C has been in use and under development since the 70s. To simplify the logic behind this, think of pointer arithmetic the same way you think about array indexing. But in that case, it is probably a lot more trivial in C than in other languages, unless you used a library. As a consequence, a string of length n requires an array of size n + 1 bytes. Another wrong one: If you want to sneak subtle bugs into a codebase, leaving out the parentheses and testing the readers attention to operator precedence is a good bet. Keep in mind that we dont have any allocated memory beyond values size, so we shouldnt dereference cptr1. 0 is evaluated as false, not true. 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. Since an int was four bytes, we can fully fit two of them in the 8 bytes offset, therefore the subtraction will output 2. Subtracting two addresses lets you compute the offset between the two addresses. 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. Are there machines, where sizeof(char) != 1, or at least CHAR_BIT > 8? which does tempt you to think that int* p, q; would give you two pointers. I still use it with some of my application development when I want speed. Strings. Checking if (ptr != NULL) lets us easily determine whether a pointer has a valid value yet or not. Find centralized, trusted content and collaborate around the technologies you use most. We know that increment operation is equivalent to addition by one. Looking for job perks? https://developer.gnome.org/glib/stable/glib-Standard-Macros.html#NULL:CAPS. With pointer arithmetic, we do the exact same thing, except the array index becomes the integer we add to the pointer, (numbers + 4). In addition various conceptual mistakes. 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. int* p; Ive always been fond of the second one, with the asterisk next to the datatype. If just having fun and challenging yourself is your goal, go with Haskell. We are still trying to undo the damage caused by theearly treatment of modularity as a language issue and, sadly,we still try to do it by inventing languages and tools. David L. Parnas. 2nd operation: p-: I only ever heard, Assembly with semicolons. Yes, there are people, professional software engineers, that fail to grasp this basic stuff. 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.. An Uncommon representation of array elements, Dynamic Memory Allocation in C using malloc(), calloc(), free() and realloc(), Subtracting two pointers of the same type. char c2 = ++*ptr; // char temp=*ptr ; ++temp ; *ptr = temp ; c2 = temp; ptr++ is equivalent to ptr + (sizeof(pointer_data_type)). 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). So when two 64 bit pointers are subtracted, the complier will use a 32 bit subtract instruction. Pointer Addition/Increment. If targeting 32bit addressing, then the size of the pointer indicates it can point to 4,294,967,296 different locations (or if 64 bits to 18,446,744,073,709,551,616 locations.) The only reason for such a policy is because people get stung by thinking that the * goes with the type instead of the variable. Lets see how the rules apply if we cast an int * to a char * and add 3 to it. Phil is correct. As integer value occupies 2-byte memory in 32-bit OS. An array name acts like a pointer constant. I agree with the bulk of it. Comparison operators on Pointers using array : In the below approach, it results the count of odd numbers and even numbers in an array. The OOP model is pretty cool and just reading the tutorials is enlightening. There are basic rules for every language. Compilers/optimizers have been intelligent enough for years now to make readable code as much efficient as cryptic code. 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. NUL, not to be confused with the NULL pointer, is simply ASCII character 0x00 or '\0'. Ada did that from its inception, but that can really slow down code, so its generally disabled after testing for production code. Yours is much more apt; almost 100%elegantlycorrect. Every language has things you can bicker and squabble over. c pointers increment 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. . C Pointer Subtraction. 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. I write provably correct code. Since the size of int is 4 bytes, therefore the increment between ptr1 and ptr2 is given by (4/4) = 1. Youre at least the second person to insist that Im Nietzsches Uberman, but Im not really convinced. So sizeof(iptr2 iptr1) should be equal to sizeof(int). But I started with BASIC and then learned assembly. How to check for #1 being either `d` or `h` with latex3? All other pointer casts are most likely severe but subtle bugs that violate strict aliasing. I do it in C. I know what Im doing, and have been doing it for many many years. It returns true for the valid condition and returns false for the unsatisfied condition. int *ptr = NULL; By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. If we wanted to advance the pointer to point to the next object of the array, we would increment it by 1. The best, most efficient solution is probably a very good optimizing C compiler plus coding in assembly language those modules identified as time-critical. NULL is 0, which is the null pointer constant, but the null pointer isnt necessarily 0. My comment was on the valid, No, I think you are on the right track, just some of the rational is off a bit. Does methalox fuel have a coking problem at all? . sizeof is a constant operator that takes a single operand and is evaluated at compile time. Take that away, and you end up (almost) with reference types, which, while having some advantages, are way less powerful than raw pointers. Step 1 : Initialize the integer values and point these integer values to the pointer. 12 bytes. Return to main()s argv parameter, which we use to retrieve the command line arguments we pass to the executable itself. No memory at address zero, so dereferencing null pointers would always yield a processor exception. In other languages there might be better arguments for writing the code in a particular way, such as efficiency, but in C you would never really get that sort of advantage because the compiler re-wrote your code the same way if you used a lot of parens, or if you used none; or in many cases, if you wrote it out as 10 clear lines, or 1 big monster of a line! 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. ; c = 22; This assigns 22 to the variable c.That is, 22 is stored in the memory location of variable c. and () have higher precedence than *. And, variable c has an address but contains random garbage value. rev2023.4.21.43403. 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. 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.. 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. "For an implementation that uses 64 bit addressing, 64 bits are needed to represent each pointer location, therefore the smallest incremental change in location will be 8 bytes." 