Just remembering that everthing is RPN is usually enough to clear my mind. 1. pushq %rbp. Im sure they meant to put an equals sign in it.. Ive been using C since the day it came out (on the PDP-11..). It is still the BEST, fastest method for getting closest to the hardware, except for assembly language. 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. `if( argc == 5 ) { strcpy ( buffer, argv[4] ); }`, The latest version of Visual Studio insists on int * i, which is like the worst of both worlds. As a side note, the string manipulation happens and stays inside that function. 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. C seemed like a gift after that. As a general rule, C doesnt go far out of its way for syntactic sugar. int* pc, c; Here, a pointer pc and a normal variable c, both of type int, is created. BASH, C, Python and Javascript are the languages I use most now. p1=p2+2; &so on. else Is there a way to make it move only 1 byte? The compiler determines the type of the operand, in this case ptrdiff_t (the difference of two pointers) and determines the size of a value of that type, which is 4 on machines with 32 bit addresses, 8 on machines with 64 bit addresses, and 2 on machines with 16 bit addresses. This site uses Akismet to reduce spam. rev2023.4.21.43403. i.e., when we increment a pointer, its value is . :-). As a bool is a type of integer, and NULL is 0, also an integer. 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. And, variable c has an address but contains random garbage value. 4. Nobody uses the second style you have adopted, so you should dump it and get with the program. +1 to you. All array subscription operations in C get decomposed to pointer arithmetic anyhow; and when dealing with either multi-dimensional arrays or arrays of structs, where you want to access an individual member *that is only specifically known at run-time*, pointer arithmetic is way more intuitive. . This always cures me. this instruction, pushq , pushes a new value onto the top of the stack . And like any other number, we can perform some basic arithmetic operations with them. C++ works great for writing microcontroller code too, even for 8 bit devices with only a few dozen kb of program + data memory. Incrementing pointer to pointer by one byte, How to create a virtual ISO file from /dev/sr0. Take that away, and you end up (almost) with reference types, which, while having some advantages, are way less powerful than raw pointers. 1 here the j address is incremented by four bytes. With pointer arithmetic, we do the exact same thing, except the array index becomes the integer we add to the pointer, (numbers + 4). Causing it to point to a memory location skipping N bytes (where N is size of pointer data type). More and more Im faced with having to somehow shoehorn CORBA IDL generated datagrams, with their alignment padding and endian-flexibility, with safety critical functions that have fixed endianess and different, unpadded or minimally padded field alignment. Actually using pp, which now has invalid value, has increasing chance of messing things up. Note that the array-to-pointer decay happens only once to the outermost dimension of the array. Like pointer addition, we can subtract a value from the pointer variable. Assuming the compiler and the rest of the toolchain is stable and trusted, it is quite possible to write very solid, bug-free, secure and robust code in just about any language. Embedded real-time systems are not the place for Java, though having a Java interpreter in a non-critical partition is ok at times. Now I work in a team of ~10 engineers, and about half of them can do C/C++ on a workable level. Ive never had problems with most computer languages. I would consider agreeing, except that they took the time to alphabetize the list and it is presumably the most stable of the declarations and wont change. are old wives tales that may have been true with respect to the microcontrollers available in the 1980s and 90s but not so applicable to modern devices and recent compilers, It all depends on how you write the code. Trying anything other than addition with an integer, or subtraction with either an integer or another pointer of the same type will result in a compiler error. Its the compilers job to make it machine-readable, its the programmers job to make it human-readable. 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. You are on the money here. The one proverbial exception to the rule that pointers are just memory addresses is the most (in)famous pointer of all: the NULL pointer. 