One has to stop somewhere, and this article decided to stop at C, as opposed to assembly. As soon as you mention the stack, you've gone beyond C and started talking about something that is not C. See my reply to your sibling comment.
I wasn't proposing adding more irrelevant detail. It's a tough line between intro and The Rabbit Hole. But I'll see what I can do there. Appreciate the feedback. Some good suggestions here that I'll add. Twisell 8 months ago root parent prev next [—]. As a beginner I was very frustrated with most approach that say "Just put that thing that is needed and will be explained latter. And it work good job attaboy! At least this guide don't let the reader in the fog wondering.
IMO, pointers are less difficult to comprehend than other abstractions, like lambdas are. If you know how to walk down a street and stop at the right street number, then you have used pointers. And if you've ever observed that one tall building may "cover" a range of street numbers, such as , then you should understand how to move from one 4-byte "value" to the next in an array in memory.
Anyway, many more analogies Maybe unions could make using pointers a bit more challenging, but again, tall buildings next to short buildings and so on. We do this kind of pointer calculation in real life. What pointers basically are is not particularly hard to grasp. What is harder to grasp it what can be done with them and how you can shoot yourself in the foot with them in non-obvious ways.
I think I only understood much of it once I learned Rust, because you realize: Ah, that thing I once did in C is something that maybe ahouldn't be possible at all without extra steps. Even if I were to nwver use Rust again, this definitly helped to understand how to use pointers more safely.
I think that pointers are a tool to control a CPU's indirect addressing modes from a higher-level language. Exposure to an assembler makes pointers easy to understand. While it's not unique to pointers, and it's in fact the case for most nontrivial programming tasks, what's unique about C is that pointers are so pervasive you can't really do anything if you don't understand how to work with them.
IME languages like Python aren't any easier than C to work with ignoring UB issues of course , but it's certainly the case that you can probably kinda sorta get your job done with Python even without understanding the first thing of what you're doing, and that's not happening if you write in C.
In C, pointers require you to think deeply about the ownership and lifetime of any "allocated object" at runtime. How long does it live, who is responsible for the deallocation, how many pointers does your program hold to that object dangling issues.
Ultimately, it can lead to a cleaner design if these issues are taken seriously up-front. I genuinely think things like that would make the language a bit more ergonomic without fundamentally changing its nature. That's why I tend to always prefer automatic and static storage to dynamic allocation wherever possible, especially in cases where you don't have "N" items or "N" cannot possibly exceed a certain small value.
That is of course fine, with the drawback that it requires a public definition of the foo type. If the type on the left hand side ever would change, this still does the right thing. Interesting, I tend to prefer a create and destroy function that allocates and frees the structure. That way you can have foo without it being initialized, and you cant have foo been freed without being de-initialized.
Where do see the value in being able to move it between different memory types? TorKlingberg 8 months ago root parent prev next [—]. Coming from the embedded world, I am far more used to the style bluetomcat showed where the caller handles allocation.
It takes more work to use, and you have to make sure to not use it before initializing or after destroying, as you said. The advantage is that the caller has full control of memory allocation.
The object can be a local variable, a static, malloc:ed, come from a custom allocator say a slab allocator , or be a part of a larger struct. That makes a lot of sense. I do some embedded development, but when I dont, I tend to prefer heap just because my memory debuggers are much more capable working with heap memory.
In the case you show, foo would have to be a struct that doesn't contain pointers to additional allocated memory, but its a entirely valid use case and pattern. Calling malloc and free, has a cost associated with it, that the stack doesn't.
But stack can in some cases, like with recursion be scary to use, because you dont know where it ends.
If malloc returns NULL you know you have found the end and can do something reasonable. Thanks for you insight! Is there some recommended good read? AlbertoGP 8 months ago root parent prev next [—]. For my own purposes, I think I can live without handling stack unwinding so I continue working on my pre-processor. No type safety and broken alignment. No thanks. What do you mean by broken alignment?
This could be applied here actually as an easy fix but it indicates even further to me that the author of the library is taking a very leisurely approach to writing conforming C. This pattern also appears in the other two functions though and I'm not sure if in those cases it's something which can be easily fixed. The other extension i would add is some sort of interface or protocol.
