Return null pointer c. – An integer 0 is not always coerced to a null pointer.

Return null pointer c Just as you can't declare a forward referenced undefined class in a header - only a pointer - you wouldn't declare a class without a new in a stack allocation. A function can return a single value that may be any type of variable, either of a primary type (such as int, float, char, etc. You would have to engineer some value that represents a null AUTO, or return some kind of optional object that can be tested (see boost There are many options: return a pointer (and nullptr is a valid return), throw an exception, return a bool and return the value via a reference parameter, have a "sentinel" value indicating a non-valid Many systems generate SIGSEGV signsl on dereferencing NULL pointer, but this is not the requirement. Here, the return type is char *, which allows direct access to the character and subsequent characters in the string from the found position. Returning pointers from functions is most often a clear indication of a bad program design. If NULL means that the function can't find where your pointer should point to - for example if you want to open a file, but it doesn't work your file pointer is returned as NULL. end(). For more information about the latter idea, you can read about the Null Object Pattern. I. I'm attempting to use fopen to open a file. (When I use tcc rather than gcc, I get 0x7fffd95a7140. for example, Win32 programmers use a VERIFY macro which runs an assertion at debug time (the macro ensures that the assertions are stripped out of release code); It's not unusual to see functions which start like this: A null pointer is guaranteed to not compare equal to a pointer to any object. 1 min read. From the C++11 standard, section on Boolean Conversions:. ; You have corrupted the heap. Here because 4 is not enough space, so you see null output. In this case your attempting to assign a pointer (NULL) to a non-pointer value item::element and getting the appropriate message. Assuming you declared your variable func0 globally There is no way to check whether the iterator is valid. The ordinary form of new will never return NULL; if allocation fails, a std::bad_alloc exception will be thrown (the new (nothrow) form does not throw exceptions, and will return NULL if allocation fails). If the character is not found, the function returns NULL. When a pointer is holding a null value, it means the pointer is not pointing at anything. Well, the actual value of null pointer in memory may not be zero, but C (and C++) standard mandates compilers to convert integral 0 literal to null pointer. c: In function I took the directory to open as the 1st argument given to the program, e. 1. 3). h”, “stdlib. If the pointer is a null pointer, however, realloc(ptr, n) will behave exactly as malloc(n). If so then you could return that. Note that the pointers you are actually storing are pointers to char literals. Return nullptr on failure. NULL is a macro, defined in as a null pointer constant. Nice answer. Or not. You can change the contents of what the pointer points to with *p = X but you can't change the caller's value of p without passing in a pointer to the point. How to safely check for null pointers. time() function in C The time() function is defined in time. If you like to return an item-pointer from a list of items, but that item does not exist, you would return a NULL pointer and check for NULL after the get* Method has been called. return a std::pair<std::vector<int>, bool>, where the pair's second field indicates success/failure. : No, it isn't. e. returning a bool (int or uint) about success and storing the result in a passed address. A "null pointer constant" is defined to be an integer expression with the value 0, or such an expression cast to type void * (C99 §6. That is plausible if line_size is very large. Follow Calling fopen on Windows core files returns Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Passing a pointer to realloc doesn't "do anything to" the pointer. Edit: The modified question is It is perfectly safe to call free on a null pointer. This does not assign it to the original copy. 22. NULL vs Void Pointer – Null pointer is a value, while void pointer is a type. What object you use to get the value that you pass to free is irrelevant; only the pointer value passed is used. Unnecessary casts can hide problems. You also reallocate to 0-sized array, probably not what you want, please use x+1 as the number of records in the reallocation. h. It needs enough bytes to contain the address of the pointed-to object. Do whatever your team does. Dereferencing In C programming, a function can be defined to have more than one argument, but it can return only one expression to the calling function. If x is anything, then &x is the I suppose - here is some (big) misunderstanding of the pointers in C from my side, but: The getcwd() function copies an absolute pathname of the current working directory to the array pointed to by buf. h, stddef. – Hogan. If you are sure you are asking malloc for a reasonable amount of memory and it returns NULL, then you're probably in enough trouble that you probably can't do anything else except return/abort immediately. an int or whatever. It is represented by the constant value “NULL” or “0”. In C & C++ pointers can have the reserved null-pointer value. pointer_name: The name of the function pointer. Now, ptr too resides in memory. For some programs, simply aborting is the