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What does pragma once mean? I found it in some header files.
What does pragma once mean? I found it in some header files. It is an extension implemented by some preprocessors to help make header files idempotent; that is, to make sure that their contents are processed exactly once even if they are #included multiple times.Some people claim that #pragma once c... 2015-12-11, 1836👍, 0💬

What does it really mean for a program to be legal or `valid or ``conforming?
What does it really mean for a program to be legal or `valid or ``conforming? Simply stated, the Standard talks about three kinds of conformance: conforming programs, strictly conforming programs, and conforming implementations. A conforming program is one that is accepted by a conforming implementa... 2015-11-23, 1835👍, 0💬

How do I send escape sequences to control a terminal or other device?
How do I send escape sequences to control a terminal or other device? If you can figure out how to send characters to the device at all , it's easy enough to send escape sequences. In ASCII, the ESC code is 033 (27 decimal), so code like fprintf(ofd, "\033[J"); sends the sequence ESC [ J . Some prog... 2015-04-20, 1834👍, 0💬

How can I write a macro which takes a variable number of arguments ....
How can I write a macro which takes a variable number of arguments, or use the preprocessor to ``turn off'' a function call with a variable number of arguments? One popular trick is to define and invoke the macro with a single, parenthesized ``argument'' which in the macro expansion becomes the enti... 2016-01-19, 1833👍, 0💬

My programs prompts and intermediate output dont always show up on the screen ...
My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program. It's best to use an explicit fflush(stdout) whenever output should definitely be visible (and especially if the text does not end with \n). Several mechanisms a... 2015-11-11, 1832👍, 0💬

What is wrong with this code?
What's wrong with this code? char c; while((c = getchar()) != EOF) ... For one thing, the variable to hold getchar's return value must be an int. EOF is an ``out of band'' return value from getchar: it is distinct from all possible char values which getchar can return. (On modern systems, it does no... 2015-11-16, 1830👍, 0💬

I mm compiling a program, and I seem to be missing one of the header files it requires
I mm compiling a program, and I seem to be missing one of the header files it requires There are several situations, depending on what sort of header file it is that's ``missing''. If the missing header file is truly a standard one (that is, one defined by the ANSI C Standard, such as &lt;stdio.... 2016-02-05, 1829👍, 0💬

I had a frustrating problem which turned out to be caused by the line ....
I had a frustrating problem which turned out to be caused by the line printf("%d", n); where n was actually a long int. I thought that ANSI function prototypes were supposed to guard against argument type mismatches like this. A:When a function accepts a variable number of arguments, its prototype d... 2016-01-11, 1829👍, 0💬

Can I use an ifdef in a #define line ...
Can I use an #ifdef in a #define line, to define something two different ways, like this? #define a b \ #ifdef whatever c d #else e f g #endif No. You can't ``run the preprocessor on itself,'' so to speak. What you can do is use one of two completely separate #define lines, depending on the #ifdef s... 2016-02-03, 1827👍, 0💬

What is a good data structure to use for storing lines of text?
What's a good data structure to use for storing lines of text? I started to use fixed-size arrays of arrays of char, but they're just too restrictive. One good way of doing this is with a pointer (simulating an array) to a set of pointers (each simulating an array) of char. This data structure is so... 2015-02-25, 1823👍, 0💬

Why doesnt C have nested functions?
Why doesnt C have nested functions? It's not trivial to implement nested functions such that they have the proper access to local variables in the containing function(s), so they were deliberately left out of C as a simplification. (gcc does allow them, as an extension.) For many potential uses of n... 2015-01-14, 1823👍, 0💬

What is the difference between const char .....
What's the difference between const char *p, char const *p, and char * const p? The first two are interchangeable; they declare a pointer to a constant character (you can't change any pointed-to characters). char * const p declares a constant pointer to a (variable) character (i.e. you can't change ... 2016-01-06, 1819👍, 0💬

I am sure I've got the trig functions declared correctly, but they are still giving me wrong answers.
I am sure I've got the trig functions declared correctly, but they are still giving me wrong answers. You weren't handing them angles in degrees, were you? C's trig functions (like FORTRAN's and most other languages) accept angles in radians. The conversion from degrees to radians is simple enough: ... 2015-06-29, 1819👍, 0💬