As soon as you can do something like "y. With this you would also be able to call some cleanup code and even a initializer. C should propose these kind of things even if it was not that conservative and it would retain a lot of coders that migrate instead giving C barely evolved from its 70's roots.
Used to be very common not sure how it is now , and very frustrating when you link against non-standard allocators. Unless you decide you use libgc, presumably? These are good points. I can sometimes feel that Python is more pointer-y? This is surprising to some people. I agree, which is why I'm glad the first language I learned was C. I don't get to write a lot of C at work, but the concepts the language teaches you are the very fundamentals of programming.
I know it's probably baseless, but I can't shake the feeling that people who learn modern languages before learning C are just making their own lives harder. I think the same goes for learning assembly before learning C. A simple assembly-based computer [1] should be the first programming target in every "introduction to programming" class.
After that, C becomes obvious and those who struggle with the fundamntals can be guided to more appropriate careers instead of dumbing down the tools everyone uses. The real difference is Perl has typed variables, such as hashes or arrays, which other languages typically do not. Perl borrows the sigil concept from Bash and other shells which makes sense, given the historical context of Perl.
In fact, the one thing that trips up JavaScript developers especially in React is that they do not understand how references work. I see senior and lead developers inadvertently doing mutation all the time. Or getting incredibly paranoid that two identical strings, for example, do not equal each other in the strictest sense in JS.
They also throw in memoization everywhere due to their fundamental lack of understanding. It's a feature! It's also similar in Java's pass-by-value vs pass-by-reference pass as well as copy , if I recall correctly. Pointers are passed by value, that is the pointer value gets copied.
References are a language level construct. ZoomZoomZoom 8 months ago parent prev next [—]. Pointers aren't that complicated but C syntax is misleading imho until someone comes along and tells you that the declaration syntax "follows use", which does not seem like the greatest idea. Then you get used to it and forget about it but when you don't know the principle behind declaration syntax, it does not help you reason about the language.
There's actually a logic to it, although not an intuitive one at all, and it's that declaration should follow use. Only the first token which must be a type is syntactically special, the rest is normal expression syntax.
Of course, that hasn't been entirely true since function declaration changed with ANSI C in the late 80s or so. But int-pointer is the type of it. There's a type and an identifier. A declaration simply specifies a name the identifier , some operations around it pointer dereferencing, array subscription, function calling, grouping parentheses and the terminal type that you get after applying all those operators in the precise order as specified by operator precedence rules.
Thank you for the concise write up, but I think lowly of the [] syntax as well. But I have trouble with the declaration site as well. Koshkin 8 months ago root parent prev next [—]. You missed the whole point.
Yes, it makes no sense until you grok the logic of it and then it starts making sense while still being unintuitive. Re-read the comments you are responding to. Pointers are like swazzles[1]. The construction is very simple. The principle behind how they work is very simple. Learning how to use one well enough that you can a consistently make it do what you want, and b not injure yourself in the process, though, is no mean feat.
Well, you've demonstrated that memory addresses aren't that hard. But you've also demonstrated how easy it is to get undefined behavior in C programs. C's pointers aren't memory addresses.
Ok, they tend to be represented as such at run time, but that's not what the spec actually says the are. And as far as compiler authors are concerned, they can do anything they want as long as it's within spec. Further, the spec even requires some additional behaviors pure memory addresses aren't capable of.
Compared to that mess, lambdas are trivial. They're just functions. And if you've ever observed that one tall building may "cover" a range of street numbers, such as I see that as a European, I have virtually no chance to understand pointers using street numbers. C is an abstraction over assembly, really benefits being that it is simpler by being more abstract and portable across CPU types.
I've always thought that an introduction to CPUs can take a simpler one as example and how they work, how memory is usually organised, and to assembly would go a long way in helping understand many programming issues and C. If you take a concrete example where memory is effectively an array and indices are addresses which holds true for most cases and, in any case is a good example then understanding pointers becomes basically common sense and notations are simply conventions of the language you're using.