right thing to do. ) Apparently gcc recognizes that your code has undefined behavior, and replaces the return statement There's a practical means to approach this, if it were desired: You'd stack allocate just the pointer with Foo *foo = new Foo(desired_size,'B') and then test for null in the following code. static_cast can change the pointer value, if you cast between object parts on different offsets: class A{ int x; }; class B{ int y; }; class C : A,B {}; C *c=new C(); B *b=c; // The B part comes after the A part in C. In C89/90 this relationship was not strict: if you called realloc with 0 It seems that the preprocess function is returning null pointers. Casting it to TYPE * yields a null pointer of type TYPE *. So you're requesting an allocation of 0 bytes. I am encountering problem with pointers. Hot Network Questions NULL is defined as 0. On a standards-conforming C++ implementation, no. If you're trying to return an pointer to an int, just have a method that returns a pointer to an int. Another problem can be found in this section: @Skrymsli: It depends on the application and the environment. At ISO C level there is nothing wrong with your example, but the results are not guaranteed to be predictable. Such a pointer is called a null pointer. But both of the code examples are poor, in some way. The argument must be a pointer to void. In C/ C++, like normal data pointers(int *, char *, etc), there can be pointers to functions. It's an attempt to return the address of nullptr. It is a type of pointer that doesn’t point to any memory location, but the zeroth memory location. More, you access index x before reallocation, which is undefined behavior as You are returning a pointer points to the stack variable. A reference in Java works more like a pointer in C++, In C, two null pointers of any type are guaranteed to compare equal. Your implementation apparently does the former. Checking NULL Pointer. Obviously for integers, this is not the case as NULL cannot be added, subtracted, etc. The first call to strtok must pass the C string to tokenize, and subsequent calls must specify NULL as the first argument, which tells the function to continue tokenizing the string you passed in first. NULL Pointers. Warning: return makes integer from pointer without a cast [enabled by default] return NULL; I am trying to return NULL in my fucntion, but it's not working out and I don't understand why it is so. function can return NULL if searching an array for a particular value and not found, or set last Example: Using Null Pointers in Function Return. That's not an allowed operation. The pointer declared by malloc() will be a value in the stack frame of the currently executing function. The easiest way to create a null pointer is to use value initialization: int main() { int* ptr {}; // ptr is now a Be aware that not all implementations of printf() handle null pointers gracefully; they are entitled to crash instead, and IMO it is better if they do. @jfs it does more than that it also limits the max size returned. I hope you can understand. It is used to initialize o pointer variable when the pointer does not point to a valid memory address. The second should not cast the return value as char * because a void * is already automatically promoted to whatever pointer it needs to be. According to the language specification, @R. A pointer that has not been initialized to anything (not even NULL) is known as a wild pointer. c: In function ‘initArray’: 2. Well, they are wrong, you are right. People assign NULL to Dereference Pointer in C. The trailing & makes the return value a "reference", which is basically a guaranteed-not-to-be-null pointer to an existing object. I'll just point out that if you write. On Linux you The normal accepted way to return 'value not found' when searching a list is to return an iterator pointing to the end of the list. NULL really is only a concept of pointers. 2. Syntax: return type (*function_pointer_name) (argument_type return const_cast<void*>(""); // non-null pointer, referand doesn't matter Perhaps the callback function has a pointer input guaranteed not to be null. It is true that on most plaforms null-pointer value is represented physically by an actual zero address value. A zero value, null pointer value, or null member pointer value is If this is the case you may run into an issue where you set the file pointer to null, and then the fopen function doesn't run, so your pointer remains null. Hello there, future programmers! Today, we're going to dive into the fascinating world of NULL pointers in C. It is true that in C99 realloc with 0 size can return null or non-null, however in C99 the relationships "null means failure" and "non-null means success" was made strict. This implementation would then try to interpret the few bytes Once a year I have a similar problem: I try to open a file with the filename in a string, obtained from a sting operation. When p is a null pointer, the location stored in p is nowhere, you're saying "give me the data at the location 'nowhere'". It is recommended that the stream pointer always be checked prior to calling this function. c:8:13: warning: assignment makes pointer from integer without a cast [enabled by default] array[i] = i*2; ^ 2. 