I have a file called myhj.txt which I hid with using this command from cmd attrib +h +s +r myhj.txt. Now I can't open the file with standart fopen() function in C, so I have decided to use GetFileAttributes() function which does not return an error. According to MSDN, when the function succeeds the return value is File Attribute Constants
I don't know how to get their values as the program crushes when it tries to run printf() down below. Also would it work stable when the file is hidden? How do I access a hidden file with all of its values?
#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
FILE *log;
int main(int argc, char *argv[]) {
if(GetFileAttributes("myhj.txt") == INVALID_FILE_ATTRIBUTES){
printf("invalid get last error %d", GetLastError());
}else{
printf("%s", FILE_ATTRIBUTE_DIRECTORY);
}
}
It crashes because %s expects a null-terminated string but the FILE_ATTRIBUTE_* constants are integers instead, so it ends up trying to read a string from an invalid memory address. Use %d instead:
printf("%d", FILE_ATTRIBUTE_DIRECTORY);
GetFileAttributes() returns a DWORD, which is an unsigned int, so use %u for it:
DWORD att = GetFileAttributes("myhj.txt");
if (att == INVALID_FILE_ATTRIBUTES) {
printf("invalid get last error %d", GetLastError());
}
else {
printf("%u", att);
}
To check for specific values, test each bit separately using the bitwise AND operator (&):
DWORD att = GetFileAttributes("file.ext");
if (att == INVALID_FILE_ATTRIBUTES) {
printf("error!\n");
}
else
{
if (att & FILE_ATTRIBUTE_DIRECTORY) printf("directory\n");
if (att & FILE_ATTRIBUTE_HIDDEN) printf("hidden\n");
...
}
If you set the read-only attribute on a file, you cannot perform destructive operations on it...
Related
(https://learn.microsoft.com/en-us/windows/win32/api/iphlpapi/nf-iphlpapi-getadaptersaddresses)
Why are some of the user friendly names in PIP_ADAPTER_ADDRESSES unprintable? (aswell as a few other attributes such as dns suffix)
By unprintable, I mean containing non-printable characters. for exmaple, the first character in one of the friendly names I tested had a unicode value fo 8207 (decimal)
A minimal complete viable example
#include <winsock2.h>
#include <iphlpapi.h>
#include <vector>
#include <iostream>
int main()
{
PIP_ADAPTER_ADDRESSES adapterAddresses;
DWORD dwReqSize;
DWORD retVal;
DWORD count = 0;
std::string tempForWstringConv;
retVal = GetAdaptersAddresses(AF_INET, GAA_FLAG_INCLUDE_PREFIX, NULL, NULL, &dwReqSize); // for knowing the required size
if (retVal != ERROR_BUFFER_OVERFLOW) {
return -1;
}
adapterAddresses = (PIP_ADAPTER_ADDRESSES)malloc(dwReqSize);
retVal = GetAdaptersAddresses(AF_INET, GAA_FLAG_INCLUDE_PREFIX, NULL, adapterAddresses, &dwReqSize); // this time actually getting the desired content
if (retVal != ERROR_SUCCESS) {
return -1;
}
for (PIP_ADAPTER_ADDRESSES adapter = adapterAddresses; adapter != NULL; adapter = adapter->Next)
{
//outLog.push_back(Adapter());
printf("\tFriendly name: %ls\n", adapter->FriendlyName);
}
return 0;
}
I finally found A solution!
meet _setmode(_fileno(stdout), _O_U16TEXT);
the problem was that the output buffer wasn't allowing these characters because the mode was incorrect. Alas, our desired output:
inorder to use this you MUST A: switch all occurences of cout to wcou; B: switch all occurences of printf to wprintf. C: include and
I have a program that I want to use to read a file and output its last N characters (could be 50 or whatever that I have coded). From my piece of code, I get output that is question marks in diamond boxes,(unsupported unicode?)