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. Of the 40 software engineers that we have, only about 5 really have a perfect understanding of C/C++, and I would call experts. Can I use my Coinbase address to receive bitcoin? I agree with the bulk of it. The asterisk denotes that this variable is a pointer, after all, which is type information, and should thus be associated with the datatype. How does compiler know how to increment different pointers? The beauty of pointers is that we can cast them to any other pointer type is incorrect. There are four arithmetic operators that can be used on pointers: ++, --, +, and -. Is it good? Is it safe to publish research papers in cooperation with Russian academics? I was reading recently that some organization (maybe Facebook) enforces a javascript style where you write if ( false == x ) rather than if ( x == false). VAR_INPUT. Else you get this shit: https://github.com/Davidslv/rogue/blob/master/rogue.h#L470. dont give compiler errors) and have defined semantics. C is not C# What REALLY happens when you don't free after malloc before program termination? An array name acts like a pointer constant. For performance, this often matches the width of the ref type, yet systems can allow less._ (per @Chux in comments) but de-referencing these locations could, and likely would lead to undefined behavior. I learned myself C and later C++. The smallest incremental change is a requirement of the alignment needs of the referenced type. Now is the tricky part. Because of how pointer arithmetics is defined in C. Let's assume we have an imaginary type 'foo'. Lets see how this looks in practice by rewriting our previous example accordingly. 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. Are there any better ways? How a top-ranked engineering school reimagined CS curriculum (Ep. Output of the program | Dereference, Reference, Dereference, Reference. The operations are: Increment: It is a condition that also comes under addition. No, Ive definitely seen code in both. The value of at most one of the members can be stored in a union at any one time. Are there any better ways? What does the power set mean in the construction of Von Neumann universe? Coming back to arrays, weve seen earlier how pointer arithmetic and array indexing are closely related and how buf[n] is identical to *(buf + n). 2nd operation: p-: It is only a coders convention rule to use NULL to represent an invalid address. The other way around, &ptr gives us the pointers address, just like with any other pointer, except the address will be of type char ***, and on and on it goes. int *ptr = NULL; Even if you write : Or better yet, He w are you going to afford to undo the mess Ive made, without MY help? Once again, its mostly a question of operator precedence and how generous we are with parentheses. Now, ptr++ is equivalent to ptr = ptr + 1. Will cause the pointer p1 to point to the next value of its type. I'd suggest you to create a pointer of char and use it to transverse your struct. could also be written: By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. takayuki.kosaka has updated components for the project titled CryingBaby (day 0). Another typical thing we do with pointers is dereference them. 12 bytes. The C++ language allows you to perform integer addition or subtraction operations on pointers. Are all data pointers the same size in one platform for all data types? Strict rules may be bad, but that includes strict rules against strict rules! It depends. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. What language are embedded RTOS written in? a) Declare f, g, and h to be of type pointer to int b) Declare an array nums of type int with 5 elements and the value 16, 9, 8,7 and 23. c) Declare s, t and v as integers. Whether we access argv via array indexing or pointer arithmetic, the output will be identical. Increment (++) and Decrement (- -) 2. Another thing we can see is a NULL pointer at the very end of argv. There are four operations that can be done on a pointer. I use many other languages for many different things, but for down and dirty hardware hacking, C is the language of choice. Content Discovery initiative April 13 update: Related questions using a Review our technical responses for the 2023 Developer Survey. Otherwise it would not be considered a practical or safe thing to do. 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. mrpendent has updated components for the project titled The Grimoire Macropad. Step 1 : Initialize the integer values and point these integer values to the pointer. 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. or Generic Doubly-Linked-Lists C implementation, English version of Russian proverb "The hedgehogs got pricked, cried, but continued to eat the cactus". regarding NULL: it is a special indicator to the compiler char buf[] decays to char *buf, and char buf[][] decays to char *buf[], but not char **buf. Yep. Phil is correct. If you find an implementation where the size of a pointer to pointer variable contains only 8 bits, (i.e. Asking for help, clarification, or responding to other answers. 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. are much easier. new. 327 likes, 4 comments - MCM | Aptitude & Coding Trainer | Memer | Mentor (@middleclassmohan) on Instagram: "C Roadmap. Ive even seen some code that uses the cursed style: NULL is not (void*)0. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. If you dont teach how to use pointers, they wont get used correctly. 