Treating C as an abstraction over assembly is a surefire way to step into all the thousands of sharp edges C has. In fact I would hazard a guess that the majority of bugs found in software written in C are a result of programmers treating it as a portable assembler instead of a language for programming an abstract machine.
So many incorrect assumptions arise as a result of telling people to treat C as a portable assembler that I think it's safe to call it an extremely bad bit of advice. I was discussing pointers. I only commented in passing about C being portable over assembler. You could rephrase this as it being a language for programming an abstract machine and it would not change anything about my comment nor would it change the fact that C is an abstraction over assembly Thank you for your reply The fact that you didn't use the term "portable assembler" when saying: "If you take a concrete example where memory is effectively an array and indices are addresses which holds true for most cases and, in any case is a good example " doesn't change the fact that the statement makes links to how actual machine memory operates on what you personally think is a usual machine.
The "very abstract" way C is taught actually prevents people from making such assumptions by not priming them to make them. The fact that people get complacent and start to lean on their understanding of what they think are real machines to write C is the result of the bugs I mentioned in the previous response. Like my professor used to say, if you ever saw a pidgeon, you know what vectors are. For anyone who wants to learn about pointers I can recommend studying a language simpler than C like for instance Oberon where pointers are more restricted.
Having a look at Oberon can also broaden your view even if you know pointers in C. Koshkin 8 months ago root parent next [—]. When pointers are used to build dynamic data structures the differences are merely syntactical. Any one remember the heyday of comp.
I wonder what goes on in there now. I wasn't sure anyone but the authors remembered C Unleashed! I wrote the chapter on binary search trees and balanced trees. I've met 5 or so of the regulars at least once.
The most famous comp. Ben Pfaff There's a name I recognize from back then. Is it beej or bj? One syllable, typically. But either works. Thanks very much! The code from the book proved very useful since C does not have a standard library of rich data structures. I bought that book ages ago. Good stuff. And Usenet is a shadow of its former self, obviously. Reddit has more traffic nowadays. That's what I remember from comp. That is an important distinction which I respect, but they should have had some kind of sister forum that's more practical.
They denied that hardware exists, etc. Good specs are derived from real usage. Yes, usenet was huge for me back in the early 90s when I had questions on C programming.
I would ask them on comp. I wish more content would be as straight to the point and easy to follow. I'm sort of a C beginner myself. I understand pointers, and I do remember they clicked in my mind suddenly. The moment before, I didn't understand at all. I also love the quirkiness of this guide. Definitely going to give this a read. Do you mean the general concept or like: a is a pointer to an array of functions which return pointers to functions which return ints and take double arrays as parameters.
This somehow never really clicked or actually it clicked and declicked somehow. For me it was the practical understanding. I understood the concept of a pointer, but I wasn't confident in writing code that used that used them or more importantly reading code.
I would see an asterisk, multiple asterisks, or ampersands, and would get confused with the code. I do think some of the issues I encountered had to do with the notation of pointers.
The asterisk serving as a symbol to both declare a pointer variable and dereference one. If you always read from right to left and put occasional braces for readability you can get through anything.
Most tricky ones are in job interviews, while ones found in the wild will often make use of typedefs to break down the complexity. Back when I was just starting to learn to code, I wanted to get right down to the low level C stuff and his guide was what I used. It was simple and approachable and I had a running TCP client by the end of the day.
It was a thrilling experience for a young novice. I see a lot of people making opinions that clearly shows that they have not audited the entire guide. For the intended audience that the author wanted to reach. I will say that he accomplished it. For anything that one finds as mistakes, the author went out of his way via references for the reader to dig further. From the creator of the famous Network programming in C guide. I've enjoyed this guide a number of times but each time I hit a brick wall trying to understand pointers.
I'm still keen to learn but it just doesnt 'click' for me Edit: poor grammar. DyslexicAtheist 8 months ago parent next [—]. It sounds silly today but what helped me are some really basic books about C. It took another couple of years until I understood what I lacked wasn't time spent reading another section on pointers but additional tooling.
