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How can I use a macro argument inside a string literal in the macro expansion?
How can I use a macro argument inside a string literal in the macro expansion? Some pre-ANSI compilers/preprocessors interpreted macro definitions like #define TRACE(var, fmt) printf("TRACE: var = fmt\n", var) such that invocations like TRACE(i, %d); were expanded as printf("TRACE: i = %d\n", i); In... 2016-01-22, 1968👍, 0💬

Why wont the Frobozz Magic C Compiler ...
Why won't the Frobozz Magic C Compiler, which claims to be ANSI compliant, accept this code? I know that the code is ANSI, because gcc accepts it. Many compilers support a few non-Standard extensions, gcc more so than most. Are you sure that the code being rejected doesn't rely on such an extension?... 2015-11-25, 1962👍, 0💬

The predefined constant M_PI seems to be missing from my machines copy of math.h.
The predefined constant M_PI seems to be missing from my machines copy of math.h. That constant (which is apparently supposed to be the value of pi, accurate to the machine's precision), is not standard; in fact a standard-conforming copy of should not #define a symbol M_PI. If you need pi, you'll h... 2015-06-24, 1960👍, 0💬

What is Duff's Device?
What is Duff's Device? It's a devastatingly devious way of unrolling a loop, devised by Tom Duff while he was at Lucasfilm. In its ``classic'' form, it was used to copy bytes, and looked like this: register n = (count + 7) / 8; /* count > 0 assumed */ switch (count % 8) { case 0: do { *to = *from++;... 2015-01-02, 1960👍, 0💬

I figured I could use scanf more safely if I checked its ...
I figured I could use scanf more safely if I checked its return value to make sure that the user typed the numeric values I expect: int n; while(1) { printf("enter a number: "); if(scanf("%d", &amp;n) == 1) break; printf("try again: "); } printf("you typed %d\n", n); but sometimes it seems to go... 2015-10-19, 1955👍, 0💬

Is there anything like an ifdef for typedefs?
Is there anything like an ifdef for typedefs? Unfortunately, no. (There can't be, because types and typedefs haven't been parsed at preprocessing time.) You may have to keep sets of preprocessor macros (e.g. MY_TYPE_DEFINED) recording whether certain typedefs have been declared. 2016-02-01, 1953👍, 0💬

How can I write a generic macro to swap two values?
How can I write a generic macro to swap two values? There is no good answer to this question. If the values are integers, a well-known trick using exclusive-OR could perhaps be used, but it will not work for floating-point values or pointers, or if the two values are the same variable. If the macro ... 2016-02-24, 1949👍, 0💬

How can I open files mentioned on the command line, and parse option flags?
How can I open files mentioned on the command line, and parse option flags? Here is a skeleton which implements a traditional Unix-style argv parse, handling option flags beginning with -, and optional filenames. (The two flags accepted by this example are -a and -b; -b takes an argument.) #include ... 2015-03-11, 1949👍, 0💬

I thought that ANSI function prototypes were supposed to guard against argument type mismatches.
I thought that ANSI function prototypes were supposed to guard against argument type mismatches. When a function accepts a variable number of arguments, its prototype does not (and cannot) provide any information about the number and types of those variable arguments. Therefore, the usual protection... 2015-11-06, 1939👍, 0💬

Why does the ANSI Standard place limits on the length and case-significance of external identifiers?
Why does the ANSI Standard place limits on the length and case-significance of external identifiers? The problem is linkers which are under control of neither the ANSI/ISO Standard nor the C compiler developers on the systems which have them. The limitation is only that identifiers be significant in... 2015-12-04, 1920👍, 0💬

I am trying to do some simple trig...
I'm trying to do some simple trig, and I am #including &lt;math.h>, but the linker keeps complaining that functions like sin and cos are undefined. Make sure you're actually linking with the math library. For instance, due to a longstanding bug in Unix and Linux systems, you usually need to use ... 2015-07-01, 1904👍, 0💬

Must I free allocated memory before the program exits?
Must I free allocated memory before the program exits? You shouldn't have to. A real operating system definitively reclaims all memory and other resources when a program exits; the system cannot afford to have memory integrity depend on the whims of random programs. (Strictly speaking, it is not eve... 2016-03-28, 1900👍, 0💬

