clock_t clock() ambiguity

[Matt]

Does clock_t clock() measure real time or process time? Unless I have
missed something, Stroustrup's treatment in TC++PL (section D.4.4.1) is
none too clear on that question.

Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows, and
MacOS?

[Phlip]

Matt wrote:

Does clock_t clock() measure real time or process time? Unless I have
missed something, Stroustrup's treatment in TC++PL (section D.4.4.1) is
none too clear on that question.

Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows, and
MacOS?

http://msdn.microsoft.com/library/de..._crt_clock.asp

Note the compatibility is "ANSI", which is MS's code for the "ISO C
Standard". All compilers worth using will have it.

Then note the first line is "Calculates the wall-clock time used by the
calling process." That "wall-clock" is jargon for real time, not process
time. And "used by the calling process" means the "epoch" starts when the
program starts.

In future, please hit a question with Google before posting it! I answered
all your questions with the first citation for [msdn clock_t].

--
Phlip
http://c2.com/cgi/wiki?ZeekLand <-- NOT a blog!!!

[mlimber]

Matt wrote:

Does clock_t clock() measure real time or process time? Unless I have
missed something, Stroustrup's treatment in TC++PL (section D.4.4.1) is
none too clear on that question.

IIRC, it's not defined by the C or C++ standards. Neither language has
the inherent concept of real time vs. process time, so implementations
will vary from platform to platform.

Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows, and
MacOS?

Depends on what you mean by "well-implemented." The function is
required by the standard, yes, but does it function the same
everywhere? No.

Cheers! --M

[Kai-Uwe Bux]

Matt wrote:

Does clock_t clock() measure real time or process time? Unless I have
missed something, Stroustrup's treatment in TC++PL (section D.4.4.1) is
none too clear on that question.

C++ inherits clock() from the standard C header time.h. The publicly
available draft of the C standard says:

7.23.2.1 The clock function

Synopsis
1 #include <time.h>
clock_t clock(void);

Description
2 The clock function determines the processor time used.

Returns
3 The clock function returns the implementation? s best approximation to the
processor time used by the program since the beginning of an
implementation-defined era related only to the program invocation. To
determine the time in seconds, the value returned by the clock function
should be divided by the value of the macro CLOCKS_PER_SEC. If the
processor time used is not available or its value cannot be represented,
the function returns the value (clock_t)-1.

Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows, and
MacOS?

I don't know, and I think it is not topical in this group.
Best

Kai-Uwe Bux

[Phlip]

Kai-Uwe Bux wrote:

3 The clock function returns the implementation? s best approximation to
the
processor time used by the program since the beginning of an
implementation-defined era related only to the program invocation. To
determine the time in seconds, the value returned by the clock function
should be divided by the value of the macro CLOCKS_PER_SEC. If the
processor time used is not available or its value cannot be represented,
the function returns the value (clock_t)-1.

Oh-kay. The MSDN page I cited claimed "wall-clock" time. Processor time is
available, inside Win32, but not used. The page did not claim the function
returns (clock_t)-1, despite the processor time is not available to the
program.

Is there a language lawyer in the house??

>Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows, and
MacOS?

I don't know, and I think it is not topical in this group.

It's not topical on any other group, either, by the same logic.

--
Phlip
http://c2.com/cgi/wiki?ZeekLand <-- NOT a blog!!!

[Default User]

Phlip wrote:

Matt wrote:

Does clock_t clock() measure real time or process time? Unless I
have missed something, Stroustrup's treatment in TC++PL (section
D.4.4.1) is none too clear on that question.

Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows,
and MacOS?

http://msdn.microsoft.com/library/de...ry/en-us/vclib
/html/_crt_clock.asp

Note the compatibility is "ANSI", which is MS's code for the "ISO C
Standard". All compilers worth using will have it.

Then note the first line is "Calculates the wall-clock time used by
the calling process." That "wall-clock" is jargon for real time, not
process time. And "used by the calling process" means the "epoch"
starts when the program starts.

That's not what it says, and that's not right.

"The clock function tells how much time the calling process has used."

It is not the wall time, it's the process time.

In future, please hit a question with Google before posting it! I
answered all your questions with the first citation for [msdn
clock_t].

