Are all data pointers the same size in one platform for all data types? For example, if the pointer refers to the second element in an array, the ++ makes the pointer refer to the third element in the array. As example, we can declare Cs main() function with either char *argv[] or char **argv parameter, there is no difference and its mainly a matter of taste which one to choose. Another wrong one: Pretty sure it can be non-pointer on Windows in C++. While if a float type pointer is decremented then it will decrement by 4(size of a float) and the new address will be 996. Once an array is declared, pointers give us an alternative way to access them, but we cannot replace the array declaration itself with a simple pointer because the array declaration also reserves memory. Whenever these strange urges come along, I visit with my old pal Alan, who has done a lot of Forth programming in his career and get him going down memory lane. Im always working in assembler when I really get deep into debugging. There are basic rules for every language. To illustrate this, lets print each and every address we can associate with argv. todays computers are far more advanced than PDP-11 but even today our smartest developers (see above) cant even figure out what a pointer is. For Example:If an integer pointer that stores address 1000 is decremented, then it will decrement by 4(size of an int) and the new address it will points to 996. Addition and subtraction Comparison Assignment The increment ( ++) operator increases the value of a pointer by the size of the data object the pointer refers to. Other than NULL, the concept remains that pointers are simply memory addresses in other words: numbers. never matter to me). The operations are: Increment: It is a condition that also comes under addition. . Difference between passing pointer to pointer and address of pointer to any function, Difference between Dangling pointer and Void pointer, Difference between NULL pointer, Null character ('\0') and '0' in C with Examples, Multidimensional Pointer Arithmetic in C/C++. It used to be Pascal, but now it is Java. new. Typically the ECC mode would be selected at the start of boot and left that way forever, but we wanted to test the robustness of the ECC, which meant turning ECC-on, writing a known 16 bit data value to somewhere in memory, switching to ECC-off mode, toggling a bit or two in the 32 bit word (made up of the 16 bit data word and its 16 bit ECC info), then switching back to ECC-on mode, and reading the 16 bit value to compare against its original value. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. This is from Expert C Programming by Peter Van Der Linden which also makes a deep dive into the differences between pointers and arrays at the nuts and bolts level. Why does the arrow (->) operator in C exist? How to make a pointer increment by 1 byte, not 1 unit, Short story about swapping bodies as a job; the person who hires the main character misuses his body. But if we are talking about style rules, go read the Torvalds coding style guidelines for the linux kernel. . So sizeof(iptr2 iptr1) should be equal to sizeof(int). Otherwise it would not be considered a practical or safe thing to do. So far I've reach a point where I don't know what to do next if it is even possible to reduce increment time. If I have a pointer tcp_option_t* opt, and I want it to be incremented by 1, I can't use opt++ or ++opt as this will increment by sizeof(tcp_option_t), which is N. I want to move this pointer by 1 byte only. Pointer Addition/Increment. (Actually, I had to instruct a programmer how to do it he was initially very much against such a bizarre concept. Note however though _[t]he smallest incremental change is [available as a by-product of] the alignment needs of the referenced type. The best description of C I ever heard was machine independent assembly. arrays) produced worse binary code, and using stupid pointer tricks compiled to the fastest implementations that you would have written were you programming in asm in the first place. As stated earlier, C uses call by value when passing parameters to a function, but adding an extra layer of pointers can be used to simulate call by reference. For as long as dereferencing yields a character with a value other than zero, we increment count and return it at the end. > Else you get this shit: https://github.com/Davidslv/rogue/blob/master/rogue.h#L470. What language are embedded RTOS written in? Just remembering that everthing is RPN is usually enough to clear my mind. But this really has nothing to do with your observation that when you increment a pointer to pointer to char that you see 8 bytes, and not 1 byte. A lot of the new-hires Ive mentored have had limited to no experience with pointers because the Java based courses they took in college did not teach them anything about them, and then when they got some stuff in C++, templates and smart pointers hid the details. Causing it to point to a memory location skipping N bytes (where N is size of pointer data type). If you dont teach how to use pointers, they wont get used correctly. In other words, by being tempted to write int* p you set yourself