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. As we have well established, pointers can point to any kind of data type, which includes other pointer types. All legal programs can only be subtracting pointers which point into the same variable. When we assign cptr1, iptr is still an int * at the time of the addition, resulting in an address offset to fit three ints, i.e. The purpose of a pointer ( eg char * ) is to store an address to an object of the same base type, in this case char. (And pedantically. a rule to only define one variable per line. This issue is on any Real Mode processors. As a bool is a type of integer, and NULL is 0, also an integer. Dont need any of those? It is still the BEST, fastest method for getting closest to the hardware, except for assembly language. Note that all this applies only to already declared arrays. The Binary Operations extension functions apply to byte arrays, to provide an easy and hopefully fast way to use the basic binary operators. never matter to me). You can make it part of a typedef, so its clearly part of a type. If you think that the pointer construct is supported by any assembly language, you dont truly understand the basic concept of assembly language, or assembly-language programming. Also, name[i] can be written as *(name + i). Hence, the need to multiply a pointer in our test program. +1 to you. 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. Also, checkout godbolt.org. And theoretically it would benefit the process of learning C, if you were read it. But when we assign cptr2, we dont use parentheses, and the operator precedence leads to a higher priority for the cast operation. You really don't want to do thator, perhaps, why on earth do you think you want to do that (because you really don't want to do that!)? Increment void pointer by one byte? "Incrementing a pointer increases its value by the number of bytes of its data type" A character(1 bytes) pointer on increment jumps 1 bytes. As long as you only use features that are cosmetically different, and dont use anything substantive, youll even get the same code size! As a result, the second output will show the full 8 bytes of the offset. There are a lot of people who never learned to program in C, even though they program in C++ all the time. And since C evaluates any value thats 0 as false, we can implement a function that returns the length of a given string with a simple loop: With every loop iteration, we dereference strings current memory location to check if its value is NUL, and increment string itself afterwards, i.e. Im not sure if they still work this way with true VM I havent programmed on a Mac since the early 90s. 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. Dereferencing cptr2 on the other hand will be fine, and will essentially extract the fourth byte of value. Its not 8 bytes of the offset! It even could generate some code check for stack crashes and maybe even to validate a pointer as not being null. Especially on most microcontrollers, it will just happily read memory at address 0, which on e.g. The type specifier for a union is identical to the struct . I am saying that it will sizeof((int *)0x1008 (int *)0x1000)) is 8, but sizeof((int *)0x1001 (int *)0x1000)) is 8 as well. That code is an extreme example, but I have no problem with it. I strongly disagree. 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. Along with argv, we get argc passed to main(), which tells us the number of entries in argv. Most of the usual objections in the vein of you cant have dynamic allocation OOP intrinsically bloats compiled code size virtual calls are slow C++ object code is slow etc. Yes it is, there are still places where C is the best choice. My current solution is opt = (tcp_option_t *) ( (char*)opt+1); but it is a bit troublesome. Dont teach this to newbies, it is plain dangerous code, especially in the gcc era we live in. Because of how pointer arithmetics is defined in C. Let's assume we have an imaginary type 'foo'. 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. Strict rules may be bad, but that includes strict rules against strict rules! Only 1 * this code, or similar ;RESET POINTER HERE MOVLW B'11111111' MOVWF COUNT1 NEXTBYTE MOVF ""THIS WOULD BE THE POINTER"", W MOVWF OUT_BYTE CALL OUTPUT ;INCREMENT POINTER HERE DECFSZ COUNT1 GOTO NEXTBYTE If I do them all individually it will obviously take up quite a lot of code lines. 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. 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..). I agree. Since incrementing a pointer is always relative to the size of its underlying data type, incrementing argv adds the size of a pointer to the memory offset, here 8 bytes. I deal with some really awful legacy code, and Id be quite pleased to be working on code like in that link. Array elements are guaranteed to be contiguous in memory, so this solution is completely portable. The C++ language allows you to perform integer addition or subtraction operations on pointers. People get stung by the precedence of *. 256 times this MOVF DATAxxx, W MOVWF OUT_BYTE CALL OUTPUT Yes, I am totally with you on this. Its just some strange syntax rules that make it sort of part of a type. Output of the program | Dereference, Reference, Dereference, Reference. The C standard requires that the C-language visible representation of the null pointer is 0. It should be: Even if you write : Incrementing an int pointer will increase its value by four because the next valid integer address is four bytes from the current location. Yep. But we wouldnt talk about it if there wasnt more to it, so lets see for ourselves what happens when we add 1 to a couple of different pointer types. 1 here the j address is incremented by four bytes. Apart from adding integer to a pointer, we can also subtract them, and as long as theyre the same type, we can subtract a pointer from another pointer. The C++ operator ____ is used to create dynamic variables. But thats still something that stay in the C coding community. I definitely disagree with the idea you should memorize precedence rules, or be expected to, or make use of such knowledge. The result of p++ and p-- is the value of p before the operation. 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. next, let's take a look at the first x86 instruction in my program: 1. As you understood pointer is an address which is a numeric value; therefore, you can perform arithmetic operations on a pointer just as you can a numeric value. Advanced C Programming De Anza College Cupertino, CA. But what happens if we increment and dereference a pointer in the same expression? C is not assembly. Exceptions. 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. rev2023.4.21.43403. For performance, this often matches the width of the ref type, yet systems can allow less. Since the size of a pointer is 64 bits on 64 bit machines, doing a pp++ will always move 8 bytes. f(NULL); //the bool variant will be called! Why? I also like to keep the .h files that Im using open, to make frequent reference to the API. So if we looked through a string and find the NUL, we know we reached its end. The sandbox prevented bugs from affecting the main function of the thing it is running on, and to make sure the interpreter had full control of the objects, pointers were hidden from the language at the source level.