptors, Access Tokens, and a whole lot of other advanced Windows security concepts.

Fortunately for me, for my current needs, I don't need the true security that the access control model provides. I can get by with basically pretending to set permissions on Windows, as long as I really set permissions on Linux.

Windows supports what they call an "ISO C++ conformant" version of chmod(2). This API is called _chmod, and it is similar to chmod(2), but more limited and not type or name compatible (of course). Windows also has a deprecated chmod, so you can't simply add chmod to Windows and use the straight chmod(2) on Linux.

I wrote the following:

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

#ifdef _WIN32
#   include <io.h>

typedef int mode_t;

/// @Note If STRICT_UGO_PERMISSIONS is not defined, then setting Read for any
///       of User, Group, or Other will set Read for User and setting Write
///       will set Write for User.  Otherwise, Read and Write for Group and
///       Other are ignored.
///
/// @Note For the POSIX modes that do not have a Windows equivalent, the modes
///       defined here use the POSIX values left shifted 16 bits.

static const mode_t S_ISUID      = 0x08000000;           ///< does nothing
static const mode_t S_ISGID      = 0x04000000;           ///< does nothing
static const mode_t S_ISVTX      = 0x02000000;           ///< does nothing
static const mode_t S_IRUSR      = mode_t(_S_IREAD);     ///< read by user
static const mode_t S_IWUSR      = mode_t(_S_IWRITE);    ///< write by user
static const mode_t S_IXUSR      = 0x00400000;           ///< does nothing
#   ifndef STRICT_UGO_PERMISSIONS
static const mode_t S_IRGRP      = mode_t(_S_IREAD);     ///< read by *USER*
static const mode_t S_IWGRP      = mode_t(_S_IWRITE);    ///< write by *USER*
static const mode_t S_IXGRP      = 0x00080000;           ///< does nothing
static const mode_t S_IROTH      = mode_t(_S_IREAD);     ///< read by *USER*
static const mode_t S_IWOTH      = mode_t(_S_IWRITE);    ///< write by *USER*
static const mode_t S_IXOTH      = 0x00010000;           ///< does nothing
#   else
static const mode_t S_IRGRP      = 0x00200000;           ///< does nothing
static const mode_t S_IWGRP      = 0x00100000;           ///< does nothing
static const mode_t S_IXGRP      = 0x00080000;           ///< does nothing
static const mode_t S_IROTH      = 0x00040000;           ///< does nothing
static const mode_t S_IWOTH      = 0x00020000;           ///< does nothing
static const mode_t S_IXOTH      = 0x00010000;           ///< does nothing
#   endif
static const mode_t MS_MODE_MASK = 0x0000ffff;           ///< low word

static inline int my_chmod(const char * path, mode_t mode)
{
    int result = _chmod(path, (mode & MS_MODE_MASK));

    if (result != 0)
    {
        result = errno;
    }

    return (result);
}
#else
static inline int my_chmod(const char * path, mode_t mode)
{
    int result = chmod(path, mode);

    if (result != 0)
    {
        result = errno;
    }

    return (result);
}
#endif

It's important to remember that my solution only provides DOS type security. This is also known as no security, but it is the amount of security that most apps give you on Windows.

Also, under my solution, if you don't define STRICT_UGO_PERMISSIONS, when you give a permission to group or other (or remove it for that matter), you are really changing the owner. If you didn't want to do that, but you still didn't need full Windows ACL permissions, just define STRICT_UGO_PERMISSIONS.

Cross-platform method of creating a folder/directory?

You'll need a #define to do this.

To keep your code looking clean, you'll want use one that will define the Linux function to translate to the equivalent Windows function when compiling for Windows.

At the top of your source file, you'll have this in a Windows specific section:

#include <windows.h>

#define mkdir(dir, mode) _mkdir(dir)

Then later on you can call the function like this:

mkdir("/tmp/mydir", 0755);

Here are some others that might be useful:

#define open(name, ...) _open(name, __VA_ARGS__)
#define read(fd, buf, count) _read(fd, buf, count)
#define close(fd) _close(fd)
#define write(fd, buf, count) _write(fd, buf, count)
#define dup2(fd1, fd2) _dup2(fd1, fd2)
#define unlink(file) _unlink(file)
#define rmdir(dir) _rmdir(dir)
#define getpid() _getpid()
#define usleep(t) Sleep((t)/1000)
#define sleep(t) Sleep((t)*1000)

Set folder permission using plain c++, or java

Function presented in MSDN article "Modifying the ACLs of an Object in C++" does the job. GetNamedSecurityInfo retrieves discretionary access control list (DACL) for the object (directory in your case). SetEntriesInAcl creates new access control list (ACL) by merging new entries (including permissions) with existing ones. SetNamedSecurityInfo assigns modified DACL back to the object.

Regarding that Modify permission is a combination of following rights: FILE_GENERIC_READ | FILE_GENERIC_WRITE | FILE_GENERIC_EXECUTE | DELETE you can call this function like here:

std::string strFullPath("C:\test");

DWORD dwRes = AddAceToObjectsSecurityDescriptor(
    const_cast<LPTSTR>(strFullPath.c_str()),
    SE_FILE_OBJECT,
    "StandardUser",
    TRUSTEE_IS_NAME,
    FILE_GENERIC_READ | FILE_GENERIC_WRITE | FILE_GENERIC_EXECUTE | DELETE,
    GRANT_ACCESS,
    NO_INHERITANCE);

Access files and folders of Internal Computer drives in CrossPlatform UWP

Figured it out, I had to remove the other storage related capabilities.

<uap:Capability Name="musicLibrary"/>
<uap:Capability Name="removableStorage"/>
<uap:Capability Name="picturesLibrary"/>
<uap:Capability Name="videosLibrary"/>
<uap:Capability Name="documentsLibrary"/>

After that I built, stopped it, unchecked and rechecked the file permissions in system settings than built and it worked.

Fixing file permissions after modifying in C++?

If you want to avoid using system(), you can use

#include <sys/stat.h>
int chmod(const char *path, mode_t mode);

It is documented in http://linux.die.net/man/3/chmod.

See also: C++ - How to set file permissions (cross platform).

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