Using a debugger and stepping through programs was the next breakthrough. Looking back today understanding my C on UNIX isn't just the language it's a whole ecosystem of tools to measure what is going on and manipulating state so that I can troubleshoot. After gdb came valgrind, strace, lsof, signal handling kill , process control, gcov, the appropriate type of CFLAGS to use e.
None of them have to do with pointers but they make life a lot easier. To become productive at this takes years but becoming good took me decades.
C imho isn't just another language but a complete career path with dozens of branches into other areas. If you stay patient with yourself and treat it as a journey instead of a milestone it can deepen your understanding of systems nod to eBPF in situations many others will bail out long before.
Don't give up and then not much will look scary any more. Thanks so much for this reply, really quite inspiring. Much appreciated. Ability to inspect is vital indeed. Koshkin 8 months ago parent prev next [—]. If you understand this, you understand pointers. The arrow operator is syntactic sugar. Pointers are easy and fun, just keep experimenting with them. What is essential is understanding that they're nothing more than like a piece of paper containing where something is instead of being part of that something, and they're the same size of the address registers of the underlying hardware as they must be able to point everywhere in the addressable memory.
Therefore, on a 32 bit system pointers will be 32 bits long, on a 64 bit system they will be 64 bits long, etc. That is, both a pointer to a single byte and a pointer to a 10GB memory chunk will be of the same size because they just hold a memory location whose value represents where that data starts.
Therefore, declaring pointers to a certain type doesn't change their size at all, it just becomes handy when one needs to go back and forth in a memory area in which objects of that type are stored one after another, so that incrementing or decrementing the pointer by a number actually means that number times the size of the objects.
Imagine asking for directions to someone and he replies "3rd door" or "3rd building" or "3rd block"; he gave you a pointer that is always the same size, but how much you have to walk will depend on the destination size.
Once grasped the above, it should become a lot more easy; I had the same problems, then one day had a flash and they became totally clear and fun. Keep also track of the pointed data values, changing it instead of the pointer, or the other way around, are common mistakes.
An old small command line program like "cdecl" can help a lot to understand complex declarations, and someone has even made an interactive webpage around it cdecl. That is fascinating, thanks for sharing. What part of pointers is it that makes it so hard for you to grasp, if you have any thoughts? Do you have any experience in other programming languages? I guess one important part is to realize that in C, variables are basically names for memory locations, that in turn hold values.
In other languages variables can be more abstract, and you have no idea how the value s are being associated with the variable name.
I started writing an example here, but I ran out of time and it wasn't good enough. EDIT: Now I've taken a loo at the relevant pages in the guide itself, and it seemed to explain the concepts very clearly and easily, so I'm not sure how to help. I'll then come back to this manual I'm sure.
Like I say - I've enjoyed it quite a bit. Its certainly me lacking - not the book! Something that might help is understanding a very fundamental thing about computers: they are very, very stupid. This gives rise to situations where in order to create greater levels of complexity, lots of unintuitive, and seemingly even pointless things need to be done. When a local variable is defined, it is not initialized by the system, you must initialize it yourself.
It is a good programming practice to initialize variables properly, otherwise your program may produce unexpected results, because uninitialized variables will take some garbage value already available at their memory location. Arrays a kind of data structure that can store a fixed-size sequential collection of elements of the same type.
An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type. Instead of declaring individual variables, such as number0, number1, A specific element in an array is accessed by an index. All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.
This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid C data type. If you omit the size of the array, an array just big enough to hold the initialization is created.
You will create exactly the same array as you did in the previous example. The above statement assigns the 5 th element in the array with a value of All arrays have 0 as the index of their first element which is also called the base index and the last index of an array will be total size of the array minus 1. An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array.
The above statement will take the 10 th element from the array and assign the value to salary variable. The following example Shows how to use all the three above mentioned concepts viz.
Arrays are important to C and should need a lot more attention. C supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.
You can pass to the function a pointer to an array by specifying the array's name without an index. You can generate a pointer to the first element of an array by simply specifying the array name, without any index. Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer.
Let's start learning them in simple and easy steps. A pointer is a variable whose value is the address of another variable, i. Like any variable or constant, you must declare a pointer before using it to store any variable address. Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. However, in this statement the asterisk is being used to designate a variable as a pointer.
