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Groups > comp.lang.c > #105651
| From | jt@toerring.de (Jens Thoms Toerring) |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Need explanation of C syntax for a Perl 6 guy |
| Date | 2017-03-09 10:16 +0000 |
| Organization | Freie Universitaet Berlin |
| Message-ID | <eicocnFmliiU1@mid.uni-berlin.de> (permalink) |
| References | <o9ql0c$di9$1@dont-email.me> |
T <T@invalid.invalid> wrote:
> Hi C!
> I am trying to write some inline C code in Perl 6. Would
> explain to me what is going on in the following two commands:
> References:
> https://tronche.com/gui/x/xlib/utilities/XStoreBuffer.html
> https://tronche.com/gui/x/xlib/utilities/XFetchBuffer.html
> man XStoreBuffer
> int XStoreBuffer(Display *display, char *bytes,
> int nbytes, int buffer);
This is a function that returns a value of type 'int',
indicating success (if it's 0) or failure. 'display'
is a pointer to a structure of type ;Display', which
contains information about the connection to the X server
For several reasons structures are often not passed to
functions by value (i.e. passing all the information they
contain) but as a pointer to the location in memory where
the structure can be found.
'buffer' is a variable of type 'pointer to char', i,e.
the address of the memory region where the data you
want to have stored reside. And since that address
doesn't tell the function how many bytes there are to
be stored you also need the 'nbytes' varialbe (of type
'int') to tell the function how many bytes from that
memory location you want to store.
Finally, 'buffer' is another 'int' variable and lets
you tell the X server in which (of up to 8 I think)
buffers you want to store those data.
> char *XFetchBuffer(Display *display,
> int *nbytes_return, int buffer);
This function returns a 'pointer to char', which either
is the address where to get the fetched data from or
which is a NULL pointer in case the function failed.
'display' is the same thing as in the othe function call.
'bytes_return' is a pointer to an int. This is used by the
function to return the number of bytes of the data it
returned by storing that number at this location. And
'buffer' is again the number of the buffer.
Here's a very primitive example:
Display *d; // must be initialized somewhere!
char mybuf[100]; // must be initialized somewhere!
char *buf;
int len;
XStoreBuffer(d, mybuf, sizeof mybuf, 0);
buf = XFetchBuffer(d, &len, 0);
With the first call you ask the X server to store all the
data form 'mybuf' ('sizeof nybuf' is lust the total size
of that array) in its buffer numbered 0.
The second call should return a pointer to wherever the
X server stored the data. The '&len' argument is the
address of your variable 'len' and the function will
put the length of the buffer the X server has stored
at that address, so after that call it will be the value
of 'len'. And the last argument is again the buffer number.
> Questions:
> 1) what does the * do to the variable?
They don't do anything to the variables, they just
tell the comppiler that they're pointers. If you have
int x;
this defines a variable named 'x' that is of type 'int'
(and has enough memory to hold all the of an 'int'). If you
have
int *pi;
then you get a pointer to a variable of type 'int', i.e.
basically the address of it. You could combine those
two lines as
1 int x = 3;
2 T *pi;
3 px = &i;
4 printf("%d\n", *pi);
In line 3 you assign the address of 'x' (the '&' is the
"address of" operator) to the pointer 'px'. And in line
4 you print out what's stored at that address - the '*'
in this context could be called the 'content of' operator.
If you define a function that acceots an 'int' as its
argument it could be
void foo(int x);
A function that expects a pointer to 'int' would be
void bar(int *pi)
if you want to call it with the address of an existing
variable of type 'int' you'ld do
int x = 13;
bar(&x);
> 2) what does the * do to the subroutine name?
It doesn't do anything with the subroutine name, it merely
says that the function will return a pointer.
> 3) what is "int nbytes, int buffer"? Is "buffer"
> the address (pointer) to the buffer? and "nbytes"
> the length of "buffer"?
'nbytes' is the number of bytes you want to store and
'buffer' is the index of an internal buffer the X server
keeps.
> 4) does "int" stand for "integer"?
It stands for a type that allows to store integers in a
range that depends on the type of machine the program is
running on. The only guarantee is that the range runs
from -32767 to 32767 (but could be quite a bit larger).
