I'm trying to print the time within my own custom system call in a human readable format (Jan 29 2015 05:53:12 for example, though any order is fine). I'm using code I know works in a standard C program because I've used it before, but for some reason it doesn't work within a system call.
#include<linux/linkage.h>
#include<linux/kernel.h>
#include<linux/sched.h>
#include<linux/time.h>
asmlinkage long sys_mycall (int someVal)
{
time_t t;
time(&t);
printk("myInt: %d", someVal);
printk("%s", ctime(&t)) ;
return 0 ;
}
I'm getting errors on implicit declaration of functions 'time' and 'ctime', even though I included linux/time.h (I also tried just time.h).
And an error on ctime returning type int when I specified %s.
I know this means that something is going wrong with the time.h, but what exactly is it? Am I not allowed to include that in a sys call?
Thanks!
The kernel coding environment is not a "hosted" implementation of C, which implies that various standard C functions like ctime() might not exist, or if they do, might not behave in the same way as the standard functions. In fact, ctime() does not exist inside the kernel at all.
Instead, you can use time_to_tm() from <linux/time.h> to convert a time_t to a broken-down time in a struct tm, and then print the components of struct tm individually.
Note that you have to supply a timezone offset to time_to_tm(), because there is no overall "current timezone" for the kernel - timezone is a display setting and is therefore handled entirely in userspace. This is one reason why the kernel typically doesn't format times to be "human-readable" itself. If you don't have a good value to supply for the timezone offset, you can use zero which will mean that the broken-down time is in UTC.
Instead of time(), to obtain the current time to seconds granuality inside the kernel use get_seconds().
The ctime() function is not available in the kernel.
In fact, there are very few date/time formatting functions available in the kernel; most of these functions are only available in user space. If you want to generate a timestamp in the kernel, don't try to format it; just return a time_t and let userspace applications handle the formatting.
If that isn't enough and you really want a readable timestamp, you'll need to define the necessary functions yourself.
Related
I am making a program to copy files from a source to a destination directory and would like to change the destination file timestamps so they match the source file timestamps.
So far I have discovered the utime() function and have manipulated the utimbuf struct with the times I would like to use.
However, the times do not take into account the nanoseconds.
For example:
If I want to copy "file1" and it has a timestamp of 123.213241, my copy will have 123.000000 when running my current program. I would like to include the nanoseconds .213241 etc.
Here is my code so far:
struct stat buf;
struct utimbuf time;
stat(filename, &buf) // get metadata of file "filename" and then store in buf
time.actime = buf.st_atim.tv_sec; // set times in time struct
time.modtime = buf.st_mtim.tv_sec;
utime(filename_copy, &time); // load file copy with time struct
How can I include nanoseconds in my file timestamps?
According to POSIX, the function you need is utimensat() (or its close relative, futimens()). Both of these take a pair of struct timespec values in an array, which allows you to specify a time to nanoseconds. The first element is the access time; the second is the modification time.
Not all file systems support nanosecond timestamps. Not all systems actually support nanosecond resolution — they might round to the nearest microsecond.
Note that modern versions of the stat() function return a structure with elements st_atim, st_ctim, and st_mtim. These are also struct timespec values. The <sys/stat.h> defines some backwards-compatibility macros:
For compatibility with earlier versions of this standard, the st_atime macro shall be defined with the value st_atim.tv_sec. Similarly, st_ctime and st_mtime shall be defined as macros with the values st_ctim.tv_sec and st_mtim.tv_sec, respectively.
For Linux, see utimensat(2). However, the documentation for stat(2) only mentions subsecond times in the Notes section near the bottom. Be cautious.
On Linux, sched.h contains the definition of
int sched_rr_get_interval(pid_t pid, struct timespec * tp);
to get the time slice of a process. However the file shipping with OS X El Capitan doesn't hold that definition.
Is there an alternative for this on OS X?
The API's related to this stuff are pretty byzantine and poorly documented, but here's what I've found.