The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to. There are a few important operations, which we will do with the help of pointers very frequently.
It is always a good practice to assign a NULL value to a pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer. The NULL pointer is a constant with a value of zero defined in several standard libraries.
In most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location.
But by convention, if a pointer contains the null zero value, it is assumed to point to nothing. Pointers have many but easy concepts and they are very important to C programming. Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function. C allows a function to return a pointer to the local variable, static variable, and dynamically allocated memory as well.
Thus a null-terminated string contains the characters that comprise the string followed by a null. The following declaration and initialization create a string consisting of the word "Hello". To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word "Hello.
Actually, you do not place the null character at the end of a string constant. Arrays allow to define type of variables that can hold several data items of the same kind. Similarly structure is another user defined data type available in C that allows to combine data items of different kinds. Structures are used to represent a record.
Suppose you want to keep track of your books in a library. To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member. The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition.
At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional.
To access any member of a structure, we use the member access operator. The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use the keyword struct to define variables of structure type.
You can pass a structure as a function argument in the same way as you pass any other variable or pointer. Now, you can store the address of a structure variable in the above defined pointer variable. Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Reading external file formats -- non-standard file formats could be read in, e.
C allows us to do this in a structure definition by putting :bit length after the variable. C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer.
If this is not the case, then some compilers may allow memory overlap for the fields while others would store the next field in the next word. A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose. To define a union, you must use the union statement in the same way as you did while defining a structure.
The union statement defines a new data type with more than one member for your program. The union tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the union's definition, before the final semicolon, you can specify one or more union variables but it is optional.
Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. It means a single variable, i. You can use any built-in or user defined data types inside a union based on your requirement. The memory occupied by a union will be large enough to hold the largest member of the union. For example, in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by a character string.
To access any member of a union, we use the member access operator. The member access operator is coded as a period between the union variable name and the union member that we wish to access.
You would use the keyword union to define variables of union type. Here, we can see that the values of i and f members of union got corrupted because the final value assigned to the variable has occupied the memory location and this is the reason that the value of str member is getting printed very well.
This structure requires 8 bytes of memory space but in actual, we are going to store either 0 or 1 in each of the variables. The C programming language offers a better way to utilize the memory space in such situations. If you are using such variables inside a structure then you can define the width of a variable which tells the C compiler that you are going to use only those number of bytes.
The above structure requires 4 bytes of memory space for status variable, but only 2 bits will be used to store the values. If you will use up to 32 variables each one with a width of 1 bit, then also the status structure will use 4 bytes. However as soon as you have 33 variables, it will allocate the next slot of the memory and it will start using 8 bytes.
An integer type that determines how a bit-field's value is interpreted. The type may be int, signed int, or unsigned int. The number of bits in the bit-field. The width must be less than or equal to the bit width of the specified type.
The variables defined with a predefined width are called bit fields. The above structure definition instructs the C compiler that the age variable is going to use only 3 bits to store the value.
If you try to use more than 3 bits, then it will not allow you to do so. The C programming language provides a keyword called typedef , which you can use to give a type a new name.
After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example. You can use typedef to give a name to your user defined data types as well.
When we say Input , it means to feed some data into a program. An input can be given in the form of a file or from the command line. C programming provides a set of built-in functions to read the given input and feed it to the program as per requirement. When we say Output , it means to display some data on screen, printer, or in any file. C programming provides a set of built-in functions to output the data on the computer screen as well as to save it in text or binary files.
C programming treats all the devices as files. So devices such as the display are addressed in the same way as files and the following three files are automatically opened when a program executes to provide access to the keyboard and screen. The file pointers are the means to access the file for reading and writing purpose. This section explains how to read values from the screen and how to print the result on the screen. The int getchar void function reads the next available character from the screen and returns it as an integer.
This function reads only single character at a time. You can use this method in the loop in case you want to read more than one character from the screen. The int putchar int c function puts the passed character on the screen and returns the same character. This function puts only single character at a time. You can use this method in the loop in case you want to display more than one character on the screen. When the above code is compiled and executed, it waits for you to input some text.
