I have created a virtual node "/dev/abc" and there are 2 applications a.c and b.c
a.c will write data in to the node,
b.c will read data from the node
In b.c I am opening the node and using select function
to verify whether data is available in the node.
I am using below code for checking the data.
But with out writing data from a.c, b.c is reading the
data.
Code:
fd_set read set;
int result;
fd=open ("/dev/abc", O_RDWR);
FD_SET (fd, &readset);
result=select (fd+1,&readset, NULL, NULL, NULL);
if(result> 0)
{
if (FD_ISSET (fd, &readset))
{
read (fd, buffer, 100);
}
}
Please suggest me how to use select function call for
the above scenario.
Regards,
Ajith Kumsi
You should call FD_ZERO() before FD_SET() to clear your readset. That's probably the reason. There is a clear example near the bottom of select man page and you can just follow it.
Related
I need to read certain statistics from iw_statistics structure, here's the code:
struct net_device *dev;
struct iw_statistics *wi_stats;
dev = first_net_device(&init_net);
while (dev)
{
if (strncmp(dev->name , "wlan",4)==0 )
{
if (dev->wireless_handlers->get_wireless_stats(dev) !=NULL ) // <--- here's where the code crashes.
{
wi_stats = dev-wireless_handlers->get_wireless_stats(dev);
printk(KERN_INFO "wi_stats = dev-wireless_handlers->get_wireless_stats(dev); worked!!! :D\n");
}
}
}
I'm working on linux kernel 2.6.35 and I'm writing a kernel module. What am I doing wrong here?
Looks like wireless_handlers struct is Null ... Just because a net device has it's name field filled doesn't mean it's configured.
This is where wireless_handlers gets set:
#ifdef CONFIG_WIRELESS_EXT
/* List of functions to handle Wireless Extensions (instead of ioctl).
* See <net/iw_handler.h> for details. Jean II */
const struct iw_handler_def * wireless_handlers;
/* Instance data managed by the core of Wireless Extensions. */
struct iw_public_data * wireless_data;
#endif
You should check the value called CONFIG_WIRELESS_EXT if it's not set , the wireless_handler struct is not set and thus you''ll be pointing to a Null and your module will get stuck
You should check that dev->wireless_handlers is not null. Can you paste the actual code snippet? What is the error you get?
I have written a kernel module which reads and writes /proc files, and it is working fine. Now I want to use permissions with it, but when I write the function for permissions shown below it gives me an error. The goal is for everyone to be able to read the file but only root can write to it.
int my_permission(struct inode *inode, int op)
{
if(op == 4||(op == 2 && current->euid = 0)) //euid is not a member of task_struct
return 0;
return -EACCES;
}
const struct inode_operations my_iops = {
.permission = my_permission,
};
The error I'm getting is:
/home/karan/practice/procf/testproc1.c: In function ‘my_permission’:
/home/karan/practice/procf/testproc1.c:50:32: error: ‘struct task_struct’ has no member named ‘euid'
I know that current is #defined to get_current(). Why is this happening? Is there a list of members of the struct returned from get_current()?
The struct task_struct is defined in include/linux/sched.h in the kernel source tree, you can view the members there. The current credentials would be in get_current()->cred , and the effective user id is get_current()->cred->euid
It's not safe to access those members directly, you must rather call current_euid() from include/linux/cred.h
http://www.kernel.org/doc/Documentation/security/credentials.txt might be of interest to you as well
I want to create a .stl file for a particular shape where each face of that shape has a different patch name like face1,face 2 etc. I have done this by overriding the StlAPI_Writer and RWStl classes in opencascade. I have used file.Append method instead of file.Build method to do so.
But I have a problem when I save the .stl file in an already existing file, it appends data to the existing one which is incorrect. I want to delete the existing data in the file and append new data face by face for a given shape.
Please help me on this.
You can use this simple function:
#include <sys/stat.h>
#include <string>
using namespace std;
bool FileExists(string strFilename) {
struct stat stFileInfo;
bool blnReturn;
int intStat;
// Attempt to get the file attributes
intStat = stat(strFilename.c_str(),&stFileInfo);
if(intStat == 0) {
// We were able to get the file attributes
// so the file obviously exists.
blnReturn = true;
} else {
// We were not able to get the file attributes.
