Ggtcf

How do I lift the insulation blower into the attic?

Is there a distance limit for minecart tracks?

1 John in Luther’s Bibel

Would this string work as string?

Do people actually use the word "kaputt" in conversation?

Mortal danger in mid-grade literature

What is the tangent at a sharp point on a curve?

What is this high flying aircraft over Pennsylvania?

Calculate Pi using Monte Carlo

Are hand made posters acceptable in Academia?

Trouble reading roman numeral notation with flats

Why is participating in the European Parliamentary elections used as a threat?

How would a solely written language work mechanically

Why does the frost depth increase when the surface temperature warms up?

Has the laser at Magurele, Romania reached the tenth of the Sun power?

Air travel with refrigerated insulin

Can a Knock spell open the door to Mordenkainen's Magnificent Mansion?

Is there any common country to visit for persons holding UK and Schengen visas?

Offset in split text content

Recursively move files within sub directories

python displays `n` instead of breaking a line

When is the exact date for EOL of Ubuntu 14.04 LTS?

Hashing password to increase entropy

Travelling in US for more than 90 days

Safely writing directly to a memory address

What's the shortest code to write directly to a memory address in C/C++?Typedef function pointer?Initializing a pointer with an absolute memory addressWrite-Only pointer typeHow does pointer type casting work in cWriting to hardware register using void *C standard compliant way to access null pointer address?Safe usage of longjmp/setjmp with volatileBest way to avoid cast integer to pointer when dealing with memory access (MISRA 2008)Memory mapping peripheral registers using pointer array

0

I'm writing to a register with a known address.

volatile uint32_t *my_address =
 (volatile uint32_t *)MY_ADDRESS;

*my_address = 0x1234;

What is the safest way of writing to memory? I've read that it should be put behind a macro and/or I should check that the pointer is not null.

Thanks

c embedded

share|improve this question

edited Mar 7 at 20:11

John Smith

asked Mar 7 at 20:08

John SmithJohn Smith

659

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Looks safe enough to me as long as there is no MMU in the way...
  
  – Eugene Sh.
  Mar 7 at 20:11
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What type of register?
  
  – Jay Elston
  Mar 7 at 20:11
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What you have right there is "non-safely derived pointer". It is implementation-defined if using non-safely derived pointers is allowed in the program. Since my gut feeling is you are writing for some sort of the embedded platform, you should rest assured that all compilers for those platforms allow non-safely derived pointers.
  
  – SergeyA
  Mar 7 at 20:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergeyA Never heard this term even though I can guess its meaning.. Any formal reference/definition?
  
  – Eugene Sh.
  Mar 7 at 20:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @EugeneSh. I am aware of C++ definition of it: eel.is/c++draft/basic.stc.dynamic.safety I am not actually sure that it is available in C (I didn't really notice the C tag)
  
  – SergeyA
  Mar 7 at 20:20
  

  
  
  
  

 | 
show 6 more comments

0

I'm writing to a register with a known address.

volatile uint32_t *my_address =
 (volatile uint32_t *)MY_ADDRESS;

*my_address = 0x1234;

What is the safest way of writing to memory? I've read that it should be put behind a macro and/or I should check that the pointer is not null.

Thanks

c embedded

share|improve this question

edited Mar 7 at 20:11

John Smith

asked Mar 7 at 20:08

John SmithJohn Smith

659

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Looks safe enough to me as long as there is no MMU in the way...
  
  – Eugene Sh.
  Mar 7 at 20:11
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What type of register?
  
  – Jay Elston
  Mar 7 at 20:11
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What you have right there is "non-safely derived pointer". It is implementation-defined if using non-safely derived pointers is allowed in the program. Since my gut feeling is you are writing for some sort of the embedded platform, you should rest assured that all compilers for those platforms allow non-safely derived pointers.
  
  – SergeyA
  Mar 7 at 20:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @SergeyA Never heard this term even though I can guess its meaning.. Any formal reference/definition?
  
