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What does it mean when the linker says that _end is undefined?
What does it mean when the linker says that _end is undefined? That message is a quirk of the old Unix linkers. You get an error about _end being undefined only when other symbols are undefined, too--fix the others, and the error about _end will disappear. 2015-07-06, 1528👍, 0💬

If I can say... why can't I say...
If I can say char a[] = "Hello, world!"; why can't I say char a[14]; Strings are arrays, and you can't assign arrays directly. Use strcpy instead: strcpy(a, "Hello, world!"); 2016-03-11, 1527👍, 0💬

When I set a float variable to, say, 3.1, why is printf printing it as 3.0999999?
When I set a float variable to, say, 3.1, why is printf printing it as 3.0999999? Most computers use base 2 for floating-point numbers as well as for integers, and just as for base 10, not all fractions are representable exactly in base 2. It's well-known that in base 10, a fraction like 1/3 = 0.333... 2015-07-03, 1525👍, 0💬

How can I invoke another program (a standalone executable, or an operating system command) from within a C program?
How can I invoke another program (a standalone executable, or an operating system command) from within a C program? Use the library function system, which does exactly that. Some systems also provide a family of spawn routines which accomplish approximately the same thing. system is more ``portable'... 2015-03-18, 1524👍, 0💬

If fflush wont work, what can I use to flush input?
If fflush wont work, what can I use to flush input? It depends on what you're trying to do. If you're trying to get rid of an unread newline or other unexpected input after calling scanf you really need to rewrite or replace the call to scanf. Alternatively, you can consume the rest of a partially-r... 2015-10-12, 1521👍, 0💬

How can a process change an environment variable in its caller?
How can a process change an environment variable in its caller? It may or may not be possible to do so at all. Different operating systems implement global name/value functionality similar to the Unix environment in different ways. Whether the ``environment'' can be usefully altered by a running pro... 2015-03-11, 1520👍, 0💬

What's the difference between...?
What's the difference between #include &lt;> and #include "" ? The syntax is typically used with Standard or system-supplied headers, while "" is typically used for a program's own header files. 2016-02-16, 1519👍, 0💬

I think somethings wrong with my compiler ...
I think something's wrong with my compiler: I just noticed that sizeof('a') is 2, not 1 (i.e. not sizeof(char)). Perhaps surprisingly, character constants in C are of type int, so sizeof('a') is sizeof(int) (though this is another area where C++ differs). 2016-03-04, 1518👍, 0💬

How do I create a directory? How do I remove a directory (and its contents)?
How do I create a directory? How do I remove a directory (and its contents)? If your operating system supports these services, they are likely to be provided in C via functions named mkdir and rmdir. Removing a directory's contents as well will require listing them and calling remove . If you don't ... 2015-04-01, 1516👍, 0💬

Which is more efficient, a switch statement or an if else chain?
Which is more efficient, a switch statement or an if else chain? The differences, if any, are likely to be slight. The switch statement was designed to be efficiently implementable, though the compiler may choose to use the equivalent of an if/else chain (as opposed to a compact jump table) if the c... 2015-02-04, 1516👍, 0💬

I cant get va_arg to pull in an argument of type pointer-to-function.
I cant get va_arg to pull in an argument of type pointer-to-function. Try using a typedef for the function pointer type. The type-rewriting games which the va_arg macro typically plays are stymied by overly-complicated types such as pointer-to-function. To illustrate, a simplified implementation of ... 2015-06-05, 1515👍, 0💬

How can I swap two values without using a temporary?
How can I swap two values without using a temporary? The standard hoary old assembly language programmer's trick is: a ^= b; b ^= a; a ^= b; But this sort of code has little place in modern, HLL programming. Temporary variables are essentially free, and the idiomatic code using three assignments, na... 2015-02-06, 1515👍, 0💬