NOTE: Though it has been deprecated to use gets function, Instead of using gets, you want to use fgets. There are many other formatting options available which can be used based on requirements. If you provide "string string" or "integer integer", then it will be assumed as wrong input.
Secondly, while reading a string, scanf stops reading as soon as it encounters a space, so "this is test" are three strings for scanf. The last chapter explained the standard input and output devices handled by C programming language. This chapter cover how C programmers can create, open, close text or binary files for their data storage.
A file represents a sequence of bytes, regardless of it being a text file or a binary file. C programming language provides access on high level functions as well as low level OS level calls to handle file on your storage devices.
This chapter will take you through the important calls for file management. You can use the fopen function to create a new file or to open an existing file. This call will initialize an object of the type FILE , which contains all the information necessary to control the stream. Opens a text file for writing. If it does not exist, then a new file is created.
Here your program will start writing content from the beginning of the file. Opens a text file for writing in appending mode. Here your program will start appending content in the existing file content. Opens a text file for both reading and writing. It first truncates the file to zero length if it exists, otherwise creates a file if it does not exist.
It creates the file if it does not exist. The reading will start from the beginning but writing can only be appended. The fclose - function returns zero on success, or EOF if there is an error in closing the file. This function actually flushes any data still pending in the buffer to the file, closes the file, and releases any memory used for the file. The EOF is a constant defined in the header file stdio. There are various functions provided by C standard library to read and write a file, character by character, or in the form of a fixed length string.
The function fputc writes the character value of the argument c to the output stream referenced by fp. It returns the written character written on success otherwise EOF if there is an error. The function fputs writes the string s to the output stream referenced by fp. It returns a non-negative value on success, otherwise EOF is returned in case of any error. Try the following example. If it is not, then before proceeding, you must create this directory on your machine. When the above code is compiled and executed, it creates a new file test.
Let us read this file in the next section. The fgetc function reads a character from the input file referenced by fp. The return value is the character read, or in case of any error, it returns EOF. The functions fgets reads up to n-1 characters from the input stream referenced by fp. It copies the read string into the buffer buf , appending a null character to terminate the string.
Let's see a little more in detail about what happened here. First, fscanf read just This because after that, it encountered a space, second call is for fgets which reads the remaining line till it encountered end of line.
Finally, the last call fgets reads the second line completely. Both of these functions should be used to read or write blocks of memories - usually arrays or structures. The C Preprocessor is not a part of the compiler, but is a separate step in the compilation process. In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation.
All preprocessor commands begin with a hash symbol. It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column. Use define for constants to increase readability. These directives tell the CPP to get stdio. The next line tells CPP to get myheader. ANSI C defines a number of macros. Although each one is available for use in programming, the predefined macros should not be directly modified. When the above code in a file test.
A macro is normally confined to a single line. The stringize or number-sign operator ' ' , when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro having a specified argument or parameter list. The token-pasting operator within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token.
This example shows the concatenation of token n into token34 and here we have used both stringize and token-pasting. The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using define. If the specified identifier is defined, the value is true non-zero.
If the symbol is not defined, the value is false zero. One of the powerful functions of the CPP is the ability to simulate functions using parameterized macros. Macros with arguments must be defined using the define directive before they can be used.
The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between the macro name and open parenthesis. A header file is a file with extension. There are two types of header files: the files that the programmer writes and the files that comes with your compiler.
You request to use a header file in your program by including it with the C preprocessing directive include , like you have seen inclusion of stdio. Including a header file is equal to copying the content of the header file but we do not do it because it will be error-prone and it is not a good idea to copy the content of a header file in the source files, especially if we have multiple source files in a program.
Both the user and the system header files are included using the preprocessing directive include. This form is used for system header files. It searches for a file named 'file' in a standard list of system directories.
You can prepend directories to this list with the -I option while compiling your source code. This form is used for header files of your own program. It searches for a file named 'file' in the directory containing the current file. The include directive works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current source file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the include directive.
For example, if you have a header file header. If a header file happens to be included twice, the compiler will process its contents twice and it will result in an error. This construct is commonly known as a wrapper ifndef.