  – Eugene Sh.
  Mar 7 at 20:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @EugeneSh. I am aware of C++ definition of it: eel.is/c++draft/basic.stc.dynamic.safety I am not actually sure that it is available in C (I didn't really notice the C tag)
  
  – SergeyA
  Mar 7 at 20:20
  

  
  
  
  

 | 
show 6 more comments

I'm writing to a register with a known address.

volatile uint32_t *my_address =
 (volatile uint32_t *)MY_ADDRESS;

*my_address = 0x1234;

What is the safest way of writing to memory? I've read that it should be put behind a macro and/or I should check that the pointer is not null.

Thanks

c embedded

share|improve this question

edited Mar 7 at 20:11

John Smith

asked Mar 7 at 20:08

John SmithJohn Smith

659

volatile uint32_t *my_address =
 (volatile uint32_t *)MY_ADDRESS;

*my_address = 0x1234;

share|improve this question

edited Mar 7 at 20:11

John Smith

asked Mar 7 at 20:08

John SmithJohn Smith

659

share|improve this question

edited Mar 7 at 20:11

John Smith

asked Mar 7 at 20:08

John SmithJohn Smith

659

asked Mar 7 at 20:08

John SmithJohn Smith

659

asked Mar 7 at 20:08

John SmithJohn Smith

659

1 Answer
1

active

oldest

votes

1

If you want to safely write/read memory-mapped, you could use an already existing piece of code designed for this very purpose, for example ARM's mmio.h header file.

The resulting code equivalent to the one in your example would then be:

#include<mmio.h>
// ...
mmio_write_32(MY_ADDRESS, 0x1234);

Because the functions included in the header are static inline, there will be no performance penalty caused by a call to a subroutine - the code will just be inserted where it is being called in your program.

Since this header file is being widely used, you can reasonably stop worrying on your register read/write operations by using it.

And because it is licensed under the BSD 3-Clause, you are free to use it in both your commercial and pet projects .

share|improve this answer

edited Mar 8 at 2:11

answered Mar 8 at 0:45

FrantFrant

84259

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But that is not necessarily safe - we don't know if the hardware permits word access there, if the address is aligned or the hardware permits it not to be, and we don't know if anything else like an ISR might see a non-atomic write half-done. It's also not meaningfully different from the code in the question - so what you are really doing is making a deference to authority over something your authority doesn't even claim to provide.
  
  – Chris Stratton
  Mar 8 at 3:59
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  First of all, I am not making a deference to any authority of any kind. I used the word 'safe' here in the sense that using mmio.h should protect against a misuse of the volatile keyword, I should have elaborated a bit: since the example code looked more like a beginner's code than the one you would probably have written yourself, I (wrongly?) assumed the question was more a basic one than the one you apparently thought it was - there was no information whatsoever on the actual target. John Smith could probably tell more precisely what he was looking for.
  
  – Frant
  Mar 8 at 14:24
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I 100% do agree with your comment's content, but for the somewhat rude part.
  
  – Frant
  Mar 8 at 14:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  ...and the ' I've read that it should be put behind a macro' is what guided my answer.
  
  – Frant
  Mar 8 at 14:29
  

  
  
  
  

add a comment | 

Your Answer

StackExchange.ifUsing("editor", function ()
StackExchange.using("externalEditor", function ()
StackExchange.using("snippets", function ()
StackExchange.snippets.init();
);
);
, "code-snippets");

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "1"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);

Sign up using Google

Sign up using Facebook

Sign up using Email and Password

Post as a guest

Name

Email

Required, but never shown

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55052026%2fsafely-writing-directly-to-a-memory-address%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

1

If you want to safely write/read memory-mapped, you could use an already existing piece of code designed for this very purpose, for example ARM's mmio.h header file.

The resulting code equivalent to the one in your example would then be:

#include<mmio.h>
// ...
mmio_write_32(MY_ADDRESS, 0x1234);

Because the functions included in the header are static inline, there will be no performance penalty caused by a call to a subroutine - the code will just be inserted where it is being called in your program.

Since this header file is being widely used, you can reasonably stop worrying on your register read/write operations by using it.