I'm using lseek to set the cursor, could someone please assist me?
int main(int argc,char *argv[]){
int fd; //file descriptor to hold open info
int count=0; //to hold value of last 200th char number
char ch; //holds read char
char* outputString = "The file does not exist!\n";
if(!access("myFile.txt",F_OK)==0){
write(2,outputString,strlen(outputString));
exit(1);
}
fd = open("myFile.txt",O_RDONLY| O_NONBLOCK);
int ret = lseek(fd,200,SEEK_END); //get position of the last 200th item
while (ret!=0) {
write(1, &ch,1);
ret--;
}
close(fd);
return(0);
}
I don't want to use <stdio.h> functions so I'm using the file descriptors not making a FILE* object.
I slightly modified your attempt. The lseek(fd, 200, SEEK_END) seeks the file 200 characters past the end of file. If you want to read last 200 character from a file, you need to seek to 200 character to end of file, ie lseek(fd, -200, SEEK_END).
I places some comments in code to help explaining.
// please include headers when posting questions on stackoverflow
// It makes it way easier to reproduce and play with the code from others
#include <unistd.h>
#include <error.h>
// I use glibc error(3) to handle errors
#include <errno.h>
#include <stdlib.h>
#include <fcntl.h>
int main(int argc,char *argv[]){
// no idea if a typo, myFile.txt != logfile.txt
if(!access("myFile.txt", F_OK) == 0) {
error(1, errno, "The file does not exist!");
exit(1);
}
int fd = open("logfile.txt", O_RDONLY | O_NONBLOCK);
if (fd == -1) {
error(1, errno, "Failed opening the file");
}
// move cursor position to the 200th characters from the end
int ret = lseek(fd, -200, SEEK_END);
if (ret == -1) {
error(1, errno, "Failed seeking the file");
}
// we break below
while (1) {
char ch = 0; // holds read char
ssize_t readed = read(fd, &ch, sizeof(ch));
if (readed == 0) {
// end-of-file, break
break;
} else if (readed == -1) {
// error handle
// actually we could handle `readed != 1`
error(1, errno, "Error reading from file");
}
// output the readed character on stdout
// note that `STDOUT_FILENO` as more readable alternative to plain `1`
write(STDOUT_FILENO, &ch, sizeof(ch));
}
close(fd);
return 0;
}
"gethostbyname" returns a pointer to this structure:
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses from name server */
};
When I try to use it, h_name points to a valid string: the partial name I supply is expanded to the correct fully qualified host name.
The value of h_addr_list is 4
h_name is valid
h_aliasis is a valid pointer to a null pointer
h_addrtype is 2 (AF_INET, IPV4)
h_length is 0 (should be 4, or perhaps a multiple of 4)
h_addr_list is 4, fails when dereferenced.
I'm running a 32 bit process (MS Office), the h_name pointer is a valid 32 bit pointer. WTF am I doing wrong? Does gethostbyname work for other people, or on other versions of OSX?
I was able to run this small example successfully on 10.10.4 (taken from paulschreiber.com)
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>
int main(int argc, char **argv) {
if (argc < 2) {
printf("Usage: %s hostname", argv[0]);
exit(-1);
}
struct hostent *hp = gethostbyname(argv[1]);
if (hp == NULL) {
printf("gethostbyname() failed\n");
} else {
printf("%s = ", hp->h_name);
unsigned int i=0;
while ( hp -> h_addr_list[i] != NULL) {
printf( "%s ", inet_ntoa( *( struct in_addr*)( hp -> h_addr_list[i])));
i++;
}
printf("\n");
}
}
However, it did segfault on 64-bit without #include <arpa/inet.h: without that, no prototype for inet_ntoa is found, the return type is assumed to be an int (when it's actually a char *), and on 64-bit this truncates the pointer and causes a segfault.
I am trying to get the VolumeDetails of my WINDOWS system- Drive label plus its respective Volume Serial number. I've tried since an hour and built a code which gone wrong in syntax. At present I am getting the following error with it-
error C2664: 'GetVolumeInformationW' : cannot convert parameter 1 from 'char []' to 'LPCWSTR'
Here is my code:
// getVolDrive.cpp : Defines the entry point for the console application.