2. void func0(void) { printf( "0\n" ); } void (*func0)(void); you actually have two completely unrelated objects with the same name func0. (pre-C17/18) If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is With the new versions of C++ like C++11 and later, we can use "nullptr" to indicate a null pointer. The NULL constant is defined in the header files stdio. Below is the code that I used to A pointer is a variable that stores the memory address of another variable. NULL pointer in C At the very high level, we can think of NULL as a null pointer which is used in C for various purposes. In C, the keyword void is used to indicate a generic pointer, and an object of type void * can be converted This will copy the value of the pointer (currently NULL) into the function's local variable pointers. Declare you array temp like this -. , malloc(). If second is not a null pointer, the returned value is also stored in the object pointed to by second. Does *d Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Note that the reason for this is that most implementations of malloc store the length of the block right before the pointer returned (allowing free to know how much memory to free). But as i know assert will be useful only in DEBUG mode. int* HonestMethod() { return pointer to whatever. The best way in your approach is to return a pointer to the actual object being stored and that can be null. [fixed by u/imaami] IMO the most maintainable solution is to write multiple separate calls rather than trying to get "clever" about it. But the thing is that i tested this software on different machines with the same OS (Windows 10) and on few of them they returns NULL, the others just download and write the file correctly. If it is then we know we have no elements and if it is not then we know there is stuff to process. floatPtr is a null pointer. When the function returns, the stack gets popped off. Definitely not, because the variable defined in the function (in "auto" storage class) will disappear as the function exits, and you'll return a dangling pointer. You should check the return value of a function that may return NULL and pass it NULL Pointer in C. 4. I'm using an absolute path, the file does exist, and it's not corrupt. Share. Pointer adjusted C *c2=static_cast<C*>(b); // Pointer gets adjusted back, points to the beginning of the C part foo() takes no arguments and does not return a value, while bar() takes no arguments and returns a generic pointer. Memory Allocation Failures: Functions like malloc (), calloc (), and realloc () As the task states, it should return either a NULL pointer or, a pointer to one student with the highest grades (I guess it is meant one element i. It's a very practical approach According to the C standard, dereferencing a NULL pointer is undefined behavior, meaning that anything could happen. However, pointers are necessary if you need to manage subclassed mapping_item's. Syntax: time_t time( time_t *sec. In your function lookup-. In C, a null pointer is a special value that points to no memory location. However, I didn't fix that in this code — (un)fortunately, the system I'm using has a printf() that prints (null) instead of crashing when given a null pointer to print as a string. – bzim. the add() function. exceeds size bytes, NULL is returned. However, returning a raw pointer raises the question of who is responsible for deleting the dynamically allocated string. And it's also very wrong to try and return a pointer to a local variable. On success it will return a file pointer as a handler. is very similar to a fairly common definition of the standard offsetof() macro, defined in <stddef. com/portfoliocourses/c-exam One way of doing it is to return a pointer to a foo: public Foo* findFoo() { return fooFound ? new fooResult() : NULL; } Another possibility to define a NullFoo of some kind, possibly as a struct extending Foo with an empty implementation. The first should return NULL rather than (0). char temp[strlen(extension)+1)]={'\0'}; // +1 for null character The C Committee draft N1570 states this about FILE*:. On modern computers it would likely get your application killed (which could lead to data loss or corruption, depending on what the application does). Declare and Initialize a NULL Pointer There's one more problem to the above code - the C++ version is inefficient. Instead, it merely contains the base address of a I was assuming that fopen returns NULL pointer if file is already open. Because malloc can return a NULL and just because it returns it doesn't mean you can dereference it. But you make a good point this is clearly the better choice for a robust program. For example, int n = 0; void* p = (void*)n; is U. The C standard (C17 7. But you're of course correct that a crash is not guaranteed. Source code: https://github. The first func0 is a function, the second func0 is a variable with type pointer-to-function. Some obvious reasons why this could happen: You have exhausted heap memory. In returnB() I'm asking to return a pointer; since a pointer points to an address, the reason why returnB() works is because I'm returning &b? In returnC() I'm asking for an address of int to be returned. h> #include <ctype. 3. NULL/0 is guaranteed to be an invalid pointer value. Strictly speaking, NULL expands to an implementation-defined null pointer constant which is defined in many header files such as “stdio. B. Passing nullptr to string::string will just make the string try to interpret (char*)nullptr as a pointer to a c-string. When you print a pointer using printf("%p", somePtr), it is printed in an implementation-defined manner, as per this quote from the POSIX printf specification (similar wording exists in the C99 specification also). h> #include <string. A NULL pointer in C is a pointer that doesn't point to any of the memory locations. Commented Oct 29, 2017 at 14:08. Some applications might be able to purge lots of allocated memory and recover in some way. As we know, that malloc() function allocates the memory; if malloc() function is not able to allocate the memory, then it returns the NULL pointer. 