(I find javascript to be a fine language by the way, but boy do people get worked up over things that To subscribe to this RSS feed, copy and paste this URL into your RSS reader. C is not C# Asking for help, clarification, or responding to other answers. . This site uses Akismet to reduce spam. Cs #define is not sophisticated at all, even with __VA_ARGS__; but I still love the language and use it most of the time. There are a lot of people who never learned to program in C, even though they program in C++ all the time. Whatever the hardware platform, I cant remember any where a value could represent an invalid memory address. Are there any better ways? A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. Return to main()s argv parameter, which we use to retrieve the command line arguments we pass to the executable itself. For a more modern and relevant example of how to effectively use pointers to pointers on modern hardware, check out the OpenSSL API. 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. is use appropriate variable types and pointer typecasts. Presumably C programmer knows to write their condition so that it would never result in UB, so this optimization can be made at compile time already. Connect and share knowledge within a single location that is structured and easy to search. This always cures me. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Another typical thing we do with pointers is dereference them. Share And I know that C doesnt try to protect the programmer from themselves. This was a nice accidental feature with the Beaglebone Black. dont give compiler errors) and have defined semantics. Pointer Addition/Increment. NULL is not (void*)0. If ptr points to an integer, ptr + 1 is the address of the next integer in memory after ptr.ptr - 1 is the address of the previous integer before ptr.. Instead, I try to re-write the code so that I dont rely on precedence, and if I still think it might be nice to make use of precedence, I consult the chart every time. The compiler makes no assumption that NULL (ie address 0) has any special meaning The result is generated by calculating the difference between the addresses of the two pointers and calculating how many bits of data it is according to the pointer data type. 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. To summarize our second part on pointers in C: pointer arithmetic happens always relative to the underlying data type, operator precedence needs to be considered or tackled with parentheses, and pointers can point to other pointers and other pointers as deep as we want. I have a structure tcp_option_t, which is N bytes. The third, fourth, ninth, etc. This follows the same principle as the null-termination of strings, indicating the end of the array. Does methalox fuel have a coking problem at all? The sizeof operator will output its size accordingly, for example 8 bytes. Yep. With int taking up 4 bytes, numbers is 40 bytes in total, with each entry 4 bytes apart. With a few exceptions: we can always cast to/from void* and we can always cast from pointer-to-type to char*. If youve put in a couple of years of assembly, C is clear, simple and you are greatfull for that. never matter to me). Improve INSERT-per-second performance of SQLite. The subtraction of two pointers gives the increments between the two pointers. Thats one of my favorite things about it. When a pointer is incremented, it actually increments by the number equal to the size of the data type for which it is a pointer. Note that all this applies only to already declared arrays. I understand why, and know the history well, but that doesnt change the fact that both int *t; and int* t; define a variable named t with type pointer to int. A union is a type consisting of a sequence of members whose storage overlaps (as opposed to struct, which is a type consisting of a sequence of members whose storage is allocated in an ordered sequence). Step 3:Initialize the count_even and count_odd. For Example:Two integer pointers say ptr1(address:1000) and ptr2(address:1004) are subtracted. A pointer is part of the type IMHO. I think a basic understanding of pointers should be required as part of any computer science curriculum even when its not part of day-to-day programming for a large percentage of professional programmers and software engineers. The reason that I would give for so many programmers who leave out notes, and poorly code, fail to provide much evidence of testing, JMP Label: Spaghetti style code (<-this is my offense, I used to go crazy not knowing enough about creating my own data types or return(Other-than NULL), so Id just JMP Label, and hope it wasnt ever caught by anyone I wanted to impress), the reason I would cite is People get stung by the precedence of *. It is only a coders convention rule to use NULL to represent an invalid address. C and C++ are different languages. I saw this article and was expecting some pointers about learning C. Guess I was wrong seems all the pointers are going everywhichway.. And as they taught me in school, O(1) ~ 0. How to combine several legends in one frame? Commonly defined as preprocessor macro (void *) 0, we can assign