How to write mmap input memory to O_DIRECT output file?

Posted by Friedrich on Stack Overflow See other posts from Stack Overflow or by Friedrich
Published on 2011-06-22T18:31:23Z Indexed on 2012/03/19 18:03 UTC
Read the original article Hit count: 358

Filed under:

linux-kernel

|

mmap

why doesn't following pseudo-code work (O_DIRECT results in EFAULT)

in_fd = open("/dev/mem");
in_mmap = mmap(in_fd);
out_fd = open("/tmp/file", O_DIRECT);
write(out_fd, in_mmap, PAGE_SIZE);

while following does (no O_DIRECT)

in_fd = open("/dev/mem");
in_mmap = mmap(in_fd);
out_fd = open("/tmp/file");
write(out_fd, in_mmap, PAGE_SIZE);

I guess it's something with virtual kernel pages to virtual user pages, which cannot be translated in the write call?

Best regards,

Friedrich

© Stack Overflow or respective owner

Related posts about linux-kernel

Out of disk space - /boot at 100%

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
My /boot is at 100%. When I run aptitude search ~ilinux-image I'm getting loads of unused images. When I try to delete one of them (after checking which one is currently in use by doing uname -r), e.g apt-get autoremove linux-image-3.2.0-44-generic I get: Reading package lists... Done Building dependency… >>> More
Linux Kernel Threads - scheduler

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi People, Is Linux Kernel scheduler a part of init process? My understanding is that it is part of Kernel threads managed internally not visible to user by either top or ps. Please correct my understanding. Is it possible to view standard kernel threads through any kernel debugger to see how standard… >>> More
Linux kernel scheduling

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I wish to know how Old Linux scheduling algorithm SJF (shortest job first) calculates the process runtime ? >>> More
The newbie's guide to hacking the Linux kernel

as seen on Internet.com - Search for 'Internet.com'
<b>TuxRadar: </b>"We asked prolific kernel hacker (and Linux Format reader!) Greg Kroah-Hartman to tell us what it takes for newbies to patch the Linux kernel - here's what he had to say." >>> More
Google open source guru says Android code will be in Linux kernel in time

as seen on Internet.com - Search for 'Internet.com'
<b>ZDNet:</b> "Google’s Android code will assume its rightful place in the Linux kernel — in good time, the company’s top open source guru says." >>> More

Related posts about mmap

ls -l freezes terminal locally and remotely

as seen on Server Fault - Search for 'Server Fault'
I've been reading other SF threads regarding ls not returning results or freezing and stalling terminal sessions and it appears they usually the fault of network problems. My problem however, occurs both over remote SSH sessions but also if I am physically at the server itself... I just installed… >>> More
PHP script keeps doing mmap/munmap

as seen on Stack Overflow - Search for 'Stack Overflow'
Hello, My PHP script contains a loop, which does nothing much more than echoing and dereferencing pointers (like in $tab[$othertab[$i]]- stuff). It was working great until yesterday, when this script starting being VERY slow (like 50 times slower than before). After using strace, i figured out… >>> More
Shell command slow when using pipe, fast with intermediate file

as seen on Server Fault - Search for 'Server Fault'
Does anyone understand this huge difference in processing time, when using an intermediate file, or when using a pipe? I'm converting tiff to pdf using standard tools on a fresh debian squeeze server. A standard way of doing this is to convert to ps first. Without pipe: root@web5:~# time tiff2ps… >>> More
Caching in mmap

as seen on Stack Overflow - Search for 'Stack Overflow'
I am using mmap call to read from a very big file using simple pointer arithmetic in C++. The problem is that when I read small chunks of data (in the order of KBs) multiple times, each read take the same amount of time as the previous one. How can I know if the disk is being accessed to fulfill my… >>> More
alignment and granularity of mmap

as seen on Stack Overflow - Search for 'Stack Overflow'
I am confused by the specification of mmap. Let pa be the return address of mmap (the same as the specification) pa = mmap(addr, len, prot, flags, fildes, off); In my opinion after the function call succeed the following range is valid [ pa, pa+len ) My question is whether the range of… >>> More