// This may mean that we don't have permission to
// access the folder which contains this file. If you
// need to do that level of checking, lookup the
// return values of stat which will give you
// more details on why stat failed.
blnReturn = false;
}
return(blnReturn);
}
I assume you use the SaveFileDialogue class. In this case you can handle the return result of the dialogue like this:
if ( saveFileDialog.ShowDialog() == ::DialogResult::OK ) {
if ( FileExist(saveFileDialog.FileName) ) {
// erase the file
}
// write the code using the Append function
}
This should work, however a easier variant must be accessible if you use something else than Append (something like Write or maybe even Append but with a parameter that specifies to rewrite the file)
HTH, JP
I am trying to create a debugfs file using the debugfs_create_file(...). I have written a sample code for this.
static int __init mmapexample_module_init(void)
{
file1 = debugfs_create_file("mmap_example", 0644, NULL, NULL, &my_fops)\
;
printk(KERN_ALERT "Hello, World\n");
if(file1==NULL)
{
printk(KERN_ALERT "Error occured\n");
}
if(file1==-ENODEV)
{
printk(KERN_ALERT "ENODEV occured\n");
}
return 0;
}
When i ran insmod i could get the Hello, World message but no the error message. So i think the debugfs_create_file worked fine. However i couldn't find any file in /sys/kernel/debug. The folder is there but it is empty. Can anyone help me with this? Thank you...
Thanks,
Bala
For debugfs to work, you actually have to have a debugfs mountpoint:
mount -t debugfs none /sys/kernel/debug
Not sure if that's what the problem is here, but may be you can check if you have a mounted debugfs
on /sys/kernel/debug
You might consider printk(KERN_ALERT "Something else happened\n") in the case that file1 is not equal to NULL or -ENODEV. That may provide some interesting results. Maybe:
if (file1 == NULL)
printk(KERN_ALERT "Error occurred\n");
else if (file1 == -ENODEV)
printk(KERN_ALERT "ENODEV occurred\n");
else
printk(KERN_ALERT "Something else occurred\n");
I'm not familiar with kernel programming libraries as much, but if there is a similar va_args interface to printk(), you could probably print the value of file1.
Now looking at this though, is there some kind of kernel errno? Or is that what debugfs_create_file() returns?
UPDATE:
The only help I can give now is to somehow discover what file1's value is and investigate what that means. You may want to do some poking around for an errno equivalent and see if its set. Equivalent to the perror() call in the kernel basically.
I am downloading a text string from a web service into an RBuf8 using this kind of code (it works..)
void CMyApp::BodyReceivedL( const TDesC8& data ) {
int newLength = iTextBuffer.Length() + data.Length();
if (iTextBuffer.MaxLength() < newLength)
{
iTextBuffer.ReAllocL(newLength);
}
iTextBuffer.Append(data);
}
I want to then convert the RBuf8 into a char* string I can display in a label or whatever.. or for the purposes of debug, display in
RDebug::Printf("downloading text %S", charstring);
edit for clarity..
My conversion function looks like this..
void CMyApp::DownloadCompleteL() {
{
RBuf16 buf;
buf.CreateL(iTextBuffer.Length());
buf.Copy(iTextBuffer);
RDebug::Printf("downloaded text %S", buf);
iTextBuffer.SetLength(0);
iTextBuffer.ReAlloc(0);
}
But this still causes a crash. I am using S60 3rd Edition FP2 v1.1
What you may need is something to the effect of:
RDebug::Print( _L( "downloaded text %S" ), &buf );
This tutorial may help you.
void RBuf16::Copy(const TDesC8&) will take an 8bit descriptor and convert it into a 16bit descriptor.
You should be able to display any 16bit descriptor on the screen. If it doesn't seem to work, post the specific API you're using.
When an API can be used with an undefined number of parameters (like void RDebug::Printf(const char*, ...) ), %S is used for "pointer to 16bit descriptor". Note the uppercase %S.
Thanks, the %S is a helpful reminder.
However, this doesn't seem to work.. my conversion function looks like this..
void CMyApp::DownloadCompleteL() {
{
RBuf16 buf;
buf.CreateL(iTextBuffer.Length());
buf.Copy(iTextBuffer);
RDebug::Printf("downloaded text %S", buf);
iTextBuffer.SetLength(0);
iTextBuffer.ReAlloc(0);
}
But this still causes a crash. I am using S60 3rd Edition FP2 v1.1
You have to supply a pointer to the descriptor in RDebuf::Printf so it should be
RDebug::Print(_L("downloaded text %S"), &buf);
Although use of _L is discouraged. _LIT macro is preferred.
As stated by quickrecipesonsymbainosblogspotcom, you need to pass a pointer to the descriptor.
RDebug::Printf("downloaded text %S", &buf); //note the address-of operator
This works because RBuf8 is derived from TDes8 (and the same with the 16-bit versions).