19 3 says NULL “expands to an implementation-defined null pointer constant” (when any of several headers have been included: <locale. It is Returning NULL from Functions: Functions may return NULL to indicate failure or the absence of a valid value. A NULL pointer is a pointer that lacks a reference to a valid memory location. In C we can also use structures to encapsulate arrays and then we can easily return them from functions. Commented Oct 28, 2017 at 13:52. void Insert(Node* head, int x) to insert node at the beginning of Linked list. Dereferencing pointer pointing to memory which doesn't belong to your program (NULL==0, and address 0 belongs to your operating system) is undefined behaviour. The problem is that when I insert the very first node and Node *head is NULL, the function Insert is not able to change the pointer address of null pointer to the newly created node. The value of the pointer is converted to a sequence of printable characters, in an implementation A shared_ptr that points to no object is called a null shared_ptr and shall not be dereferenced. Wanting the immediate pointer value in the [0] slot of the resulting vector will get you (a) an indeterminate pointer value, and (b) lose any reference the actual allocation outright since nowhere do you persist the original result address. The behavior of such an allocation is implementation-defined; it can return a null pointer, or it can return a unique non-null pointer, similar to malloc(1) except that you can't necessarily dereference it. " – In C/ C++, like normal data pointers(int *, char *, etc), there can be pointers to functions. The actual claim made on the guide reads like. 3 Files The address of the FILE object used to control a stream may be significant; a copy of a FILE object need not serve in place of the original. h” etc. h>, <stddef. char *strchr( const char *s, int c ); I understand that strchr locates the first occurrence of character c in string s. The below is the generic syntax of function declaration: return_type (*pointer_name) (parameter_types);return_type: The type of the value that the function returns. [3] Prior to C23, the preprocessor macro NULL was provided, defined as an implementation-defined null pointer constant in <stdlib. Unlike C++, Java has no value semantics for objects, in Java returning Person always returns a reference to an object. 7. It returns references to unique_ptrs. test* ptr = fun(); // return a pointer of type test return a std::vector<int>* pointer, optionally wrapped in std::unique_ptr for lifetime management. 21. Why? #include <stdio. 17. – 2. h> //Call the functions void printFirstWord(char inputString[]); char People assign NULL to pointer to indicate that it points to nothing. Consistently Getting Null Value in C String using getcwd. Below is my code, but I will not post the entire thing since I am only having problems with the return statement. 4: Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is Should I declare a Mystruct variable, define the properties of Mystruct, assign a pointer to it, and return the pointer. If you want to MSVC documentation says "If stream is NULL, the invalid parameter handler is invoked, as described in Parameter Validation. Following is the syntax to return an In C NULL is generally defined as the following. h> for input-output operations. In your calling function test On the other hand, in plain C, NULL == 0 would return true IIRC because NULL is just a macro returning 0, which is always an invalid address (AFAIK). Since my_int_ptr is filled with garbage, it can be any address. Segmentation fault when accessing specific pointer. Wild pointer in C. This can't be a temporary object since you aren't returning I went with switching the restype to c_void_p and then casting to POINTER(POINTER(c_char)), because that mostly matched up what I was doing before. [5] Since C23, a null pointer is Your problem can be solved without the use of std::function altogether. Check for null in Objects of pointer type merely hold pointer values. The reason that you can't return NULL here is because you've declared your return type as Attr&. NULL is a perfectly valid pointer-to-char which happens to not point to anything, so it is stored in the vector. Follow A constant expression of type int with the value 0, or an expression of this type, cast to type void * is a null pointer constant, which if converted to a pointer becomes a null pointer. h> void Fopen(const char * restrict filePath, const char * It depends on the broader circumstances. c:11:3: warning: function returns address of local variable [-Wreturn-local-addr] 2. It is guaranteed by the standard to compare unequal to any pointer to any object or function. The actual representation of null-pointer value has nothing to do with any "zeros". . Why does fopen() return a null pointer? I'm trying to save the parameters after BRIDGE and after LAN (respectively 4 and 5) in cont_br and cont_lan, but fopen() doesn't work #include <strin Dereferencing just means accessing the memory value at a given address. In C++, returning Person by-value always returns a copy of it (except for RVO, which is not applicable here). The memory will still be freed, so you can no longer safely access it, but your A pointer variable is an object that can point to another object. In general I prefer the way Jon Skeet proposed, ie. struct item { struct elem* element; }; An explanation of the NULL pointer value in C, including common use cases of the NULL pointer value. Per 7. So before you dereference anything returned from malloc, you have to check that it's not null. h> (in C) or <cstddef> (in C++). When you dereference a pointer p, you say "give me the data at the location stored in "p". , and in particular needs not make p a null pointer. On the other hand, if you use just the single initial assignment, int *my_int_ptr = 2;, the program will try to fill the contents of the memory location pointed to by my_int_ptr with the value 2. There are three separate concepts which must be dealt with: NULL, a null pointer, and a null pointer constant. It does something to the object pointed to by it. h> #include <stdlib. That pointer may point to a pointer. But sometimes my pointer is NULL. When I print the name it seems OK, but fopen() returns a null pointer. Pointers are advisable if instances are large or need to be shared. " – I'm just starting out with pointers, and I'm slightly confused. The point is: malloc returning NULL doesn't mean you're out of memory. h>, <time. In the unlikely event malloc returned NULL, your program would end up To answer "what is happening to the pointer returned by malloc()?". But your function is very similar to strtol, so I think it is a good idea to use the same (or similar) API for your function. Commented Jul 23, 2017 at 19:23. I assume that your intent is for mystrstr() to follow that specification. The unary & operator is the address-of operator. But it becomes to an infinite loop and it still says, that the pointer is NULL. If you give realloc an uninitialized pointer, it would think it was a valid pointer (a pointer is a pointer, all realloc can do is trust you). – Such a not handled NULL Pointer leads to undefinded behaviour ( and oh boy that sucks when your PC may drink your last beer, steals your car and may sends the "homework" folders to your family members), because NULL is a legit address. h (ctime in either a null pointer is returned to indicate an error, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object. When the function returns, myPointer is unchanged. 0. So when you have a pointer to something, to dereference the pointer means to read or write the data that the pointer points to. We just have to assign the NULL value. you have a pointer. For some applications, the right thing to do is to shrink caches and try the malloc again. struct node* create_empty_list() { struct node* head = NULL; return head; } The value of head, which happens to be NULL (0), is copied into the stack before function create_empty_list returns. For some multithreaded programs, just waiting (to give other threads a chance to free memory) and retrying will work. Don't worry if you're new to programming; I'll guide you through this concept step by step, just like I've done for countless students over my years of While doing programming i am using assert as well as NULL pointer validation. The return value of the function returns a C Why is it comparing the pointer to NULL. Otherwise, a NULL pointer is Interestingly, in create_file(), you could move the return regularTxt; out of the if statement and use it in place of the return NULL; — simply return its value, NULL or not, but don't use it unless it is non-null. A null pointer stores a defined value, but one that is defined by the environment to not be a valid address for any member or object. 59. h”, “stddef. So now what to do? I made a loop, that tries to allocate it again and again if the pointer is NULL. Multidimensional pointers aren't really a thing. If you really need pointers, consider using boost::shared_ptr<> rather than raw pointers, both inside your class and as parameter types to e. Alternatively, you could return a pointer, which can be null. The number of asterisks tell you how many times the resulting object has been dereferenced -- implicitly, what it actually does is tell you the type of the object is a pointer to a pointer to a pointer to . C 2018 7. You're allocating memory for SIZE_TABLE pointers. const char* filen However, in skipWords() I am supposed to return a pointer value of NULL if the amount of words that you wish to skip is greater than the amount of words in the string you input. It seems like your intent is to have element be a pointer here so try the following . Since we have a container we can check something else, namely, whether or not the container is empty. The stack variable no longer exists, the pointer become a dangling pointer. Although some implementations of printf will print (null) when you pass a NULL pointer, this is undefined behavior. (It's not a "pointer to null"; the pointer is null. False otherwise. argv[i] with i<argc should never be a NULL pointer. reinterpret_cast<void*>(1) is supposed to look ugly, because it's not guaranteed to work. 1. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company With an iterator I can't return null to indicate the container's empty or the search failed. On some older C++ compilers (especially those that were released before the language was standardized) or in situations @AjayBrahmakshatriya NULL/0 as a pointer value doesn't necessarily mean a 0 representation, so a system where an all-bits-zero pointer would be valid would have to provide an alternative way to write it for strict conformance. In the above code, we use the library function, i. I know & means the address of a variable and that * can be used in front of a pointer variable to get the value of the object that is pointed to by the pointer. So while declaring a pointer we can simply assign NULL to it in following way. NULL is not a null pointer; it is required to be defined as a "null pointer constant" (which in C++, cannot be a pointer, and in C, traditionally is not a pointer). In this case, returning a std::unique_ptr would be the No, c_str() will always return a null-terminated C-style string, assuming no prior undefined behaviour. Every function created in a program gets an address in memory since pointers can A pointer pointing to nothing or no memory location is called null pointer. How do I fix this? Here is the C file: so in those cases fopen will return 0 (a NULL pointer). Rather than using double pointer we can just allocate a new pointer and just return it, no need to pass double pointer I think you are mixing up naming and defining function pointers. If your goal is to keep the changed value in myPointer , you need to either pass-by-reference or pass a pointer to your int*** (NOTE: this level of indirection is already a code smell!) &nullptr is not a "reference" to a null pointer. h>, <string. The only help is printing the name with delimiters showing the exact beginning and end of realloc can change the base address, but array pointer is passed by value, so a local reallocation is not visible in the main and generates some trouble. The parenthesis around the pointer_name is The type of NULL may be either an integer type or void *. Anyhow, the function doesn't return pointesr anyhow. Casting a pointer to a long is perfectly well defined, although it often is easier to work only with pointers to p = &i; That also won't work. When the stack frame (and thus *tileCoordinate) goes out of scope at function return, that pointer will cease to exist. If I was doing a code review I would wonder why the function is returning a null pointer in the first place, or why it is returning a pointer a character at all but never question the null check itself. h>, <stdlib. At that point, you should instead set the first character of the string to '\0' and return it since the surrounding code expects to get the string returned in all cases. This is my code: #include <stdio. The warning you were getting was to make sure you realized you were turning an int into a void pointer. Detecting if a object is owned by a smart pointer. Incorrect pointer created within a function. So A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or Basically, any uninitialized pointer, if in a program, is made NULL and then used differently, for ex. There are 2 important operators that we will use in pointers malloc will return the null pointer when it fails. When I return c is the & operator automatically "appended" c? In returnC2() I'm asking again for an address of int to be returned. A null pointer can signal an error or a special condition. On the other hand, what you could do if you insist to return an iterator is having a method in Wrapper to check the validity of the iterator. A null pointer is one that points to nowhere. According to the documentation for curl_easy_setopt, when the second argument is CURLOPT_WRITEFUNCTION, the third argument should be a pointer to a function having the signature. In this case link. In C++11, std::unique_ptr<SomeResource> or std::shared_ptr<SomeResource> could be returned also. Return Value of strchr() The strchr() function returns a pointer to the first occurrence of the character in the string. But things work differently when you're working with arrays, strings or when you're calling functions with a pointer copy of a variable. In the following program, file_ptr is NULL, yet it is being initialized properly. I have function. @tmyklebu: Having all pointer arithmetic (other than comparisons) on a null pointer trap would IMHO be a good thing; whether memcpy() should be allowed to perform any pointer arithmetic on its arguments prior to ensuring a non-zero count is another question [if I were designing the standards, I would probably specify that if p is null, p+0 meaning that your function is returning a null pointer. This is because the C standard allows it to be defined as either an integer constant expression or the result of a cast to void *. if realloc returns null in C99, the original memory is guaranteed to be not deallocated. You are assigning NULL to the local copy of the variable (ptr). – jfs. To Time Complexity: O(N), where N denotes the returned array size. It's actual value is system dependent and may vary depending on the type. Every function created in a program gets an address in memory since pointers can be used in C/C++, so a pointer to a function can also be created. Only an integral literal 0 can be treated as a null pointer as described. Examples of strchr() in C In C, NULL is an "implementation-defined null pointer constant" (C99 §7. Notice though that an empty shared_ptr is not necessarily a null shared_ptr, and a null shared_ptr is not necessarily an empty shared_ptr. The former would correspond to value semantics in that the resource would have to be copied, while the latter would correspond to option 1 (but probably requires refactoring SomeClass's content_ to contain