And because it is licensed under the BSD 3-Clause, you are free to use it in both your commercial and pet projects .

share|improve this answer

edited Mar 8 at 2:11

answered Mar 8 at 0:45

FrantFrant

84259

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But that is not necessarily safe - we don't know if the hardware permits word access there, if the address is aligned or the hardware permits it not to be, and we don't know if anything else like an ISR might see a non-atomic write half-done. It's also not meaningfully different from the code in the question - so what you are really doing is making a deference to authority over something your authority doesn't even claim to provide.
  
  – Chris Stratton
  Mar 8 at 3:59
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  First of all, I am not making a deference to any authority of any kind. I used the word 'safe' here in the sense that using mmio.h should protect against a misuse of the volatile keyword, I should have elaborated a bit: since the example code looked more like a beginner's code than the one you would probably have written yourself, I (wrongly?) assumed the question was more a basic one than the one you apparently thought it was - there was no information whatsoever on the actual target. John Smith could probably tell more precisely what he was looking for.
  
  – Frant
  Mar 8 at 14:24
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I 100% do agree with your comment's content, but for the somewhat rude part.
  
  – Frant
  Mar 8 at 14:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  ...and the ' I've read that it should be put behind a macro' is what guided my answer.
  
  – Frant
  Mar 8 at 14:29
  

  
  
  
  

add a comment | 

1

If you want to safely write/read memory-mapped, you could use an already existing piece of code designed for this very purpose, for example ARM's mmio.h header file.

The resulting code equivalent to the one in your example would then be:

#include<mmio.h>
// ...
mmio_write_32(MY_ADDRESS, 0x1234);

Because the functions included in the header are static inline, there will be no performance penalty caused by a call to a subroutine - the code will just be inserted where it is being called in your program.

Since this header file is being widely used, you can reasonably stop worrying on your register read/write operations by using it.

And because it is licensed under the BSD 3-Clause, you are free to use it in both your commercial and pet projects .

share|improve this answer

edited Mar 8 at 2:11

answered Mar 8 at 0:45

FrantFrant

84259

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But that is not necessarily safe - we don't know if the hardware permits word access there, if the address is aligned or the hardware permits it not to be, and we don't know if anything else like an ISR might see a non-atomic write half-done. It's also not meaningfully different from the code in the question - so what you are really doing is making a deference to authority over something your authority doesn't even claim to provide.
  
  – Chris Stratton
  Mar 8 at 3:59
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  First of all, I am not making a deference to any authority of any kind. I used the word 'safe' here in the sense that using mmio.h should protect against a misuse of the volatile keyword, I should have elaborated a bit: since the example code looked more like a beginner's code than the one you would probably have written yourself, I (wrongly?) assumed the question was more a basic one than the one you apparently thought it was - there was no information whatsoever on the actual target. John Smith could probably tell more precisely what he was looking for.
  
  – Frant
  Mar 8 at 14:24
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I 100% do agree with your comment's content, but for the somewhat rude part.
  
  – Frant
  Mar 8 at 14:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  ...and the ' I've read that it should be put behind a macro' is what guided my answer.
  
  – Frant
  Mar 8 at 14:29
  

  
  
  
  

add a comment | 

If you want to safely write/read memory-mapped, you could use an already existing piece of code designed for this very purpose, for example ARM's mmio.h header file.

The resulting code equivalent to the one in your example would then be:

#include<mmio.h>
// ...
mmio_write_32(MY_ADDRESS, 0x1234);

Because the functions included in the header are static inline, there will be no performance penalty caused by a call to a subroutine - the code will just be inserted where it is being called in your program.

Since this header file is being widely used, you can reasonably stop worrying on your register read/write operations by using it.

And because it is licensed under the BSD 3-Clause, you are free to use it in both your commercial and pet projects .

share|improve this answer

edited Mar 8 at 2:11

answered Mar 8 at 0:45

FrantFrant

84259

#include<mmio.h>
// ...
mmio_write_32(MY_ADDRESS, 0x1234);

share|improve this answer

edited Mar 8 at 2:11

answered Mar 8 at 0:45

FrantFrant

84259

answered Mar 8 at 0:45

FrantFrant

84259

answered Mar 8 at 0:45

FrantFrant

84259