Questions tagged [c]
C is a general-purpose computer programming language used for operating systems, games and other high performance work and is clearly distinct from C++. It was developed in 1972 by Dennis Ritchie for use with the Unix operating system.
880 questions
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Do I need to define an absolute path to be able to compile the DRM / Panfrost subtree code of JSM on the RockPro64 / Khadas Edge-V / RK3399?
I'm trying to compile the drm-subtree of JSM because I want to enable the panfrost driver on the RockPro64 / KHADAS EDGE-V / RK3399. The code is here:
https://github.com/jsm222/drm-subtree
He improved ...
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2
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1
answer
155
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Bash exception handling with a stack trace and cleanup code - how to implement?
When using the set -e flag in Bash, then it's difficult to get user-friendly error messages (a stack trace) and it's difficult to add cleanup code that executes before your script exits with an error ...
- 192
0
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1
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148
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In one process, any way to know whether there exists any other same program is running? [duplicate]
So I am designing a feature to redirect log file if there are multiple processes of this very program are running.
I guess this requires me to somehow get to know whether there exists yet unfinished ...
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If fprinting to stderr fails, which error code should a C program return? [closed]
Assume the following C snippet showing a piece of error handling:
#include <sysexits.h>
#include <stdio.h>
…
int main(int argc, char argv*[]) {
int retval=0;
…
if(3!=argc) { /* wrong ...
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26
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3
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Why does gmtime() give days 1-31, but months 0-11?
The 'gmtime()' standard library function returns a struct that has days numbered 1-31, but months numbered 0-11.
Why is that? Presumably there is some historic reason?
7
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1
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546
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Syscalls required by glibc calls
Are there any lists compiled that provide a list of linux system calls used per function in a standard glibc build?
For example, free() requires mmap, munmap, mprotect, prlimit64, and brk.
If ...
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2
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2
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131
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Sudo doesn't work in my C wrapper
I’m trying to write a C wrapper to run a bash process. The goal of this wrapper is to apply a seccomp policy to restrict certain syscalls.
Here is the code:
#define _GNU_SOURCE
#include <stdio.h>...
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0
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0
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107
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Panfrost kernel driver on FreeBSD for rk3566?
We are two FreeBSD lovers and a stubborn system administrators.
We are trying to project a phone based on FreeBSD for several months.
Emanuel Vadot and Jesper Schmitz Mouridsen have enabled the ...
0
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2
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124
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when opening a FIFO for reading+writing, and blocking - how to tell if other side opened for reading or writing?
If I open a fifo for reading+writing in blocking mode:
fd = open("foo", "O_RDWR");
Then it blocks until someone on the other end opens it for either reading or writing. But how do ...
- 117
0
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0
answers
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Window move bound to display area
Trying to move a window by grip-able button. On 2 other window managers, and no manager it works fine. Windows move anywhere you want pass display edge bounds. On GNOME Shell and Mutter(Gala) the ...
0
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1
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66
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Failed to register hook to tracepoint of signal_generate in Linux?
I am testing FTRACE in Linux VM (ubuntu 24.04) the kernel is Linux VirtualBox 6.11.0-26-generic.
I wrote a kernel module to register a hook (probe) to kernel tracepoint of signal_generate, here is the ...
- 651
0
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2
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87
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Do race conditions occur during pathname resolution with constant string absolute path literals?
According to The Open Group Base Specifications Issue 8, on the rationale of the open() and openat() functions:
The purpose of the openat() function is to enable opening files in directories other ...
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1
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flock not working between forks on the same fd
I am working on a process that forks several times. To debug it, I am using a debug file for which I open a fd and which stays the same for all child forks. Then I have a function print_debug, that ...
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0
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Why does there appear to be a discrepancy when trying to programmatically access files associated with different tasks in Linux from kernel space?
As a way to better understand Linux kernel structures, interacting with and pulling information from these (and also getting more experience coding in C), I've been building simple kernel modules to ...
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How many environments are stored in the stack? [duplicate]
When I used the getenv() function to get the address of USER, I got 0x7ffceae5e923 but when randomly checking the address using gdb, the $USER variable was stored at 0x7ffffffffe90e [ I started a ...
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1
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Why PAM modules are restricted from accessing keys on root keyring?