Why does the call char scanf work?
Why does the call char s[30]; scanf("%s", s); work? I thought you always needed an & on each variable passed to scanf. You always need a pointer; you don't necessarily need an explicit &amp;. When you pass an array to scanf, you do not need the &, because arrays are always passed to func... 2015-10-30, 1900👍, 0💬

Why doesnt strcat work? ....
Why doesn't strcat(string, '!'); work? There is a very real difference between characters and strings, and strcat concatenates strings. A character constant like '!' represents a single character. A string literal between double quotes usually represents multiple characters. A string literal like "!... 2016-03-14, 1898👍, 0💬

I dont understand why I cant use const values in initializers ....
I don't understand why I can't use const values in initializers and array dimensions, as in const int n = 5; int a[n]; The const qualifier really means ``read-only''; an object so qualified is a run-time object which cannot (normally) be assigned to. The value of a const-qualified object is therefor... 2016-01-08, 1898👍, 0💬

So what could go wrong? Are there really any systems where void main doesnt work?
So what could go wrong? Are there really any systems where void main doesnt work? It has been reported that programs using void main() and compiled using BC++ 4.5 can crash. Some compilers (including DEC C V4.1 and gcc with certain warnings enabled) will complain about void main(). 2015-12-18, 1895👍, 0💬

How can I determine whether a machines byte order is big-endian or little-endian?
How can I determine whether a machines byte order is big-endian or little-endian? The usual techniques are to use a pointer: int x = 1; if(*(char *)&amp;x == 1) printf("little-endian\n"); else printf("big-endian\n"); or a union: union { int i; char c[sizeof(int)]; } x; x.i = 1; if(x.c[0] == 1) p... 2015-02-16, 1891👍, 0💬

If I have a char * variable pointing to the name of a function ...
If I have a char * variable pointing to the name of a function, how can I call that function? Code like extern int func(int, int); char *funcname = "func"; int r = (*funcname)(1, 2); or r = (*(int (*)(int, int))funcname)(1, 2); doesn't seem to work. By the time a program is running, information abou... 2015-02-20, 1889👍, 0💬

I a'm getting strange syntax errors inside lines I have ifdeffed out.
I a'm getting strange syntax errors inside lines I have ifdeffed out. Under ANSI C, the text inside a ``turned off'' #if, #ifdef, or #ifndef must still consist of ``valid preprocessing tokens.'' This means that the characters " and ' must each be paired just as in real C code, and the pairs mustn't ... 2016-01-29, 1888👍, 0💬

What was noalias and what ever happened to it?
What was noalias and what ever happened to it? noalias was another type qualifier, in the same syntactic class as const and volatile, which was intended to assert that an object was not pointed to (``aliased'') by other pointers. The primary application, which is an important one, would have been fo... 2015-12-02, 1885👍, 0💬

People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed
People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed, but my compiler can't even replace i/=2 with a shift. A: Was i signed or unsigned? If it was signed, a shift is not equivalent (hint: think about the result if i is negative and odd), s... 2015-02-06, 1885👍, 0💬

What does the message Automatic aggregate intialization is an ANSI feature mean?
What does the message Automatic aggregate intialization is an ANSI feature'' mean? My compiler is complaining about valid ANSI code. Messages like these are typically emitted by pre-ANSI compilers which have been upgraded just enough to detect (but not properly translate) new C features which were i... 2015-11-30, 1881👍, 0💬

How can I use a preprocessorif expression to ? ....
How can I use a preprocessor #if expression to tell whether a machine's byte order is big-endian or little-endian? You probably can't. The usual techniques for detecting endianness involve pointers or arrays of char, or maybe unions, but preprocessor arithmetic uses only long integers, and there is ... 2016-02-01, 1878👍, 0💬

I can write C code that looks more like Pascal? ....
Here are some cute preprocessor macros: #define begin { #define end } With these, I can write C code that looks more like Pascal. What do y'all think? Use of macros like these, though perhaps superficially attractive, is generally discouraged; in severe cases the practice is called ``preprocessor ab... 2016-02-24, 1872👍, 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