First, the datatypes related to RR scheduling seem to be in /usr/include/mach/policy.h, around line 155. There's this struct:
struct policy_rr_info {
...
integer_t quantum;
....
};
The quantum is, I think, the timeslice (not sure of units.) Then grepping around for this or related types defined in the same place, I found the file /usr/include/mach/mach_types.def, which says that the type struct thread_policy_t contains a field policy_rr_info_t on line 203.
Next, I found in /usr/include/mach/thread_act.h the public function thread_policy_get, which can retrieve information about a thread's policy into a struct thread_policy_t *.
So, working backwards. I think (but haven't tried at all) that you can
Use the thread_policy_get() routine to return information about the thread's scheduling state into a thread_policy_t
That struct seems to have a policy_rr_info_t sub-substructure
That sub-structure should have a quantum field.
That field appears to be the timeslice, but I don't know about the units.
There are no man pages for this part of the API, but this Apple Developer page explains at least a little bit about how to use this API.
Note that this is all gleaned from just grepping the various kernel headers, and I've definitely not tried to use any of these APIs in any actual code.
I came across a method named do_nanosleep() in C that I don't understand how it is used? One thing I know that it has to do with the suspending the execution of the calling thread, but that task is handled by nanosleep() in C. If that's true, then what is the need of do_nanosleep() here and how it is different from nanosleep()?
For reference, this is what it does.
/* arguments are seconds and nanoseconds */
inline void
do_nanosleep(time_t tv_sec, long tv_nsec)
{
struct timespec req;
req.tv_sec = tv_sec;
req.tv_nsec = tv_nsec;
nanosleep(&req, NULL);
}
Since do_nanosleep() is not a standard function, you will have to track it in your source code, or in the manuals for your system, to see what it does. It might be a portability wrapper which uses nanosleep() when it is available, and something else (usleep() or even sleep()) when it is not. It might do something completely unrelated to sleeping, too — but it probably does do what its name suggests.
Google has not (yet — 5 minutes after it was asked) indexed your question, and it does not know anything about do_nanosleep(). That suggests the code should be in your source somewhere, rather than in a system manual.
With the function definition in the question, we can see that instead of requiring the user to create a struct timespec, they can call do_nanosleep() with two arguments, the first for the seconds and the second for the fractions of a second (0..999,999,999 measured in nanoseconds). It then calls nanosleep(). So, in the minds of the people who wrote the software, do_nanosleep() presents a slightly more convenient interface to the underlying nanosleep() function. Since it is inline, the declarations for struct timespec must still be in scope, so I'm not convinced I agree with the authors, but it is not automatically a wrong decision.
It looks like it's just a simplified (and crippled) wrapper around POSIX nanosleep.
The first parameter is the number of seconds, and the second is the number of nanoseconds.
Like, do_nanosleep(3, 500000000) would (hopefully) sleep for 3 and a half seconds.
Since the function completely ignores return values... Your mileage may vary.
I currently have a double containing a POSIX time stamp, and I am successfully using gmtime and asctime to display a calendar date via a time_t struct:
time_t input = posix;
printf("%s",asctime(gmtime(&input)));
This works well, except (obviously) when the POSIX time falls outside the time.h library's limits (i.e. 1901-2038), in which case it returns a date in 1901. Are there any easy alternatives to gmtime/asctime/time.h, or am I simply going to have to work with the raw figures?
Edit: I should add that having a result that falls outside those limits is fairly likely, as the posix double is the result of a calculation, rather than an instantiation of the current system time.
Read the Wikipedia entry on Unix Time:
The POSIX and Open Group Unix specifications include the C standard library, which includes the time types and functions defined in the <time.h> header file. The ISO C standard states that time_t must be an arithmetic type, but does not mandate any specific type or encoding for it.
If you need to handle a large range of times in a portable manner, you will probably have to use your own time library. I don't think you can even count on struct tm covering a range beyond 1901 to 2038.
You might also want to read this StackOverflow answer regarding time_t.
