close(int fd) not recognized in QT both fcntl.h and unistd.h are included

vicky_mac

hi i am doing some memory read and write using mmap and open function in my application.

But when i am going for closing the file descriptor close() functions is not recognized and program densest compile.

#include"mainwindow.h"
#include<unistd.h>
#include<fcntl.h>

MainWindow::ReadMem()
{
int fd = open("/dev/mem",O_RDWR|O_SYNC);
int* add =(int*)mmap(NULL,2048,PROT_READ | PROT_WRITE,MAP_SHARED,dh,0);
//now some operation as needed
munmap(add,2048);
close(fd);/// this line not compiling
}

// Nomatching function to call function close```
code_text

KroMignon

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

Hi jsulm,
Sorry to bother again but i don't know which namespace to use in this case i tried std:: that also doesn't work.
Can you please suggest which namespace to be used. Thank you

In C++, there are namespaces to prevent name conflicts (cf. https://en.cppreference.com/w/cpp/language/namespace).
When you are writing code in a C++ class, the default namespace is the current class.
So when you are calling a close() function, the compiler first try to find this function in the current namespace.

To avoid this, you have to specify full namespace. In case of "standard functions" like close(int), the namespace is empty. So the fullname of standard open(int), in C++ is ::open().

This are C++ basics, it is always helpful to learn the basics ;)

KroMignon

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

hi i am doing some memory read and write using mmap and open function in my application.
But when i am going for closing the file descriptor close() functions is not recognized and program densest compile.

You have to specify the namespace :: to use standard open() function:

int fd = ::open("/dev/mem",O_RDWR|O_SYNC);

EDIT, of course the same is also for close() and all standard functions, to ensure using std function and not same called function is current name space:

int fd = ::open("/dev/mem",O_RDWR|O_SYNC);
int* add =(int*)::mmap(NULL,2048,PROT_READ | PROT_WRITE,MAP_SHARED,dh,0);
//now some operation as needed
::munmap(add,2048);
::close(fd);

JonB

@KroMignon
The OP claims his (only) error is on close().

@vicky_mac
Depending (C, C++), either what have you #included or do you need ::close()?

jsulm

@vicky_mac

man 2 close

and include the header file mentioned there.

KroMignon

@jsulm said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

and include the header file mentioned there.

According to thread title, unistd.h has been included, so it should not be the problem.

jsulm

@KroMignon Yeah, careful reading is something I should do more often :-)

JonB

@KroMignon
I had not noticed that information in the title. (Was it added later?! :) )

@vicky_mac
What you do not say is where --- inside which class --- your code is located. You are likely to need ::close() as @KroMignon has said, did you at least try it?

vicky_mac

@JonB hi Joh
I have edited my question with include and function detail. Please advise what exactly needs to be done.

JonB

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

Please advise what exactly needs to be done.

@JonB said:

You are likely to need ::close() as @KroMignon has said, did you at least try it?

Your plain close() will be MainWinow::close().

vicky_mac

@JonB
Hi Jon,

I am getting the same error even after MainWindow::close(fd)

jsulm

@vicky_mac @JonB did not say you need to use MainWindow::close(fd)! What he means is that your MainWindow probably has its own close() method nd if you write close(...) that one is used.
Why don't you simply try what @KroMignon suggested?!

vicky_mac

@jsulm

Hi jsulm,

Sorry to bother again but i don't know which namespace to use in this case i tried std:: that also doesn't work.
Can you please suggest which namespace to be used. Thank you

Christian Ehrlicher

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

Can you please suggest which namespace to be used. Thank you

So hard to read @KroMignon 's answer?

You have to specify the namespace :: to use standard open() function:
int fd = ::open("/dev/mem",O_RDWR|O_SYNC);

Basic c++ stuff ...

jsulm

@vicky_mac Did you read what @KroMignon suggested?

::close(fd);

KroMignon

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

Hi jsulm,
Sorry to bother again but i don't know which namespace to use in this case i tried std:: that also doesn't work.
Can you please suggest which namespace to be used. Thank you

In C++, there are namespaces to prevent name conflicts (cf. https://en.cppreference.com/w/cpp/language/namespace).
When you are writing code in a C++ class, the default namespace is the current class.
So when you are calling a close() function, the compiler first try to find this function in the current namespace.

To avoid this, you have to specify full namespace. In case of "standard functions" like close(int), the namespace is empty. So the fullname of standard open(int), in C++ is ::open().

This are C++ basics, it is always helpful to learn the basics ;)

vicky_mac

@KroMignon
Thanx for the reply kromignon.

I am familiar with C but not much C++. I think i'll have to read the basics of C++ for using QT

KroMignon

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

I think i'll have to read the basics of C++ for using QT

Yes, or you will have a very hard time.
I know it, because I had the same learning way. I start with C/Assembler for years before programming in C++.
Event if they seem identical, they are definitively not.

Programming C++ as a C developer is not the right way to do.
Take time to learn C++ basics will save you many days later.

Qt is a C++ framework, so it is mandatory to understand C++ to be able to use Qt.

JonB

@vicky_mac said in close(int fd) not recognized in QT both fcntl.h and unistd.h are included:

I am getting the same error even after MainWindow::close(fd)

Both @KroMignon and I said from the start and multiple times to try ::close(fd);. Same with ::open(). How much clearer than that can one be?

If you want to be "safe", every single C (not C++) function you want to use should be ::function().