I'm writing a special PAM module that is supposed to authenticate user with a password stored in kernel keyring.
The key is added by running the following command as root:
keyctl add user some-key 123 ...
0
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0
answers
35
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How does printf() actually work? [duplicate]
When I execute the c file
#include <stdio.h>
#include <stdlib.h>
int main(){
printf("before the system call. I am too excited!\n");
system("ps");
...
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0
votes
1
answer
68
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how does Visual Studio Code resolve includes & macros for autocomplete & build?
If using only the C/++ Extension from Microsoft in Visual Studio Code, then how does preprocessing of includes & macros for autocomplete work? Many include headers (such as sys/statvfs.h), depend ...
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0
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Wrong attributes bitmask in READDIR requests on NFSv4.1
I'm struggling the following problem.
I have an NFS v4.1 mount, where I have a directory with a couple of thousands files. I'm trying to list their names and types. Even with a minimal example program ...
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1
answer
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Changing all strcpy to strscpy
I am learning about Linux kernel development. I have cloned into the stable branch of Linux. I watched a tutorial from Greg KH on youtube that explains you can use the checkpatch.pl script to find ...
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0
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Does a systemd service send org.freedesktop.DBus.Properties.PropertiesChanged signal at boot-up?
I have 2 systemd services.
a.service
b.service
b depends on a but I don't want to explicitly mention the dependency in b.service file.
Instead I am waiting on the
org.freedesktop.DBus.Properties....
0
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1
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147
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Use libnm or popen(nmcli) to configure network programmatically?
We have a system where we need the user to be able to permanently change the network settings from the interface (via our C code), that used to be achieved by generating a new dhcpcd.conf file and ...
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0
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0
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33
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Hit noise when playing part of wave file with ALSA PCM interface
I am working a WAVE file playing with ALSA PCM interface in Linux, and I heard noise when I played the file quickly and partially.
Here is my playing function.
static int playback_function(uint8_t *...
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0
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1
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31
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Cannot see "file,line,func" fields in the journal
I am trying to use sd_journal_print_with_location() function.
#include <systemd/sd-journal.h>
int main() {
sd_journal_print_with_location(LOG_INFO, "CODE_FILE=tst_file.c", "...
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2
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1
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310
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Automatic flush of stream when reading input
I read in Advanced Programming in Unix Environment this:
Line buffering comes with two caveats. First, the size of the buffer
that the standard I/O library uses to collect each line is fixed, so
I/O ...
- 135
0
votes
0
answers
115
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Send UDP data from one NIC and receive on another NIC on same machine
I have multiple NIC's on my Ubuntu machine.
$ ifconfig
docker0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.17.0.1 netmask 255.255.0.0 broadcast 172.17.255.255
inet6 ...
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0
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1
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645
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Get output into journal in a C written service
I have a small service written in C on Debian.
This service automatically starts with Linux and does some file operations and other things as expected.
The service prints some informational messages ...
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0
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There is no file linux/init.h
I'm trying to write my own pci-device driver on an Ubuntu system.
The example I found says that I need:
#include <linux/init.h>
#include <linux/module.h>
#include <linux/pci.h>
I'm ...
0
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1
answer
46
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Is mmap holding a reference to the OFD specified by POSIX, or a Linux extension, and where is it documented?
I am using Open File Description (OFD) owned locks on Linux (fcntl with command F_OFD_SETLK). After locking a file, I memory mapped it, and closed the file descriptor. Another process tried to lock ...
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0
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1
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252
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setuid not working with C code and system() call [duplicate]
I have NICE-DCV installed in RHEL-8.10 and as root doing a dcv list-sessions shows all sessions on the system. But if a user does dcv list-sessions then the output from that is only their session ...
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1
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175
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Implement UDP packets filtering in C
I have a question on how to setup the filtering as described in the following scenario:
I see that systemd-resolved listens on port 53 and sends all the DNS requests to the real DNS resolvers.
I am ...
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3
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2
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310
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shell script to match a function in a file and replace it with another function in another file
I have files that contains multiple functions like below.
file1.c:
static void function1()
{
code
}
...
static void functionX()
{
code
}
...
static void lastFunction()
{
code
}
file2.c:
...
