473,372 Members | 947 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,372 software developers and data experts.

need to be able to write in a shared mem segment.

hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..
i am using open() , to open a file, and then use mmap to get a void*
file_memory pointer. then i use fwrite(aStruct,structSize,1,(((FILE*)
file_memory) + anOffset)) to try to write to the file but it crashes.

could it be that fwrite does not understand the file_memory pointer?
what else can i use to do the writing?
thankyou for your help
nass

Sep 23 '06 #1
7 2808
nass wrote:
hi all,
[OT redacted]
You might try a group where your question can be better answered.

http://www.parashift.com/c++-faq-lit...t.html#faq-5.9
Sep 23 '06 #2
nass wrote :
hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..
You could use boost.interprocess (used to be boost.shmem)
Sep 23 '06 #3
nass schrieb:
hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..
This is offtopic here, but...
i am using open() , to open a file, and then use mmap to get a void*
file_memory pointer. then i use fwrite(aStruct,structSize,1,(((FILE*)
file_memory) + anOffset)) to try to write to the file but it crashes.
The fourth parameter of fwrite must be a _unchanged_ FILE*, that you got by
calling fopen. You can't cast any pointer to a FILE* and write to that like
a file.

And you don't need to. You said that, mmap returns a memory pointer as
void*. So simply use it, just like you would use memory from a malloc() call.
could it be that fwrite does not understand the file_memory pointer?
exactly.

--
Thomas
http://www.netmeister.org/news/learn2quote.html
Sep 23 '06 #4


Thomas J. Gritzan wrote:
nass schrieb:
hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..

This is offtopic here, but...
i am using open() , to open a file, and then use mmap to get a void*
file_memory pointer. then i use fwrite(aStruct,structSize,1,(((FILE*)
file_memory) + anOffset)) to try to write to the file but it crashes.

The fourth parameter of fwrite must be a _unchanged_ FILE*, that you got by
calling fopen. You can't cast any pointer to a FILE* and write to that like
a file.

And you don't need to. You said that, mmap returns a memory pointer as
void*. So simply use it, just like you would use memory from a malloc() call.
nope i cant do that, the compiler gives me an error saying
error: invalid conversion from `void*' to `FILE*'
its gcc btw.. any other posibility?

could it be that fwrite does not understand the file_memory pointer?

exactly.

--
Thomas
http://www.netmeister.org/news/learn2quote.html

nass

Sep 24 '06 #5
nass wrote:
>
Thomas J. Gritzan wrote:
>nass schrieb:
>>hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..
This is offtopic here, but...
>>i am using open() , to open a file, and then use mmap to get a void*
file_memory pointer. then i use fwrite(aStruct,structSize,1,(((FILE*)
file_memory) + anOffset)) to try to write to the file but it crashes.
The fourth parameter of fwrite must be a _unchanged_ FILE*, that you got by
calling fopen. You can't cast any pointer to a FILE* and write to that like
a file.

And you don't need to. You said that, mmap returns a memory pointer as
void*. So simply use it, just like you would use memory from a malloc() call.
nope i cant do that, the compiler gives me an error saying
error: invalid conversion from `void*' to `FILE*'
its gcc btw.. any other posibility?
mmap returns a pointer to the shared memory.
You do not use a 'FILE *' to write to that memory.
After the mmap you read/update it like any other memory
buffer.

You'll get better support in these newsgroups:

comp.os.linux.development.apps
comp.os.linux.development.system

Larry
>
>>could it be that fwrite does not understand the file_memory pointer?
exactly.

--
Thomas
http://www.netmeister.org/news/learn2quote.html


nass
Sep 25 '06 #6
for some reason i can not 'open' these 2 sites u recommended from a
newsgroups reader..
anyhow, igues you are refering to doing buffer read-update with
sprintf()... i would like to use that function too but the problem is i
have a struct that i need to save in the buffer, and i do not now what
type to instruct the sprintf function to 'think' my struct is....
the struct is not always the same.. but it contains in8, uint8, int32,
floats, doubles and even strings.
how could i use sprintf in order to save this struct?
nass

Larry I Smith wrote:
nass wrote:

Thomas J. Gritzan wrote:
nass schrieb:
hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..
This is offtopic here, but...

i am using open() , to open a file, and then use mmap to get a void*
file_memory pointer. then i use fwrite(aStruct,structSize,1,(((FILE*)
file_memory) + anOffset)) to try to write to the file but it crashes.
The fourth parameter of fwrite must be a _unchanged_ FILE*, that you got by
calling fopen. You can't cast any pointer to a FILE* and write to that like
a file.

And you don't need to. You said that, mmap returns a memory pointer as
void*. So simply use it, just like you would use memory from a malloc() call.
nope i cant do that, the compiler gives me an error saying
error: invalid conversion from `void*' to `FILE*'
its gcc btw.. any other posibility?

mmap returns a pointer to the shared memory.
You do not use a 'FILE *' to write to that memory.
After the mmap you read/update it like any other memory
buffer.

You'll get better support in these newsgroups:

comp.os.linux.development.apps
comp.os.linux.development.system

Larry
>could it be that fwrite does not understand the file_memory pointer?
exactly.