#include "stdafx.h"
#include <iostream>
#include <windows.h>
#include <direct.h>
#include <stdio.h>
#include <conio.h>
#include <tchar.h>
#include <sstream>
#include <string>
#include <ctype.h>
#include <algorithm>
using namespace std;
//wchar_t mydrives[5];// = " A: ";
char mydrives[] = " A: ";
string retVolSno(char drives[]) //wchar_t drives[]
{
DWORD dwSerial;
stringstream ss;
cout<<drives<<endl;
if(!GetVolumeInformation(drives, NULL, 0, &dwSerial, NULL, NULL, NULL, 0))
{
ss<<"Error: "<<GetLastError();
}
else
{
ss<<hex<<dwSerial;
}
return ss.str();
}
int _tmain(int argc, _TCHAR* argv[])
{
string cVolSno;
ULONG DriveMask = _getdrives();
if(DriveMask == 0)
printf("_getdrives() failed with failure code: %d\n", GetLastError());
else
{
printf("This machine has the following logical drives:\n");
while (DriveMask)
{
cout << "In While" << endl;
if(DriveMask & 1)
printf("%s", mydrives);
wcout << mydrives << endl;
cVolSno = retVolSno(mydrives);
cout<<cVolSno<<endl;
++mydrives[1];
DriveMask >>= 1;
}
}
//std::transform(cVolSno.begin(), cVolSno.end(),cVolSno.begin(), ::toupper);
//cout<<cVolSno<<endl;
_getch();
return 0;
}
I've also tried replacing char with wchar_t, I didn't got any build errors, but while executing the application, got Error Code 3- Path not found!.
CODE MODIFIED:
// getVolDrive.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <windows.h>
#include <direct.h>
#include <stdio.h>
#include <conio.h>
#include <tchar.h>
#include <sstream>
#include <string>
#include <ctype.h>
#include <algorithm>
using namespace std;
//wchar_t mydrives[5];// = " A: ";
char mydrives[] = " A:\\\\ ";
string retVolSno(char drives[]) //wchar_t drives[]
{
DWORD dwSerial;
stringstream ss;
wchar_t text[10];
mbstowcs(text,drives,100); //strlen(drives)+1
LPWSTR ptr = text;
if(!GetVolumeInformation(ptr, NULL, 0, &dwSerial, NULL, NULL, NULL, 0))
{
ss<<"Error: "<<GetLastError();
}
else
{
ss<<hex<<dwSerial;
}
return ss.str();
}
int _tmain(int argc, _TCHAR* argv[])
{
string cVolSno;
ULONG DriveMask = _getdrives();
if(DriveMask == 0)
printf("_getdrives() failed with failure code: %d\n", GetLastError());
else
{
printf("This machine has the following logical drives:\n");
while (DriveMask)
{
if(DriveMask & 1)
printf("%s \n", mydrives);
cVolSno = retVolSno(mydrives);
std::transform(cVolSno.begin(), cVolSno.end(),cVolSno.begin(), ::toupper);
cout<<cVolSno<<endl;
++mydrives[1];
DriveMask >>= 1;
}
}
//std::transform(cVolSno.begin(), cVolSno.end(),cVolSno.begin(), ::toupper);
//cout<<cVolSno<<endl;
_getch();
return 0;
}
OUTPUT:
This machine has the following logical drives:
ERROR: 123
ERROR: 123
C:\\
ERROR: 123
D:\\
ERROR: 123
E:\\
ERROR: 123
I see at least these main issues:
1) wchar_t is the right type because you're compiling for UNICODE, you can write generic code using TCHAR macro or explicitly declare your buffer as wchar_t but that's what to do.
2) You have that error because you're passing wrong path to GetVolumeInformation() (trailing backslash is required so A: must become A:\).
Moreover please note that you have a little bit more easy way to achieve same result, you can use GetLogicalDriveStrings() to directly get a NULL delimited string list. Split it using, for example, this (don't forget UNICODE) and use c_str() with each entry.
EDIT about your modified code:
Why you drive path is A:\\ (escaped to A:\\\\)? Just one trailing backslash is needed so mydrives has to be declared as:
wchar_t mydrives[] = L"A:\\";
EDIT 2: there are more errors in your code so I'll post a reviewed version. There are more things I'd change but I'll point out just what doesn't actually work.
Function retVolSno to read volume serial number. Original version were almost right, in your modified version you perform useless character conversion. What you had to do was just to accept a wchar_t drive path.