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. For simplicity, lets pretend value is located at address 0x1000, so we will get the following output: We can see a clear difference between those two additions, which is caused by Cs operator precedence. Share At the very least actually mention strict aliasing! Strings. In the latter case, the result will be the number of elements of the pointers underlying data type that fully fit in the memory area between the two pointers. Im always working in assembler when I really get deep into debugging. b) you cant make mistakes of forgetting the second = (eg. What does "up to" mean in "is first up to launch"? A string is a one-dimensional array of characters terminated by a null(\0).When we write char name[] = "Srijan";, each character occupies one byte of memory with the last one always being \0.. Yours is much more apt; almost 100%elegantlycorrect. Pointer arithmetic is, IMHO, one of the greatest strengths of C pointers. The type of (iptr2 iptr1) is an int. 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. Id have preferred not to have chars, pointers to chars, arrays of chars, and pointers to arrays of chars all mixed up in the same declaration. The difference between address is 4 bytes. This follows the same principle as the null-termination of strings, indicating the end of the array. 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. Did the drapes in old theatres actually say "ASBESTOS" on them? Other advanced topics, like how to support poymorphism in C by following some simple rules when defining data structures also involve an understanding of pointers. 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. This is a C vs C++ difference. I dont see any comipler will catch this as an eror. Any pointer assigned to a null pointer constant is henceforth a null pointer. The language definition says that but I never seen any system/compiler where NULL is not of value 0 Dont teach this to newbies, it is plain dangerous code, especially in the gcc era we live in. Advanced C Programming De Anza College Cupertino, CA. As a result, the second output will show the full 8 bytes of the offset. 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. The CPU address (as opposed to the location in the actual DRAM) of our data word changed as we toggled the ECC mode; with ECC-off, the CPU address was twice what it was with ECC-on. The type specifier for a union is identical to the struct . char c2 = ++*ptr; // char temp=*ptr ; ++temp ; *ptr = temp ; c2 = temp; As the ++ applies to (*ptr) it also increments the value pointed before assigning the value to c2. 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 always cast your memory access to a char pointer and have it raw, using offsets. The -- decrement operator subtracts 1 from its pointer operand. Therefore, if the integer pointer has a value of 62fe30, incrementing the pointer will result in a new address of 62fe34. And since any value other than 0 is evaluated as true in C, we can write it even shorter as if (ptr). Is there a generic term for these trajectories? delete. Subtracting any number from a pointer will give an address. // I can use ptr Yeah, it is a bit crazy. I want to be able to increment the pointer address in an easy way. All other pointer casts are most likely severe but subtle bugs that violate strict aliasing. If i were to make k address to be incremented by 4 bytes, how do i typecast k to be int now on in the expression so that it will increment the address by 4 bytes? I want to process this data somewhere else in my program so I want to use pointers. Every few years I entertain thoughts of screwing around with forth. The * is part of the type, not the variable name. In most cases this results in strict pointer aliasing violations. pushing a value onto the stack. Im your age. (And pedantically. Find centralized, trusted content and collaborate around the technologies you use most. I know it because this summer I worked in a C analyzer and found that detail. 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 How you choose to cuddle the asterisk for pointer types is your own affair, and is one of the nearly religious issues that make no real difference to the quality of the code. If I have unix running and ruby or python at my fingertips, there are few things I would ever do in C. It is all about picking the right tool for the job. Pointers to pointers were called Handles and allowed their memory management routines to move things around in not-quite-virtual memory without losing track of them: the first dereference was to the memory map, the second to the current location of the block of memory. (My bad if this already came up just too much to read. NULL is 0, which is the null pointer constant, but the null pointer isnt necessarily 0. I have a structure tcp_option_t, which is N bytes. typo in this one in the box in the article, char c3 = *ptr++; // c3 = *ptr; ptr + ptr + 1;. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The other languages are still not that useful from my point of view apart from small simple applications, or something specific ie php is great for web sites and scripting, ruby has uses as well. So yeah, I would expect all University-trained programmers, at least in the US, to know little to no C at all, unless they learned it outside of school. It should always go with the variable, because: Just about every codebase Ive worked on has had policies discouraging the declaration of multiple variables per statement. To understand why having any UB is indeed UB: compiler is free to decide that the effect of the code, which can be proven to have UB, is nothing, or is never reached. ptr++ is equivalent to ptr + (sizeof(pointer_data_type)). My phone's touchscreen is damaged. I spot 3 cases of undefined behavior, 2 silly ones and 1 serious. Not all written the way Id do it, but it doesnt even disgust me. Step 2 :Declare the pointer variable and point it to the first element of an array. The result of ++p and --p is the value of p after the operation. Even worse. I mean, what does it even mean to beep?! Learn how your comment data is processed. Parabolic, suborbital and ballistic trajectories all follow elliptic paths. etc etc The main thing I HATE about people writing production systems in some other languages (ie java) is that you pretty need one machine for each application, and sometimes even that isnt enough. Are there really people who have this much trouble with C? It used to be Pascal, but now it is Java. For the first 10 years or so of my C programming career, I still thought in PDP-11 assembler when writing C. I worked for a company developing a C compiler and tools in the later 1980s (Mark Williams Co.) for the Atari-ST (M68000). Returns a pointer to the first occurrence of character in the C string s. The terminating null-character is considered part of the C string. Increment void pointer by one byte? ptrdiff_t is printed with %td, not %ld. Left for a proper University . And contrast the coding rules for the Gnu project. Below is the implementation to illustrate the Subtraction of Two Pointers: Pointer Arithmetic on Arrays:Pointers contain addresses. The result of such a subtraction will be of type ptrdiff_t, a platform dependent integer type defined in stddef.h. C is not just assembly with different syntax! I definitely disagree with the idea you should memorize precedence rules, or be expected to, or make use of such knowledge. People here like to bicker and squabble. I do it in C. I know what Im doing, and have been doing it for many many years. I used the worst possible example to verify my false assumption. Its 0. nullptr (in the newer standard) is a proper pointer. Of course if you write int* p; then maybe you should add the stupid rule to compliment that style 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. On whose turn does the fright from a terror dive end? Does methalox fuel have a coking problem at all? Gen Tojos Teeth: Morse Code Shows Up In The Strangest Places, Keebin With Kristina: The One With The Music Typewriter, Vintage Computer Festival East Was A Retro Madhouse, Hackaday Podcast 216: FETs, Fax, And Electrochemical Fab, This Week In Security: Session Puzzling, Session Keys, And Speculation, Making Hydrogen With Solar Energy, With Oxygen And Heat A Bonus, When Your Smart Light Switches Stop Working, Build Your Own, The Worlds First Agricultural Right To Repair Law, Building A Receiver With The ProgRock2 Programmable Crystal, Puya PY32: The Cheapest Flash Microcontroller You Can Buy Is Actually An ARM Cortex-M0+. Second, if you really want to see pointers-to-pointers in use, take a look at the Macintosh API at least the early versions. The best description of C I ever heard was machine independent assembly. Or something of the sort. But for simplicity and understanding we can also use %u to get the value in Unsigned int form. By using our site, you It isnt math, so how do I know if it would be easy for a computer? By the time the addition is performed, iptr is already a char *, resulting in a three byte offset. The operations are slightly different from the ones that we generally use for mathematical calculations. The payoff is huge. 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. It should be: How to make a pointer increment by 1 byte, not 1 unit. No, that's exactly the right way to do it. increment. Then, I can consult the precedence chart when writing the macro, and not having memorized it then has O(1) cost. In that case I highly recommend getting a boxed set of The Art of Computer Programming series, because you can point right at it, and say, its mine! and many people will be impressed, not only by the size of the volumes, but by the authors name, and the obvious gravitas of the binding. What REALLY happens when you don't free after malloc before program termination? For a more modern and relevant example of how to effectively use pointers to pointers on modern hardware, check out the OpenSSL API. 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. https://developer.gnome.org/glib/stable/glib-Standard-Macros.html#NULL:CAPS. Why is 'this' a pointer and not a reference? This is especially tricky in C++ with function overloading: NULL is a relic of the past, and should die. So when two 64 bit pointers are subtracted, the complier will use a 32 bit subtract instruction. >printf(%ld\n, sizeof(iptr2 iptr1));
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