If NULL and 0 are equivalent as null pointer constants, which should I use?
If NULL and 0 are equivalent as null pointer constants, which should I use? Many programmers believe that NULL should be used in all pointer contexts, as a reminder that the value is to be thought of as a pointer. Others feel that the confusion surrounding NULL and 0 is only compounded by hiding 0 b... 2015-05-11, 1514👍, 0💬

I am trying to define a few simple little function-like macros ...
I'm trying to define a few simple little function-like macros such as #define square(x) x * x but they're not always working. There are three important rules to remember when defining function-like macros: 1. The macro expansion must always be parenthesized to protect any lower-precedence operators ... 2016-02-26, 1513👍, 0💬

Why does this code crash?
Why does this code: char *p = "hello, world!"; p[0] = 'H'; crash? String constants are in fact constant. The compiler may place them in nonwritable storage, and it is therefore not safe to modify them. When you need writable strings, you must allocate writable memory for them, either by declaring an... 2015-05-27, 1509👍, 0💬

Does C have anything like the `substr (extract substring) routine present in other languages?
Does C have anything like the `substr (extract substring) routine present in other languages? Not as such. To extract a substring of length LEN starting at index POS in a source string, use something like char dest[LEN+1]; strncpy(dest, &amp;source[POS], LEN); dest[LEN] = '\0'; /* ensure \0 term... 2015-08-19, 1507👍, 0💬

I have a pre-ANSI compiler, without stdarg.h What can I do?
I have a pre-ANSI compiler, without stdarg.h What can I do? There's an older header, &lt;varargs.h>, which offers about the same functionality. Here is the vstrcat function, rewritten to use &lt;varargs.h>: #include &lt;stdio.h> #include &lt;varargs.h> #include &lt;string.h> exte... 2015-06-10, 1506👍, 0💬

What is the best style for code layout in C?
What is the best style for code layout in C? While providing the example most often copied, also supply a good excuse for disregarding it: The position of braces is less important, although people hold passionate beliefs. We have chosen one of several popular styles. Pick a style that suits you, the... 2015-05-18, 1501👍, 0💬

I came across some code that puts a (void) cast before each call to printf. Why?
I came across some code that puts a (void) cast before each call to printf. Why? printf does return a value (the number of characters printed, or an error code), though few programs bother to check the return values from each call. Since some compilers (and lint) will warn about discarded return val... 2015-05-13, 1500👍, 0💬

How can I do serial comm port I O?
How can I do serial (``comm'') port I/O? It's system-dependent. Under Unix, you typically open, read, and write a device file in /dev, and use the facilities of the terminal driver to adjust its characteristics.Under MS-DOS, you can use the predefined stream stdaux, or a special file like COM1, or s... 2015-04-22, 1500👍, 0💬

What are pragmas and what are they good for?
What are pragmas and what are they good for? The #pragma directive provides a single, well-defined ``escape hatch'' which can be used for all sorts of (nonportable) implementation-specific controls and extensions: source listing control, structure packing, warning suppression (like lint's old /* NOT... 2015-12-11, 1498👍, 0💬

How can I trap or ignore keyboard interrupts like control-C?
How can I trap or ignore keyboard interrupts like control-C? The basic step is to call signal, either as #include &lt;signal.h> signal(SIGINT, SIG_IGN); to ignore the interrupt signal, or as extern void func(int); signal(SIGINT, func); to cause control to transfer to function func on receipt of ... 2015-03-06, 1498👍, 0💬

I am using scanf c to read a Y/N response
I'm using scanf %c to read a Y/N response, but later input gets skipped. You wanted scanf %c to read a single character, and it tried to, but when you tried to type that single character at it, before the rest of the input system would accept it, you had to hit the RETURN key, too. scanf read only t... 2015-10-21, 1496👍, 0💬

My compiler is complaining that printf is undefined ...
My compiler is complaining that printf is undefined! How can this be? It's the world's most popular C function... Allegedly, there are C compilers for Microsoft Windows which do n ot support printf, on the argument that printf is for printing to old-fashioned terminals, while under Windows the right... 2015-07-03, 1490👍, 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