shared_ptrs also). I can return vector::end(), but cplusplus. It doesn't make a lot of sense to mix the C++ style vector with the C-style char . Instead of holding a direct value, it holds the address where the value is stored in memory. Do you have any ideas? my question is return; the same as return NULL; in C++?. Your code exhibits undefined behaviour . – That doesn't explain why you would need to return such a pointer, rather than use a pointer passed as one of the function's parameters, which is the convention in C. Here, you are returning the value of a local variable, which is OK:. The above expression will return true if the pointer is a NULL pointer. The null pointer value is special: it means "hey, this pointer is not valid, don't use it for anything". function can return NULL if searching an array for a particular value and not found, or set last element of an array You can; the null pointer is implicitly converted into boolean false while non-null pointers are converted into true. size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata); This C Tutorial Explains NULL Pointer in C with Examples. My question is suppose i have a internal pointer which i am sure cant be NULL example function returning a pointer(but the pointer is not a member of class) in such cases i can use assert. I understand that in C++, return NULL; is the same as return 0; in the context of pointers. name, ID, the grade of 1 student) depending on a specific condition. Here int *ptr = NULL; declares ptr as a pointer object, that potentially points to an object of int. If you return an empty object, initial use of it may work, but you may get errors later. Note that the language doesn't guarantee (or even imply) that a null pointer has the value 0, though it very commonly does. Also, all system users have full access to read/write the file. In c11, there is strnlen_s(str, strsz) that returns zero if str is a null pointer. To get a null int pointer you would do . A common use case for null pointers is in functions that return pointers. ptrName == NULL or ptrName == nullptr. A pointer is initialized to NULL to avoid the unpredicted behavior of a program or to prevent segmentation fault errors. p is a local variable so setting it has no effect on the caller's pointer value. parameter_types: The types of the parameters the function takes. If "crashing" immediately is required then I'd prefer an explicit abort rather than carrying on running an indeterminate amount of time until some use of the null pointer causes a crash as a side-effect. But it looks fopen does not return NULL in case file is already open in &quot;w&quot; mode. @DevSolar There are quite often reasons to return NULL-Pointers. The calling function would typically copy this value into some other variable. – An integer 0 is not always coerced to a null pointer. – Imagine a type erased smart pointer that must call a vtable method prior to getting the T*, how to return an null tr1::shared_ptr and test if it is null. After I allocate the memory, I check if the pointer is NULL (that's because there isn't that big memory space or I don't know why). Let's say I have sorted the array so that the highest student's data (name, ID, grade) stood at the top. pass the std::vector by a reference parameter, and then return a bool for success/failure. We can check whether a pointer is a NULL pointer by using the equality comparison operator. An advantage of the double-indirect pointer approach is that it can return status information beyond pass/fail, and if pointers are always null whenever they're not valid it can implement "allocate if needed" semantics. Code Explanation: We begin the C code example by including the essential header file <stdio. if in a program, is made NULL and then used differently, for ex. g. h>, [4] which in C99 can be portably expressed as ((void *)0), the integer value 0 converted to the type void* (see pointer to void type). ; The pointer returned by fopen() points to a FILE structure and the contents of that structure are implementation-specific (which means they are different in NULL Pointer in C. Problem creating a pointer to a given address. – user2100815. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog The other answers assume you wanted to return a pointer to the integer, which is very different than the integer as a pointer. A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. If c is found, a pointer to c in s is returned. The problem is, for some reason fopen returns NULL, and because of that, my program is crashing at the fwrite statement. However, because you return the pointer value to the calling function, it now Function Pointer Declaration. It's typically used to indicate that a pointer doesn't have a specific memory address assigned, often serving as a placeholder or You don't have a vector of strings -- you have a vector of pointer-to-char. h As a previous poster mentioned, null return values will cause code to fail straight away, If you've made a mistake in your code and returning null leads to null pointer exceptions, then the sooner you catch that the better. Obviously, it can't do this, so it throws a null pointer exception. And that it is encouraged by some to use 0 instead of NULL for pointers because it is more convenient for portability. strtok is part of the C library and what it does is splitting a C null-delimited string into tokens separated by any delimiter you specify. char *p = argv[1]; (I set the pointer p to point to the directory given on the command line) Then when I strcat (entry, p); I first put the directory in entry and then add the separator and filename with strcat (entry, "/"); and strcat (entry, dir->d_name); Let me know if you are still stuck. 