Global variable mydrives. You actually don't need any global variable for that. It must be wchar_t and space before/after path are useless. One trailing backslash is needed. Line where you increment character value (++mydrives[0];) must be changed accordingly (index 0 instead of 1).
Check for drive availability. After if(DriveMask & 1) you did forget { then you won't print drive name but you'll perform GetVolumeInformation() even on unavailable drives (error 123). That's why indentation is important...
You're mixing UNICODE/NOT UNICODE and C/C++ stuff. I strongly suggest you pick one of them and you keep it (C or C++? UNICODE or NOT UNICODE?). For example you used C function printf() to print stuff and you have both std::string and wchar_t things.
Let's put everything together to have a working version. First the function to read serial number given drive path:
wstring getVolumeSerialNumber(const wchar_t* drivePath)
{
DWORD dwSerial;
wstringstream ss;
if (!GetVolumeInformation(drivePath, NULL, 0, &dwSerial, NULL, NULL, NULL, 0))
ss << L"Error: " << GetLastError();
else
ss << hex << dwSerial;
return ss.str();
}
It's almost the same as your original version, just changed to work with UNICODE characters. Then main function that cycles through available drives and print out their serial number:
int _tmain(int argc, _TCHAR* argv[])
{
wchar_t drive[] = L"A:\\";
ULONG driveMask = _getdrives();
if (driveMask == 0)
wcout << L"_getdrives() failed with failure code: " << GetLastError() << endl;
else
{
wcout << L"This machine has the following logical drives:" << endl;
while (driveMask)
{
if (driveMask & 1)
{
wcout << drive << endl;
wcout << getVolumeSerialNumber(drive) << endl;
}
++drive[0];
driveMask >>= 1;
}
}
wcin.ignore();
return 0;
}
From the documentation , the first parameters should be with trailing slash if drive letter is passed.
lpRootPathName [in, optional]
A pointer to a string that contains the root directory of the volume to be described.
If this parameter is NULL, the root of the current directory is used.
A trailing backslash is required.
For example, you specify \\MyServer\MyShare as \\MyServer\MyShare\, or the C drive as C:\
I have two questions as I'm trying device drivers as a beginner.
I created one module , loaded it, it dynamically took major number 251 say. Number of minor devices is kept 1 only i.e minor number 0. For testing , I tried echo and cat on the device file (created using mknod) and it works as expected. Now if I unload the module but don't remove /dev entry and again load the module with same major number and try writing/reading to same device file which was used previously, kernel crashes. I know we shouldn't do this but just want to understand what happens in this scenario which causes this crash. I think something that VFS does.
When I do cat on device file, the read keeps on happening indefinitely. why? To stop that needed to use offset manipulation. This looks to be because buffer length is coming as 32768 as default to read?
EDIT: further in this I added one ioctl function as below, then I'm getting error regarding the storage class of init and cleanup function, which work well if no ioctl is defined. Not getting the link between ioctl and the init/cleanup functions' storage class. Updated code is posted. Errors are below:
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:95:12: error: invalid storage class for function ‘flow_init’
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c: In function ‘flow_init’:
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:98:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c: In function ‘flow_ioctl’:
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:112:13: error: invalid storage class for function ‘flow_terminate’
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:119:1: error: invalid storage class for function ‘__inittest’
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:119:1: warning: ‘alias’ attribute ignored [-Wattributes]
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:120:1: error: invalid storage class for function ‘__exittest’
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:120:1: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:120:1: warning: ‘alias’ attribute ignored [-Wattributes]
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:120:1: error: expected declaration or statement at end of input