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1
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0
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Arch Linux version & headers package don't line up
I'm trying to write a kernel module in C on ArchLinux. Installing the linux-headers package gives me the version 6.10.5-arch1-1 but running uname -r returns 6.10.5-arch2-1
How can I install the ones ...
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1
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0
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Waking Qemu From Suspend With USB Device
I am doing research with Linux Power Management interactions with USB devices. Which is why I would like to be able to wake a qemu vm from suspend with a passed through USB device. I have been able to ...
1
vote
1
answer
174
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Gedit Bracket match is out of range
Gedit matches brackets like { with } and tells us where the matching opening/closing bracket is. If it is more than 10000 characters, it says "Bracket match is out of range" instead. How ...
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2
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Is it possible for SIGALRM to be delivered after setitimer() disables the timer?
I have some code like this that has failed once on CI and once locally, though I cannot reproduce it reliably. The code is like this:
#include <stddef.h>
#include <signal.h>
#include <...
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3
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2
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2k
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Why is off_t signed?
On my journey to create a 16 EiB sized file,
I encountered the following issue:
POSIX defines files to have their size represented using type off_t,
and that "blkcnt_t and off_t shall be signed ...
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9
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1
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Does mtrace() still work in modern distros?
tldr: Does mtrace still work or am I just doing it wrong?
I was attempting to use mtrace and have been unable to get it to write data to a file. I followed the instructions in man 3 mtrace:
t_mtrace.c:...
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0
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135
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File descriptors get corrupted when program runs in background
I have written a program for an embedded Linux system(petalinux). It runs fine when started from the terminal or TCF debugger.
However, I want it to run when the system starts, so I use the start-stop-...
- 111
1
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1
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659
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Where do i trace NVME IO within the Linux driver?
I want to write a small Linux driver extension. More specific: I want to write all the communication between the host and a M.2-nvme-ssd into a userspace file. The nvme driver is pretty big though and ...
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0
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Does bash explicitly allow commands like passwd, su, etc., to run in setuid while disabling other custom binaries? [duplicate]
I know that /bin/bash has certain built-in protections that prevent the abuse of the Set-UID mechanism. So if I create a custom binary and execute it, it won't run with SUID 0 (refer the below image). ...
0
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0
answers
270
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How to keep a shell alive after it gets started from inside a program?
I was not sure where to ask this question, since it is a mix of both programming and Linux, but hopefully this is the right place. I have written a simple program, which I'm buffer overflowing. I have ...
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1
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0
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132
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What safety CFLAGS does the Linux kernel use?
I have seen it said that the Linux kernel is built with -fwrapv to define signed integer overflow as wraparound, potentially sacrificing a tiny bit of performance in return for preventing the ...
- 111
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1
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414
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Help with Makefile for C program
I'm trying to generate a new library that includes another one I have created previously.
In my current folder I have my source files inside a src folder and my headers inside an include folder.
This ...
38
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3
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7k
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Single '#' in old C files
I started studying old C code (Unix v6) and I'm wondering what is the purpose of a single # in the beginning of the .c file. For example from https://github.com/lsahn-gh/unix-v6/blob/master/sys%2Fken%...
- 701
9
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5
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Print all C comments to a separate text file
I would like to print all my C comments to a separate text file.
Using awk, sed, grep, or bash
output all multi-line C comments between /* ... */ (inclusive)
output lines with // comments
Optional: ...
- 299
0
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1
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194
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compile error with tiny http server c code gcc debian
My distro is MX Linux (Debian)
GCC was already installed with distro.
I am trying to compile some open source code from Nir Lictman.
His Minimalist C web server code compiles in his YouTube video.
...
- 105
0
votes
3
answers
602
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Making changes to the shared library
Suppose we have a C program that uses the shared library.
If you make changes to a shared library and rebuild it, all programs using that library will automatically receive those changes the next time ...
- 3
0
votes
0
answers
174
views
How do I identify the connected USB device?
If I write the rules, for example, in this form:
ACTION=="remove", GOTO="serial_end"
SUBSYSTEM!="tty", GOTO="serial_end"
KERNEL=="ttyUSB[0-9]*", RUN+=...
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1
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1
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535
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Reboot command doesn't invoke reboot system call?
OS is Debian.
As per my question here:
Is there a way to determine what process is rebooting a server?
I wrote a kernel module to intercept the reboot syscall via modifying the syscall table.
...
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