Why are some ANSI ISO Standard library functions showing you
Why are some ANSI/ISO Standard library functions showing up as undefined, even though I've got an ANSI compiler? It's possible to have a compiler available which accepts ANSI syntax, but not to have ANSI-compatible header files or run-time libraries installed. (In fact, this situation is rather comm... 2015-11-30, 1817👍, 0💬

What are the complete rules for header file searching?
What are the complete rules for header file searching? The exact behavior is implementation-defined (which means that it is supposed to be documented; Typically, headers named with &lt;> syntax are searched for in one or more standard places. Header files named with "" syntax are first searched ... 2016-02-12, 1816👍, 0💬

Why isnt there a numbered, multi-level break statement to break out
Why isn't there a numbered, multi-level break statement to break out of several loops at once? What am I supposed to use instead, a goto? A:First, remember why it is that break and continue exist at all--they are, in effect, ``structured gotos'' used in preference to goto (and accepted as alternativ... 2015-01-30, 1816👍, 0💬

I am starting to think about multinational character sets ....
I'm starting to think about multinational character sets, and I'm worried about the implications of making sizeof(char) be 2 so that 16-bit character sets can be represented. If type char were made 16 bits, sizeof(char) would still be 1, and CHAR_BIT in &lt;limits.h> would be 16, and it would si... 2016-03-02, 1815👍, 0💬

I am trying to use the ANSI stringizing preprocessing operator ....
I'm trying to use the ANSI ``stringizing'' preprocessing operator `#' to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value. It turns out that the definition of # says that it's supposed to stringize a macro argument immediately, w... 2016-01-22, 1814👍, 0💬

Is there a way to have non-constant case labels (i.e. ranges or arbitrary expressions)?
Is there a way to have non-constant case labels (i.e. ranges or arbitrary expressions)? No. The switch statement was originally designed to be quite simple for the compiler to translate, therefore case labels are limited to single, constant, integral expressions. You can attach several case labels t... 2015-02-02, 1813👍, 0💬

Why doesn't sizeof tell me the size of the block of memory pointed to by a pointer?
Why doesn't sizeof tell me the size of the block of memory pointed to by a pointer? sizeof tells you the size of the pointer. There is no portable way to find out the size of a malloc'ed block. (Remember, too, that sizeof operates at compile time 2016-03-21, 1811👍, 0💬

How can I clear the screen? How can I print text in color? How can I move the cursor to a specific x, y position?
How can I clear the screen? How can I print text in color? How can I move the cursor to a specific x, y position? Such things depend on the terminal type (or display) you're using. You will have to use a library such as termcap, terminfo, or curses, or some system-specific routines, to perform these... 2015-04-27, 1807👍, 0💬

Having dynamically allocated an array , can I change its size?
Having dynamically allocated an array , can I change its size? Yes. This is exactly what realloc is for. Given a region of malloced memory its size can be changed using code like: dynarray = realloc(dynarray, 20 * sizeof(int)); Note that realloc may not always be able to enlarge memory regions in-pl... 2016-03-21, 1804👍, 0💬

But what about main's third argument, envp?
But what about main's third argument, envp? It's a non-standard (though common) extension. If you really need to access the environment in ways beyond what the standard getenv function provides, though, the global variable environ is probably a better avenue (though it's equally non-standard). 2015-12-20, 1803👍, 0💬

Why cant I open a file by its explicit path?
Why can't I open a file by its explicit path? The call fopen("c:\newdir\file.dat", "r") is failing. The file you actually requested--with the characters \n and \f in its name--probably doesn't exist, and isn't what you thought you were trying to open. In character constants and string literals, the ... 2015-04-08, 1802👍, 0💬

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.NET C C++ CSS DBA General HTML Java JavaScript JSP JUnit MySQL Networking Oracle Perl Perl PHP PL/SQL RSS Software QA SQL Server Struts Unix Windows XHTML XML