> 5) does "Display" stand for a special variable or a
> routine?
No, it's a structure containing information about which
X server the function should interact with and its capa-
bilities. This could be a server on a different machine
and your program could also be interacting with several X
servers. The details shouldn't be of concern to you. But
if you really want to know what that structure is look
into '/usr/include/X11/Xlibint,h' and search for the
declaration of 'struct _XDisplay' (Display is just an
alias for it).
> 6) why does "*display" have a * in front of it?
Because the function wants a pointer to a 'Display'
structure.
> 7) what does "char" do to "char *XFetchBuffer"?
The 'char *' bit just indicates that the function will
return a pointer to char, the first char of a set of
'nbytes_return' chars.
One word of warning: as long as you're still struggling
with basic concepts of Cd, as pointers, buffers etc. you
have a big chance of making mistakes that can result in
all kinds of weird errors. And then mixing that with the
very low-level stuff from Xlib is a perfect recipe to
get completely confused. If you use pointers in C you must
be very aware of what you're doing, one mistake and all
kinds of strange things can happen (with a segmentatuon
fault being the most begnin!). I'm not trying to discourage
you but before you start messing around with Xlib (which is
in part that arcane that few people use it directly but
instead some wrapper library) you better have a rather
firm grasp of how C works. And once you start messing
around with Xlib you will need good documentation (not
easy to find, though), not just some random pages from
the internet. "XLIB Programming Manual" by Adrian Nye
is probably your best bet. Perhaps you better check
carefully if what you want to achieve (which I don't
know) can't be done with the graphics package you're
rather likely already using.
Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\__________________________ http://toerring.de
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Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-08 20:20 -0800
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-08 20:57 -0800
Re: Need explanation of C syntax for a Perl 6 guy Barry Schwarz <schwarzb@dqel.com> - 2017-03-09 00:46 -0800
Re: Need explanation of C syntax for a Perl 6 guy gordonb.txjrp@burditt.org (Gordon Burditt) - 2017-03-09 03:29 -0600
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-09 12:39 -0800
Re: Need explanation of C syntax for a Perl 6 guy jt@toerring.de (Jens Thoms Toerring) - 2017-03-09 10:16 +0000
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-09 12:40 -0800
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-09 13:25 -0800
Re: Need explanation of C syntax for a Perl 6 guy jt@toerring.de (Jens Thoms Toerring) - 2017-03-10 21:02 +0000
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 17:49 -0800
Re: Need explanation of C syntax for a Perl 6 guy Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-09 10:22 +0000
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-09 12:40 -0800
Re: Need explanation of C syntax for a Perl 6 guy GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-03-09 12:48 +0100
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-09 12:43 -0800
Re: Need explanation of C syntax for a Perl 6 guy James Kuyper <jameskuyper@verizon.net> - 2017-03-10 10:22 -0500
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 11:47 -0800
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 12:42 -0800
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 19:52 -0800
Re: Need explanation of C syntax for a Perl 6 guy raltbos@xs4all.nl (Richard Bos) - 2017-03-09 12:09 +0000
Re: Need explanation of C syntax for a Perl 6 guy Keith Thompson <kst-u@mib.org> - 2017-03-09 09:05 -0800
Re: Need explanation of C syntax for a Perl 6 guy John Bode <jfbode1029@gmail.com> - 2017-03-09 12:31 -0800
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-09 12:41 -0800
Re: Need explanation of C syntax for a Perl 6 guy Paul <nospam@needed.invalid> - 2017-03-10 04:06 -0500
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 12:42 -0800
Re: Need explanation of C syntax for a Perl 6 guy jt@toerring.de (Jens Thoms Toerring) - 2017-03-10 21:35 +0000
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 17:48 -0800
Re: Need explanation of C syntax for a Perl 6 guy John Bode <jfbode1029@gmail.com> - 2017-03-10 14:54 -0800
Re: Need explanation of C syntax for a Perl 6 guy T <T@invalid.invalid> - 2017-03-10 17:43 -0800
Re: Need explanation of C syntax for a Perl 6 guy James Kuyper <jameskuyper@verizon.net> - 2017-03-10 21:11 -0500
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