and other people up for thinking they could just add a ,q and get two pointers. With pointer arithmetic, we do the exact same thing, except the array index becomes the integer we add to the pointer, (numbers + 4). It is an integer pointer so it has incremented by 2 bytes, when it was 200 then it became 202, if it is float pointer then it will move by 4 bytes because float takes 4 bytes and if it is a character pointer then it will move by 1 byte so, how many bytes pointer will move forward, it depends on the data type of that pointer. It isnt math, so how do I know if it would be easy for a computer? Yes it is, there are still places where C is the best choice. Now there are lots of programmers who never learned anything else except maybe Python or Ruby, and those programmers may be quite competent, but they are limited in their understanding of the underlying structure of the data they work with and are generally unaware of the efficiency, or lack thereof, of the data either at rest or in flight. C always uses call by value when passing parameters to a function, so calling strlen(ptr) will create a copy of ptr when passing it to the function. Ada did that from its inception, but that can really slow down code, so its generally disabled after testing for production code. I spot 3 cases of undefined behavior, 2 silly ones and 1 serious. However, it also makes things possible that are otherwise slow/impossible to do. We know that increment operation is equivalent to addition by one. The allocation function alloca() and the pitfalls inherent in using it, and maybe some guidelines of how and when it can be used safely would be a public service. mrpendent has updated the project titled the C.A.T.. mrpendent has added details to the C.A.T.. mrpendent has updated the log for The Grimoire Macropad. delete. the value will become 1, and hence the pointer will point to the memory location 1. Placement of the asterisk in pointer declarations. f(NULL); //the bool variant will be called! How does compiler know how to increment different pointers? Above code is likely to do what you want, even though that last memcpy already triggers undefined behavior, because it copies invalid value to a pointer (that is enough for it to be UB). int *ptr = malloc(100); How to check for #1 being either `d` or `h` with latex3? So if that last memcpy is inside if, and compiler can prove UB occurs if condition is true, it may just assume condition is never true and optimize whole if away. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Keep in mind that we dont have any allocated memory beyond values size, so we shouldnt dereference cptr1. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Of the 40 software engineers that we have, only about 5 really have a perfect understanding of C/C++, and I would call experts. >Since an int was four bytes, we can fully fit two of them in the 8 bytes offset, therefore the subtraction will output 2. Of course if you write int* p; then maybe you should add the stupid rule to compliment that style Are we saying the same thing ? >int *iptr1 = 0x1000; With int taking up 4 bytes, numbers is 40 bytes in total, with each entry 4 bytes apart. Since an int was four bytes, we can fully fit two of them in the 8 bytes offset, therefore the subtraction will output 2. next, let's take a look at the first x86 instruction in my program: 1. Not really In the definition of glib it is #define NULL (void *)0. The . Also, checkout godbolt.org. When a pointer is subtracted with a value, the value is first multiplied by the size of the data type and then subtracted from the pointer. (BTW, the test program ran in a different physical memory with a different memory controller that wasnt affected by the ECC switch.). For performance, this often matches the width of the ref type, yet systems can allow less. When a pointer is decremented, it actually decrements by the number equal to the size of the data type for which it is a pointer. There is a course much later, perhaps for graduate students that tries to teach them C. I am told that many students have big problems with it. Since it is undefined, your compiler is free to optimize it away (or do anything it likes, really). if ( ptr != NULL ) And as they taught me in school, O(1) ~ 0. Are there really people who have this much trouble with C? Incrementing an int pointer will increase its value by four because the next valid integer address is four bytes from the current location. Its 0. nullptr (in the newer standard) is a proper pointer. 