Unfortunately, you answered them incorrectly.

Brian

[Default User]

mlimber wrote:

Matt wrote:

Does clock_t clock() measure real time or process time? Unless I
have missed something, Stroustrup's treatment in TC++PL (section
D.4.4.1) is none too clear on that question.

IIRC, it's not defined by the C or C++ standards. Neither language has
the inherent concept of real time vs. process time, so implementations
will vary from platform to platform.

From the C99 draft standard:

7.23.2.1 The clock function

Synopsis

[#1]

#include <time.h>
clock_t clock(void);

Description

[#2] The clock function determines the processor time used.

Returns

[#3] The clock function returns the implementation' s best
approximation to the processor time used by the program
since the beginning of an implementation-defined era related
only to the program invocation. To determine the time in
seconds, the value returned by the clock function should be
divided by the value of the macro CLOCKS_PER_SEC. If the
processor time used is not available or its value cannot be
represented, the function returns the value (clock_t)-1.252)

252In order to measure the time spent in a program, the
clock function should be called at the start of the
program and its return value subtracted from the value
returned by subsequent calls.

Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows,
and MacOS?

Depends on what you mean by "well-implemented." The function is
required by the standard, yes, but does it function the same
everywhere? No.

No, but it should be reasonably close. Unlike tine(), the semantics for
clock() are fairly well defined. It does waffle a bit, with "best
approximation", but it's not unspecified like time() and time_t.


Brian

[Phlip]

Default User wrote:

"The clock function tells how much time the calling process has used."

It is not the wall time, it's the process time.

Despite, uh, saying "wall" time at the top of the page?

Please Google [wall-clock time]. If it's meaning isn't related to the
movement of a clock's hands while the program runs, then I'm wrong.
Otherwise, the MSDN page is inconsistent with itself.

A Win32 test involving Sleep() and two clock() calls would instantly answer
questions about the actual implementation.

And please be more careful with your own claims.

--
Phlip
http://c2.com/cgi/wiki?ZeekLand <-- NOT a blog!!!

[Default User]

Phlip wrote:

Default User wrote:

"The clock function tells how much time the calling process has
used."

It is not the wall time, it's the process time.

Despite, uh, saying "wall" time at the top of the page?

It's poorly (or incorrectly) worded in that section. The Remarks and
the example code get it right.

Please Google [wall-clock time]. If it's meaning isn't related to the
movement of a clock's hands while the program runs, then I'm wrong.

I know what it means, but that not what clock() measures. That's what
time() measures.

Otherwise, the MSDN page is inconsistent with itself.

Good thing it's not the Standard.

A Win32 test involving Sleep() and two clock() calls would instantly
answer questions about the actual implementation.

And please be more careful with your own claims.

As you're incorrect, I'd watch the atty-tude.

Brian

[Kai-Uwe Bux]

Phlip wrote:

Kai-Uwe Bux wrote:

>3 The clock function returns the implementation? s best approximation to
the
processor time used by the program since the beginning of an
implementati on-defined era related only to the program invocation. To
determine the time in seconds, the value returned by the clock function
should be divided by the value of the macro CLOCKS_PER_SEC. If the
processor time used is not available or its value cannot be represented,
the function returns the value (clock_t)-1.

Oh-kay. The MSDN page I cited claimed "wall-clock" time.

If so, that page is mistaken or describes a non-compliant implementation of
clock(). Pick your poison.

Processor time is available, inside Win32, but not used.

Huh? And that relates to the clock() function in C or C++ like how?

The page did not claim the function returns (clock_t)-1, despite the
processor time is not available to the program.

Is there a language lawyer in the house??

I think, no lawyer is needed. The quote from the draft standard is pretty
clear, isn't it? I do not have a copy of the final version, but I would not
imagine that it changed substantially.

>>Is clock() well-implemented for the major compilers g++, VC++, and
Macintosh compilers and other C++ compilers on Linux/Unix, Windows, and
MacOS?

I don't know, and I think it is not topical in this group.

It's not topical on any other group, either, by the same logic.

Well, that's too bad then. How about splittig the question and asking in a
Linux forum about Linux, in a Windows forum about Windows, in a gcc forum
about gcc, ...?
Best

Kai-Uwe Bux