Is there a way in C++'s standard libraries (or linux sys/stat.h, sys/types.h, sys/.... libraries) to set the file permissions of a file when creating it with ofstream (or using something else from those libraries)?
When I create a file it just gets created with some default set of file permissions (I assume whatever the current umask is), but I want to explicitly set the permissions to something other than the default (ex. 600), and I can't just set the umask before starting the program (b/c others will run it).
// Example of creating a file by writing to it
ofstream fp(filename.c_str())
/* write something to it */
Is there a way to do this in C++ or if not, a way to set the umask within the C++ program?
For example, in C's standard library you can just do:
open(filename, O_RDWR|O_CREAT, 0666)
but I don't want to resort to using the C function, as it'd be nice to be able to use the functions associated with fstream objects.
(Side Note: there was a question whose title was exactly what I was looking for, but it turned out to be unrelated.)
You cannot. The standard library must be platform agnostic, and permissions like 0600 are meaningless on Windows for example. If you want to use platform-specific features, you'll need to use platform-specific APIs. Of course, you can always call umask() before you open the file, but that's not part of the C++ standard library, it's part of the platform API.
Note: open() isn't part of the C standard library either. It's a platform API. The C standard library function to open a file is fopen().
In C++17, std::filesystem::permissions was introduced. It will enable you to get and set permissions in a platform-agnostic manner.
Get permissions:
using fs = std::filesystem;
bool owner_can_read =
fs::status("my_file.txt").permissions() & fs::perms::owner_read != fs::perms::none;
Set permissions (add all permissions for owner and group, that is, add modes 0x770 on unix):
using fs = std::filesystem;
fs::permissions("my_file.txt",
fs::perms::owner_all | fs::perms::group_all,
fs::perm_options::add);
Example based on an example from cppreference.
I'm writting a C++ program in order to make some static analyse and modifications over a website. I don't change the project files, but the files are copied, analysed and modified in a new folder.
Is there a way of checking, for example, using boost::filesystem, if I (the program/the user executing it) have permissions enough to read (files), execute (for processing the directory hierarchy) and write (files or create new folders) on a current folder and its files (or at least under unix systems)?
No, you can not check file permissions with boost program options library. File permissions can be queried with boost filesystem library:
#include <boost/filesystem.hpp>
#include <stdio.h>
namespace bfs=boost::filesystem;
int main(int argc,char * argv[])
{
if (argc < 2)
return;
bfs::path p(argv[1]);
bfs::file_status s = status(p);
printf("%X\n",s.permissions());
}
The values of permissions flags are as enum perms in
boost/filesystem/v3/operations.hpp
IF you're using linux you can use stat function for get info about the file, including the mode permissions, the owner's ID and the owner's group ID of such file.
Then you can use getuid to compare the permissions of the user running the program with those obtained with stat.
Both links (that are actually man pages) has examples on how to use these functions.
Also you can try to do some operation over the selected file, lets say write, and then handle the exception properly. You don't have to access the file, just try to open it for writting, if you get and exception and errno is equal to EACCESS(permission denied) well, you don't have shuch permissions.
I'm saving my data in the executable file of the program. I copy it to a temporary file, overwrite a part starting at a 'magic string' and rename it to the original. I know this is a bad idea, but I'm doing it just for experimenting.
I got everything to work so far, except for that I have to re-enable "Allow running as an executable" each time the file is replaced. What ways are there to solve this?
Additional information: I use linux.
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).
If you include stdlib.h, you can use system("command").
Try it:
system("chmod 755 yourExeFile")
How can I programmatically lock/unlock, or otherwise prevent/enable editing, a source file on Linux using C++.
I want to be able to lock source file so that if I open it in an editor it will not allow me to save back to the same source file.
I am thinking of maybe changing the permissions to read-only (and change it back to read-write later): how do I do that from C++?
Try man fchmod:
NAME
chmod, fchmod - change permissions of a file
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
int fchmod(int fildes, mode_t mode);
Why aren't you using a source code management tool like CVS or Subversion? CVS does nice locking (so does Subversion). More importantly, you have the history of changes. Better still (with CVS anyway) you have to make the step of doing a "checkout" to make the file writeable.