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c: At top level:
/home/diwakar/Documents/my_modules/first_test_module/flowTest.c:73:13: warning: ‘flow_ioctl’ defined but not used [-Wunused-function]
Below is the code:
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <linux/cdev.h>
#include <linux/kdev_t.h>
#include <linux/errno.h>
#include <linux/ioctl.h>
#define SUCCESS 0
#define BUF_LEN 80
#define FLOWTEST_MAGIC 'f'
#define FLOW_QUERY _IOR(FLOWTEST_MAGIC,1,int)
MODULE_LICENSE("GPL");
int minor_num=0,i;
int num_devices=1;
int fopen=0,counter=0,ioctl_test;
static struct cdev ms_flow_cd;
static char c;
///// Open , close and rest of the things
static int flow_open(struct inode *f_inode, struct file *f_file)
{
printk(KERN_ALERT "flowtest device: OPEN\n");
return SUCCESS;
}
static ssize_t flow_read(struct file *f_file, char __user *buf, size_t
len, loff_t *off)
{
printk(KERN_INFO "flowtest Driver: READ()\nlength len=%d, Offset = %d\n",len,*off);
/* Check to avoid the infinitely printing on screen. Return 1 on first read, and 0 on subsequent read */
if(*off==1)
return 0;
printk(KERN_INFO "Copying...\n");
copy_to_user(buf,&c,1);
printk(KERN_INFO "Copied : %s\n",buf);
*off = *off+1;
return 1; // Return 1 on first read
}
static ssize_t flow_write(struct file *f_file, const char __user *buf,
size_t len, loff_t *off)
{
printk(KERN_INFO "flowtest Driver: WRITE()\n");
if (copy_from_user(&c,buf+len-2,1) != 0)
return -EFAULT;
else
{
printk(KERN_INFO "Length len = %d\n\nLast character written is - %c\n",len,*(buf+len-2));
return len;
}
}
static int flow_close(struct inode *i, struct file *f)
{
printk(KERN_INFO "ms_tty Device: CLOSE()\n");
return 0;
}
///* ioctl commands *///
static long flow_ioctl (struct file *filp,unsigned int cmd, unsigned long arg)
{
switch(cmd) {
case FLOW_QUERY:
ioctl_test=51;
return ioctl_test;
default:
return -ENOTTY;
}
///////////////////File operations structure below/////////////////////////
struct file_operations flow_fops = {
.owner = THIS_MODULE,
.llseek = NULL,
.read = flow_read,
.write = flow_write,
.unlocked_ioctl = flow_ioctl,
.open = flow_open,
.release = flow_close
};
static int flow_init(void)
{
printk(KERN_ALERT "Here with flowTest module ... loading...\n");
int result=0;
dev_t dev=0;
result = alloc_chrdev_region(&dev, minor_num,
num_devices,"mod_flowtest"); // allocate major number dynamically.
i=MAJOR(dev);
printk(KERN_ALERT "Major allocated = %d",i);
cdev_init(&ms_flow_cd,&flow_fops);
cdev_add(&ms_flow_cd,dev,1);
return 0;
}
static void flow_terminate(void)
{
dev_t devno=MKDEV(i,0); // wrap major/minor numbers in a dev_t structure , to pass for deassigning.
printk(KERN_ALERT "Going out... exiting...\n");
unregister_chrdev_region(devno,num_devices); //remove entry from the /proc/devices
}
module_init(flow_init);
module_exit(flow_terminate);
1- You're missing cdev_del() in your cleanup function. Which means the device stays registered, but the functions to handle it are unloaded, thus the crash. Also, cdev_add probably fails on the next load, but you don't know because you're not checking return values.
2- It looks ok... you modify offset, return the correct number of bytes, and then return 0 if offset is 1, which indicates EOF. But you should really check for *off >= 1.
EDIT-
The length passed into your read handler function comes all the way from user-land read(). If the user opens the device file and calls read(fd, buf, 32768);, that just means the user wants to read up to 32768 bytes of data. That length gets passed all the way to your read handler. If you don't have 32768 bytes of data to supply, you supply what you have, and return the length. Now, the user code isn't sure if that's the end of the file or not, so it tries for another 32768 read. You really have no data now, so you return 0, which tells the user code that it has hit EOF, so it stops.
In summary, what you're seeing as some sort of default value at the read handler is just the block size that the utility cat uses to read anything. If you want to see a different number show up at your read function, try using dd instead, since it lets you specify the block size.
dd if=/dev/flowtest of=/dev/null bs=512 count=1
In addition, this should read one block and stop, since you're specifying count=1. If you omit count=1, it will look more like cat, and try to read until EOF.
For 2, make sure you start your module as a char device when using mknod.
mknod /dev/you_device c major_number minor_number