The preprocessor will skip over the entire contents of the file, and the compiler will not see it twice. Sometimes it is necessary to select one of the several different header files to be included into your program. For instance, they might specify configuration parameters to be used on different sorts of operating systems. But as it grows, it becomes tedious, instead the preprocessor offers the ability to use a macro for the header name.
This is called a computed include. Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a 'long' value into a simple integer then you can type cast 'long' to 'int'. It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value.
Type conversions can be implicit which is performed by the compiler automatically, or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.
Integer promotion is the process by which values of integer type "smaller" than int or unsigned int are converted either to int or unsigned int. Here, the value of sum is because the compiler is doing integer promotion and converting the value of 'c' to ASCII before performing the actual addition operation. The usual arithmetic conversions are implicitly performed to cast their values to a common type.
Here, it is simple to understand that first c gets converted to integer, but as the final value is double, usual arithmetic conversion applies and the compiler converts i and c into 'float' and adds them yielding a 'float' result. As such, C programming does not provide direct support for error handling but being a system programming language, it provides you access at lower level in the form of return values.
It is set as a global variable and indicates an error occurred during any function call. So a C programmer can check the returned values and can take appropriate action depending on the return value. It is a good practice, to set errno to 0 at the time of initializing a program. A value of 0 indicates that there is no error in the program. The C programming language provides perror and strerror functions which can be used to display the text message associated with errno. The perror function displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value.
The strerror function, which returns a pointer to the textual representation of the current errno value. Let's try to simulate an error condition and try to open a file which does not exist. Here I'm using both the functions to show the usage, but you can use one or more ways of printing your errors.
Second important point to note is that you should use stderr file stream to output all the errors. It is a common problem that at the time of dividing any number, programmers do not check if a divisor is zero and finally it creates a runtime error. Recursion is the process of repeating items in a self-similar way. In programming languages, if a program allows you to call a function inside the same function, then it is called a recursive call of the function.
The C programming language supports recursion, i. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop. Recursive functions are very useful to solve many mathematical problems, such as calculating the factorial of a number, generating Fibonacci series, etc. Sometimes, you may come across a situation, when you want to have a function, which can take variable number of arguments, i.
The C programming language provides a solution for this situation and you are allowed to define a function which can accept variable number of parameters based on your requirement. The following example shows the definition of such a function. It should be noted that the function func has its last argument as ellipses, i. To use such functionality, you need to make use of stdarg. Define a function with its last parameter as ellipses and the one just before the ellipses is always an int which will represent the number of arguments.
This type is defined in stdarg. When the above code is compiled and executed, it produces the following result. It should be noted that the function average has been called twice and each time the first argument represents the total number of variable arguments being passed. Only ellipses will be used to pass variable number of arguments. This chapter explains dynamic memory management in C.
The C programming language provides several functions for memory allocation and management. This function allocates an array of num elements each of which size in bytes will be size. While programming, if you are aware of the size of an array, then it is easy and you can define it as an array.
But now let us consider a situation where you have no idea about the length of the text you need to store, for example, you want to store a detailed description about a topic. So you have complete control and you can pass any size value while allocating memory, unlike arrays where once the size defined, you cannot change it.
When your program comes out, operating system automatically release all the memory allocated by your program but as a good practice when you are not in need of memory anymore then you should release that memory by calling the function free.
Alternatively, you can increase or decrease the size of an allocated memory block by calling the function realloc. You can try the above example without re-allocating extra memory, and strcat function will give an error due to lack of available memory in description. It is possible to pass some values from the command line to your C programs when they are executed. These values are called command line arguments and many times they are important for your program especially when you want to control your program from outside instead of hard coding those values inside the code.
The command line arguments are handled using main function arguments where argc refers to the number of arguments passed, and argv[] is a pointer array which points to each argument passed to the program.
When the above code is compiled and executed with single argument, it produces the following result. When the above code is compiled and executed with a two arguments, it produces the following result.
When the above code is compiled and executed without passing any argument, it produces the following result. If no arguments are supplied, argc will be one, and if you pass one argument then argc is set at 2.
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