NULL like any other pointer. Notice that at the end of the file are a bunch of one-line declarations of various types that appears to be how less-stable declarations are being written. >printf(%ld\n, sizeof(iptr2 iptr1)); Of course if you write int* p; then maybe you should add the stupid rule to compliment that style char c2 = ++*ptr; // *ptr = *ptr + 1 ; c2 = *ptr; A good rules : A good C coder is NOT the the one who knows operator precedence by heart. Where do stupid policies like that come from? As you said, in some systems, address 0 will be valid, and the null pointer will not be 0, since the null pointer should not be equal to a valid pointer. compilers optimiser are (most-)always better than you are. Pointers can be incremented like. The only people graduating Universities who I would expect to know C would be people with EE degrees. As a side note, the string manipulation happens and stays inside that function. The type specifier for a union is identical to the struct . Find centralized, trusted content and collaborate around the technologies you use most. ARR37-C-EX1: Any non-array object in memory can be considered an array consisting of one element. Is it good? which does tempt you to think that int* p, q; would give you two pointers. NULL is 0, which is the null pointer constant, but the null pointer isnt necessarily 0. What does the power set mean in the construction of Von Neumann universe? For Example: if an array named arr then arr and &arr[0] can be used to reference array as a pointer. There are two ways in which we can initialize a pointer in C of which the first one is: Method 1: C Pointer Definition datatype * pointer_name = address; The above method is called Pointer Definition as the pointer is declared and initialized at the same time. *((float *) ((char *) (struct_array + 2) + 11)) looks fishy and is likely a violation of strict aliasing. C seemed like a gift after that. Generic Doubly-Linked-Lists C implementation, English version of Russian proverb "The hedgehogs got pricked, cried, but continued to eat the cactus". rev2023.4.21.43403. But I expect to get back to Java soon. [] and . 2. Every language has things you can bicker and squabble over. On the other hand, people who insist on cuddling if with the open paren and putting extra space inside the parens, as in if( expression ) should be shunned. As a result, the second output will show the full 8 bytes of the offset. I was reading recently that some organization (maybe Facebook) enforces a javascript style where you write if ( false == x ) rather than if ( x == false). new. 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. Pointer challenges galore so that all you C is memory. I appreciate this series of posts, not because Im learning new things about C pointers, which Im not (see my other replies), but because its good to see both C advocacy and a desire to take the mystery out of pointers. Some may issue a warning. Dereferencing cptr2 on the other hand will be fine, and will essentially extract the fourth byte of value. And like any other number, we can perform some basic arithmetic operations with them. One of my philosophies for evaluating opinions on this stuff; the people blaming the C language are always wrong, and the people blaming the programmer are often right; but sometimes they blamed the programmer for the wrong thing. Personally, I dislike alloca() because it makes static stack analysis useless, but Ive used it on some projects anyway. C doesnt really know the concept of an actual string data type, but works around it by using a null-terminated char array as alternative. How does compiler know how to increment different pointers? Compilers/optimizers have been intelligent enough for years now to make readable code as much efficient as cryptic code. Not the answer you're looking for? 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. Counting and finding real solutions of an equation, Generate points along line, specifying the origin of point generation in QGIS, Effect of a "bad grade" in grad school applications. The address it references is therefore still the same, but the original pointer remains unchanged. Pointer increment operation increments pointer by one. I agree with the bulk of it. And thus may be implicitly casted by the compiler. It should be noted that any expression used as the operand of sizeof is not evaluated at runtime, so something like uint64_t* ptr3 = NULL; printf(sizeof(uint32_t) = %u\n, (unsigned int)sizeof *ptr3); will print 4 on any CPU (64 or 32 bit); it doesnt matter that ptr3 is NULL, since it is not accessed in that printf() statement. increment. The C++ operator ____ is used to destroy dynamic variables. The C++ language allows you to perform integer addition or subtraction operations on pointers. I write provably correct code. This takes only 1 byte! Of course dereferencing an invalid pointer is UB. 