--
Thomas
http://www.netmeister.org/news/learn2quote.html

nass
Sep 25 '06 #7
nass wrote:
for some reason i can not 'open' these 2 sites u recommended from a
newsgroups reader..
anyhow, igues you are refering to doing buffer read-update with
sprintf()... i would like to use that function too but the problem is i
have a struct that i need to save in the buffer, and i do not now what
type to instruct the sprintf function to 'think' my struct is....
the struct is not always the same.. but it contains in8, uint8, int32,
floats, doubles and even strings.
how could i use sprintf in order to save this struct?
nass

Larry I Smith wrote:
>nass wrote:
>>>

Thomas J. Gritzan wrote:
nass schrieb:
hi all,
i am running slackware linux and need to use some function that will
will enable me to write and read from a shared mem segment..
This is offtopic here, but...

i am using open() , to open a file, and then use mmap to get a void*
file_memory pointer. then i use fwrite(aStruct,structSize,1,(((FILE*)
file_memory) + anOffset)) to try to write to the file but it crashes.
The fourth parameter of fwrite must be a _unchanged_ FILE*, that you got by
calling fopen. You can't cast any pointer to a FILE* and write to that like
a file.

And you don't need to. You said that, mmap returns a memory pointer as
void*. So simply use it, just like you would use memory from a malloc() call.

nope i cant do that, the compiler gives me an error saying
error: invalid conversion from `void*' to `FILE*'
its gcc btw.. any other posibility?
mmap returns a pointer to the shared memory.
You do not use a 'FILE *' to write to that memory.
After the mmap you read/update it like any other memory
buffer.

You'll get better support in these newsgroups:

comp.os.linux.development.apps
comp.os.linux.development.system

Larry
>>>>could it be that fwrite does not understand the file_memory pointer?
exactly.

--
Thomas
http://www.netmeister.org/news/learn2quote.html

nass
Please do not top post.

One more time...

FORGET ABOUT 'FILE *'.

Once a file is mapped into memory - it's just memory.

The sprintf() example was just an example of one (of many)
ways to access the memory.

If you want to put a 'struct stuff' in the mapped memory,
just do it:

struct stuff * pStuff;

/* assuming that 'pMem' is the pointer returned by
* mmap() -AND- that the mapped memory is large
* enough to hold a 'struct stuff', this next line
* allows your code to treat the memory at 'pMem'
* as a 'struct stuff' - via 'pStuff'.
* anything put into '*pStuff' will be visible to
* all processes that have the same memory mapped.
*/
pStuff = (struct stuff *) pMem;

/* now write and read the members of the
* 'struct stuff' that resides at the beginning
* of the mapped memory, eg:
*/
pStuff->some_int_field = 55;
This is NOT a C++ language issue so - Post to a
unix/linux newsgroup in the future.

READ THE DOCS ON THE MEMORY MAPPING FUNCTIONS.
It's all explained therein.

Larry
Sep 26 '06 #8

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

5
by: Confused User | last post by:
I am working on device that utilizes a Motorola 68HC16 microcontroller. I am using an old unsupported piece of crap Whitesmith's / Intermetrics / Tasking compiler. The embedded compiler business...
4
by: Zachary Hilbun | last post by:
I need to write an ASP that requires a user to give a User Name and Password to run it. Whenever I've used this on the Web it displays a standard dialog and then offers to save the User Name and...
2
by: Just Me | last post by:
Need to write a program that wakes up every 15 minutes and does something. I guess I could just create a Windows application and schedule it to run every 15 minutes. Or I could have a program...
0
by: jens Jensen | last post by:
Hello, I need to write a webservice that will authenticate via x509. The problem here is , i will configure it not to use SOAP. Just use http post. How can i add x509 authentication? ...
29
by: jens Jensen | last post by:
Hello, I got this "breath taking" task to write a an http server to which "xml data" will be posted to and will answer with xml data. The logic behind the xml processing is not a matter here. ...
13
by: Pradeep Vasudevan | last post by:
hai i am a student and need to write a simple web crawler using python and need some guidance of how to start.. i need to crawl web pages using BFS and also DFS... one using stacks and other...
2
kamill
by: kamill | last post by:
i need to write content of one text file into another text file. My code is working ,if i choose both files from same directory where my program reside..BUT,its not working if i select files from...
8
seshu
by: seshu | last post by:
hi every body This is seshu, here iam able write a code in vb6 on controle arrays ofcourse while designing it self i made my controls as arrays but here in .net i dint find that...
1
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
0
by: Faith0G | last post by:
I am starting a new it consulting business and it's been a while since I setup a new website. Is wordpress still the best web based software for hosting a 5 page website? The webpages will be...
0
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome former...
0
by: ryjfgjl | last post by:
In our work, we often need to import Excel data into databases (such as MySQL, SQL Server, Oracle) for data analysis and processing. Usually, we use database tools like Navicat or the Excel import...
0
by: taylorcarr | last post by:
A Canon printer is a smart device known for being advanced, efficient, and reliable. It is designed for home, office, and hybrid workspace use and can also be used for a variety of purposes. However,...
0
by: Charles Arthur | last post by:
How do i turn on java script on a villaon, callus and itel keypad mobile phone
0
by: ryjfgjl | last post by:
In our work, we often receive Excel tables with data in the same format. If we want to analyze these data, it can be difficult to analyze them because the data is spread across multiple Excel files...
1
by: Sonnysonu | last post by:
This is the data of csv file 1 2 3 1 2 3 1 2 3 1 2 3 2 3 2 3 3 the lengths should be different i have to store the data by column-wise with in the specific length. suppose the i have to...
0
by: Hystou | last post by:
There are some requirements for setting up RAID: 1. The motherboard and BIOS support RAID configuration. 2. The motherboard has 2 or more available SATA protocol SSD/HDD slots (including MSATA, M.2...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.