3/1) says: If the size of the space requested is zero, the behavior is implementation defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned strstr() doesn't return an index (or a pointer to an index) - it returns a pointer to the character in the string passed as the first parameter where the instance of str2 is found (if it is found). Auxiliary Space: O(1), as the array is static once it's size is declared it can't be changed further so it remains constant. It will be even funnier when you free such a pointer! But this is another story. ), a pointer to a variable of primary or user−defined type, or a pointer to any variables. Please note that ISO standards do not define many things, leaving implementation details up to implementations. Accessing or manipulating the content that is stored in the memory address pointed by the pointer using dereferencing or indirection operator (*) is called dereferencing the pointer. com says: If the container is empty, vector::end() function returns the same as vector::begin() and then for vector::begin() it says: If the container is empty, the returned iterator value shall not be dereferenced. The strings are not copied. This is exactly how the std::find algorithm from <algorithm> behaves. Hardly surprising when it contains a line return NULL;. For doing this we simply assign NULL to the pointer. Return an Array in C Using Structures. You could just return a unique_ptr that has no managed object. If the pointer is uninitialized or invalid, the call has undefined behavior and bad things will happen. For the statement, (ignoring, for Because the pointer is copied by value to your function. You simply misunderstood the point. In C, the unary * operator is the dereferencing operator. Most architectures don't mind you doing that, but To keep the issues clearer: NULL is a macro, defined in <stddef. If x is a pointer, then *x is what x points to. Improve this answer. It will still copy the mis-pointed c-string to its managed storage getcwd() with pointer returns "null" 0. You can't assume that without extra information. ) That's not what I would have expected; I expected a invalid garbage pointer value. int* p = 0; A null pointer will be returned by malloc on failure. When a pointer is assigned the value NULL, it means that it is When a NULL value is assigned to the pointer, then it is considered as a Null pointer. Then, inside the main() function, which is the entry Sidebar: You may also find that methods which return values via parameters are difficult to reason about and more likely to contain bugs. 0 is a null pointer constant. I was asking for a "fairly In particular the file is written in C++ and when i try to download a file using the following code the fopen function returns NULL. h>, <stdio. char temp[strlen(extension)]; // basically char temp[3] and with loop you fill full array , without leaving space for '\0' and then print using %s and passing it to strcmp will also cause UB. Therefore, it is necessary to add the condition which will check whether the value of a pointer is null or not, if the value of a pointer is not null means that the memory is allocated. h as well as stdlib. One solution is to allocate the appropriate amount of memory in the main function, and pass the pointer to the memory to the helper function. malloc and realloc return a pointer value, and what object or objects you store it in are irrelevant to its operation or to the operation of free. What is a Null Pointer in C? A null pointer in the C programming language signifies a pointer that lacks a valid memory address, meaning it doesn't reference any variables. In C and C++ programming, two null pointers are guaranteed to compare equal; ANSI C guarantees that any null pointer will be equal to 0 in a comparison with an integer type; furthermore the macro NULL is defined as a null pointer constant, that is value 0 (either as an integer type or converted to a pointer to void), so a null pointer will Output: intPtr is a null pointer. #define NULL ((void*)0) This means that it's a pointer value. It can be absolutely anything appropriate for a given platform. c:11:3: warning: return from incompatible pointer type [enabled by default] return array; ^ 2. malloc(0) may return a null pointer. throw an exception on failure. That could happen if there are errors in the code that you are running, but have removed for the purpose of asking this question. I would expect no one to have trouble reading code with either of those. If execution is allowed to continue, fclose sets errno to EINVAL and returns EOF. h> (and some other headers). The pointer may Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company In particular, I wonder if am I allowed to overload the basic plain form of ::operator new to always return null pointer, and therefore expect that all plain new-expressions that use that operator will now return null pointers as well. The value initially stored into this pointer object is NULL (it is initialized to NULL, so ptr does not point to any object). This avoids having to call a "itemExists()", with the need of twice looping the list, or having the Apart from the simple "no", what you're wanting to do doesn't make sense anyway. jpo ppspuax afjwrk uhd gpk kcuudj sups rtzp ogez niwl