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. These single-byte memory cells are ordered in a way that allows data representations larger than one byte to occupy memory cells that have consecutive addresses. You can make it part of a typedef, so its clearly part of a type. The operations are: Increment: It is a condition that also comes under addition. Since such a variable cannot be larger than 32 bits in size, the difference must also fit into 32 bits. The author wrote the very readable book while employed at SUN Microsystems to work on compilers. The purpose of a pointer ( eg char * ) is to store an address to an object of the same base type, in this case char. For example, *p.f is the same as *(p.f) as opposed to *(p).f, Also, int *q[] is int *(q[]) as opposed to int (*q)[]. For Example: If an integer pointer that stores address 1000 is incremented, then it will increment by 4 ( size of an int) and the new address it will points to 1004. This issue is on any Real Mode processors. They did that because the local University had already switched to Java a couple years earlier, and most of the programming students were intending to transfer. We know that increment operation is equivalent to addition by one. The hardware implementation can have some other encoding, but your code is always allowed to compare with 0. Arithmetic operation on type char seems like ordinary arithmetic because the size of char type is 1 byte. Making statements based on opinion; back them up with references or personal experience. Since it is undefined, your compiler is free to optimize it away (or do anything it likes, really). As integer value occupies 2-byte memory in 32-bit OS. The ++ increment operator adds 1 to its pointer operand. That is, it will increment the pointer by an amount of sizeof (*p) bytes, regardless of things like pointee value and memory alignment. Step 2 :Declare the pointer variable and point it to the first element of an array. Same as mjacobs. I still use it with some of my application development when I want speed. Or something of the sort. The Binary Operations extension functions apply to byte arrays, to provide an easy and hopefully fast way to use the basic binary operators. For as long as dereferencing yields a character with a value other than zero, we increment count and return it at the end. Which of the following arithmetic operations is allowed on pointer variables? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. There is no language that guarantees bug-free code; that is the responsibility of the engineers who design, write, and test the code. this instruction, pushq , pushes a new value onto the top of the stack . Use your high priced time to make code maintainable. Looking for job perks? It returns true for the valid condition and returns false for the unsatisfied condition. When you increment or decrement a pointer, it will always increment by the number of bytes occupied by the type it points to. The language definition says that but I never seen any system/compiler where NULL is not of value 0 ), 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. C does have some problems, but theyre not disqualifying. Usually the bug isnt because you remembered wrong, but because since you presumed your memory to mean you got it right, you then wrote an excessively complex construction where it is easy to slip and write it out wrong. Fixed now, thanks. Are there really people who have this much trouble with C? 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 then we have done some very careful selection to get those guys. Thats the sad reality of these days. Taking both prefix and postfix increment into account, we end up with four different options: If youre not fully sure about the operator precedence, or dont want to wonder about it every time you read your code, you can always add parentheses and avoid ambiguity or enforce the execution order as we did in the fourth line. At 32, Im not *that* old, am I? Yes, there are people, professional software engineers, that fail to grasp this basic stuff. Youre at least the second person to insist that Im Nietzsches Uberman, but Im not really convinced. 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! >int *iptr1 = 0x1000; Kuba Sunderland-Ober wrote a comment on Multislope ADC. This is totally untrue. but it is a bit troublesome. :-). But I started with BASIC and then learned assembly. When we declare char **ptr, we declare nothing but a pointer whose underlying data type is just another pointer, instead of a regular data type. Agreed. ++ increments a char * by 1. Then, I can consult the precedence chart when writing the macro, and not having memorized it then has O(1) cost. Step 2 : Now, check the condition by using comparison or relational operators on pointer variables. A string is an array of char objects, ending with a null character '\ 0 Not quite. C Pointers and Strings with Examples. are much easier. It is 8 bytes which are taken by the type of the result of pointer subtraction (most probably size_t). the cast, although allowing a compile without complaining, is simply masking a problem. Will cause the pointer p1 to point to the next value of its type. The compiler replaced the expression sizeof *ptr3 with a constant value that, due to the typecast, will be an unsigned integer with the value 4. Are there machines, where sizeof(char) != 1, or at least CHAR_BIT > 8?
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