My code is:
#include <unistd.h>
(void)alarm(unsigned int 0);
error: expected expression before 'unsigned'
But I'm getting the following error:
Error: expected expression before 'unsigned', due to: (void)alarm(unsigned int 0);
I'm not sure if it is my syntax or use of (void) or something else. What's going on?
I'm using Code::Blocks under Windows.
Uh, is this meant to be a function call, or is it meant to be a declaration?
If you meant it to be a declaration, then it should be:
void alarm( unsigned int i );
If you meant it to be a function call, then it should be:
(void) alarm( 0 );
(Back in my C++ days I used to cast function results to void when I wanted to document the fact that I do not care what the function returned.)
EDIT: Then again, if what you are trying to do is to just declare a variable, then try this:
unsigned int alarm = 0;
Or if you are just trying to set a variable to zero, then things are even more simple:
alarm = 0;
8-)
It looks like you are trying to call the POSIX alarm function which takes an unsigned int and returns and unsigned int.
A correct form of the call would be:
alarm(0);
There is not normally a need to cast the return value to void although it can silence a warning on some compilers.
There is normally no need to explicitly cast 0 to unsigned int. The correct form would be (unsigned int)0. It is usually simpler to use a suffix where necessary, e.g. 0U has type unsigned int but in this instance plain 0 will work fine.
Additionally, as a function call is not a declaration it must appear inside a function body.
E.g.
void foo()
{
alarm(0);
}
You're not giving us enough context to figure out just what you're trying to do.
I'm going to assume that you want to call the alarm function with an argument of 0. According to the man page (type man alarm or man 2 alarm, or follow this link), alarm(0) will cancel any existing alarm without setting a new one.
On my system (Ubuntu, a Linux. i.e., Unix-like system), the following compiles, links, and executes without error:
#include <unistd.h>
int main(void) {
alarm(0);
return 0;
}
I saved the program in a file called c.c, and I compiled and linked it with the following command:
gcc c.c -o c
and executed it with:
./c
The implementation of the alarm function happens to be in the standard C library, which is linked by default. That might or might not be the case on your system, but if it's Linux or some other Unix-like system, it probably is.
(This isn't a particularly useful program, but it could be a starting point for something useful.
EDIT :
I see now that you're using Windows. The alarm() function is defined by the POSIX standard, and is (mostly) specific to Unix-like systems. Windows probably doesn't provide it by default. There are Unix-like emulation layers that run under Windows, such as Cygwin.
But if you want to develop code under Windows, you might consider avoiding non-portable constructs that Windows doesn't (directly) support.
Why do you want to call alarm()? Do you have a requirement to do what that particular function does, or are you just trying to learn the basics?
Agreed with Chiron that this belongs to StackOverflow.
You don't need to cast return to void just simply ignore it if you choose but better yet don't
Why are you declaring variable 0 inside the function call? Call should be:
(void)alarm(0);
#include <unistd.h>
int main() {
alarm(0);
}
I think the question is to write a simple program which calls the alarm standard function. (See the comment on #MikeNakis's question).
You can't just copy the code from the man page into a program and compile it. You must make a complete program, like the one I have given here.
Since you've included unistd.h, I'm assuming you're trying to call the alarm() function declared in that header:
unsigned int alarm(unsigned int seconds);
If you want to call alarm with an argument of 0 seconds, simply do alarm(0). You don't need to cast 0 to an unsigned int, but if you really wanted to you would just have alarm( (unsigned)0 );.
If you don't care about the return type, then just don't assign it to a variable. You don't need to add (void) to the start of the function call to ignore the return value. (void)alarm(0); is perfectly legal, but also pointless.
And I'm not sure if your snippet is a chopped up example or your actual code, but you can't just call alarm from outside of a function like that.
Also, from the man pages:
If seconds is zero, no new alarm() is scheduled.
But perhaps you may be wanting to cancel previous alarms:
In any event any previously set alarm() is canceled.
EDIT: Didn't realise until Keith pointed it out a few minutes ago that you're using Windows. alarm() is a *nix function, if you wanted to call it from Windows you could use tools such as Cygwin (which is commonly used) to emulate a *nix environment.