2nd operation: p-: The problem as it stands now is that even some of most prestigious computer science programs, by chasing the latest Language Du jour, are turning out dilettantes who themselves keep chasing, and forever; why not?thats the example thats been set for them. And thus may be implicitly casted by the compiler. The pointer will be increased or decreased by N times the number of byte (s) of the type of the variable. Suppose that the uint8_t pointer p points to some location in memory and you want to obtain the next four bytes and interpret them as an int32_t value; then you could try these: int32_t N = *p; // NO. NULL is a macro guaranteed to give a null pointer constant. If p1 is an integer pointer with an initial value, say 2800, then after with an initial value, the value of p1 will be 2902, & not 2801. and because in this case it is designed to point to char, each address differs by one byte. Increment void pointer by one byte? Note that. And as an added benefit, anybody else that wants to reuse your code would have to port it back to plain C, so you wont get pestered with a bunch of email questions or pull requests. Youd think theyd been touched inappropriately, or something. Iterate the for loop and check the conditions for number of odd elements and even elements in an array. Multiple variables defined on 1 line is pretty much a no-go except for simple primatives. [] and . For Example:Two integer pointers say ptr1(address:1000) and ptr2(address:1004) are subtracted. Just strive for clarity and whatever you do dont invent some offbeat style, read enough good code that you can adopt the style used by the masters. The hardware implementation can have some other encoding, but your code is always allowed to compare with 0. Doing anything is C/C++ is generally 3 to 10 times harder then in a protected language. Subtracting any number from a pointer will give an address. C Pointer Subtraction. The author wrote the very readable book while employed at SUN Microsystems to work on compilers. Sorry to say that, but your edit did no good: >Subtracting two pointers will result in another pointer of the same type, so the sizeof operation will print the size of a regular pointer, 8 bytes in this case. The proof indeed is in the pudding, but most of the pudding was in fact written in C. Even programs that claim to be written in other languages often have the majority of their actual functionality handled by C libraries, or even by parts of the operating system written in C. I doubt a program that generates a beep is going to be trivial! or NUL, not to be confused with the NULL pointer, is simply ASCII character 0x00 or '\0'. Subtracting two addresses lets you compute the offset between the two addresses. C++ Pointer Arithmetic. On incrementing, a pointer will point to the memory location after skipping N bytes, where N is the size of the data type(in this case it is 4). Programming in C / By Code Window A pointer is an address, means it is a numerical value. else "Signpost" puzzle from Tatham's collection. int c = *ptr; Thats the sad reality of these days. Unclear as to OP's goal at this point. a) its strange to see it that way so you pay more attention to the expected value ), From http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.htm, Dereferencing a NULL Pointer: contrary to popular belief, dereferencing a null pointer in C is undefined. I also like to keep the .h files that Im using open, to make frequent reference to the API. p1=p2+2; &so on. My biggest gripe about the language is the preprocessor; the macro facility is not very powerful compared to whats available in most macro-assemblers multi-line function like macros that are able to analyse and even decompose their arguments and construct code based on them. Another typical thing we do with pointers is dereference them. Are there machines, where sizeof(char) != 1, or at least CHAR_BIT > 8? I was reading recently that some organization (maybe Facebook) enforces a javascript style where you write if ( false == x ) rather than if ( x == false). So if we looked through a string and find the NUL, we know we reached its end. I want to process this data somewhere else in my program so I want to use pointers. Yep. The beauty of pointers is that we can cast them to any other pointer type, and if we do so during an arithmetic operation, we add plenty of flexibility in comparison to array indexing. My current solution is opt = (tcp_option_t *) ( (char*)opt+1); but it is a bit troublesome. Its the compilers job to make it machine-readable, its the programmers job to make it human-readable. Find centralized, trusted content and collaborate around the technologies you use most. 2. The only difficult thing with multiple languges is recalling which syntax or format youre using. Ive never had problems with most computer languages. 8. In the 98/99 school year I was taking first year programming at a community college and it was the last year that they taught it using C/C++; the next year all the same classes were Java, and the only C class was a 300-level Operating Systems class where you wrote your own simple OS. Does methalox fuel have a coking problem at all? If you dont know what the compiler is doing, you really need to read up before using it. As long as you only use features that are cosmetically different, and dont use anything substantive, youll even get the same code size! I am saying that it will sizeof((int *)0x1008 (int *)0x1000)) is 8, but sizeof((int *)0x1001 (int *)0x1000)) is 8 as well. A foo* pointer points to the first element of an array of foo objects. I still use it with some of my application development when I want speed. It is always good practice to initialize otherwise uninitialized pointers with NULLto let the compiler know, but it helps us too.
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