Yes, it is a bit hard to tell what you are looking for
Security against other users editing you files -> use "chmod, fchmod"
Security against you yourself accidentally messing with your source files -> you should really change your thinking and use a source control tool. Like Subversion (SVN) or even better Mercurial.
I am using C++ ofstream to write out a file. I want to set the permissions to be only accessible by the user: 700. In unix; I suppose I can just issue a system("chmod 700 file.txt"); but I need this code to work on Windows as well. I can use some Windows api; but what is the best c++ cross platform way to do this?
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.
There is no cross-platform way to do this. Windows does not support Unix-style file permissions. In order to do what you want, you'll have to look into creating an access control list for that file, which will let you explicitly define access permissions for users and groups.
An alternative might be to create the file in a directory whose security settings have already been set to exclude everyone but the user.
Cross-platform example to set 0700 for a file with C++17 and its std::filesystem.
#include <exception>
//#include <filesystem>
#include <experimental/filesystem> // Use this for most compilers as of yet.
//namespace fs = std::filesystem;
namespace fs = std::experimental::filesystem; // Use this for most compilers as of yet.
int main()
{
fs::path myFile = "path/to/file.ext";
try {
fs::permissions(myFile, fs::perms::owner_all); // Uses fs::perm_options::replace.
}
catch (std::exception& e) {
// Handle exception or use another overload of fs::permissions()
// with std::error_code.
}
}
See std::filesystem::permissions, std::filesystem::perms and std::filesystem::perm_options.
The system() call is a strange beast. I have been bitten by a NOP system() implementation on a Mac many moons ago. It's implementation defined meaning the standard doesn't define what an implementation (platform/compiler) is supposed to do. Unfortunately, this is also about the only standard way of doing something outside the scope of your function (in your case -- changing the permissions).
Update: A proposed hack:
Create a non-empty file with appropriate permissions on your system.
Use Boost Filesystem's copy_file to copy this file out to your desired output.
void copy_file(const path& frompath, const path& topath): The contents and attributes of the file referred to by frompath is copied to the file referred to by topath. This routine expects a destination file to be absent; if the destination file is present, it throws an exception. This, therefore, is not equivalent to the system specified cp command in UNIX. It is also expected that the frompath variable would refer to a proper regular file. Consider this example: frompath refers to a symbolic link /tmp/file1, which in turn refers to a file /tmp/file2; topath is, say, /tmp/file3. In this situation, copy_file will fail. This is yet another difference that this API sports compared to the cp command.
Now, overwrite the output with actual contents.
But, this is only a hack I thought of long after midnight. Take it with a pinch of salt and try this out :)
No idea if it would work, but you could look into using the chmod.exe executable that comes with Cygwin.
There's no standard way to do this in C++, but for this special requirement you should probably just write a custom wrapper, with #ifdef _WIN32. Qt has a permission wrapper in it's QFile class, but this would of course mean depending on Qt ...
I just found a couple of ways to do chmod 700 easily from the Windows command line. I'm going to post another question asking for help coming up with an equivalent win32 security descriptor structure to use (if I can't figure it out in the next few hours).
Windows 2000 & XP (messy- it always seems to prompt):
echo Y|cacls *onlyme.txt* /g %username%:F
Windows 2003+:
icacls *onlyme.txt* /inheritance:r /grant %username%:r
EDIT:
If you had the ability to use the ATL, this article covers it (I don't have Visual Studio available):
http://www.codeproject.com/KB/winsdk/accessctrl2.aspx
Actually, it says the sample code includes non-ATL sample code as well- it should have something that works for you (and me!)
The important thing to remember is to get r/w/x for owner only on win32, you just have to wipe all of the security descriptors from the file and add one for yourself with full control.
You can't do it in a cross-platform manner. In Linux, you should use the function chmod(2) instead of using system(2) to spawn a new shell. On Windows, you'll have to use the various authorization functions to make an ACL (access-control list) with the proper permissions.