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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, 591👍, 0💬

What does it mean?
Is char a[3] = "abc"; legal? What does it mean? It is legal in ANSI C (and perhaps in a few pre-ANSI systems), though useful only in rare circumstances. It declares an array of size three, initialized with the three characters 'a', 'b', and 'c', without the usual terminating '\0' character. The arra...
2015-12-09, 590👍, 0💬

Since array references decay into pointers ...
Since array references decay into pointers, if arr is an array, what's the difference between arr and &arr? The type. In Standard C, &arr yields a pointer, of type pointer-to-array-of-T, to the entire array. (In pre-ANSI C, the & in &arr generally elicited a warning, and was generall...
2015-12-09, 555👍, 0💬

Why cant I perform arithmetic on a void pointer?
Why cant I perform arithmetic on a void pointer? The compiler doesn't know the size of the pointed-to objects. (Remember that pointer arithmetic is always in terms of the pointed-to size; Therefore, arithmetic on void *'s is disallowed (though some compilers allow it as an extension). Before perform...
2015-12-07, 534👍, 0💬

What is the difference between memcpy and memmove?
What is the difference between memcpy and memmove? memmove offers guaranteed behavior if the memory regions pointed to by the source and destination arguments overlap. memcpy makes no such guarantee, and may therefore be more efficiently implementable. When in doubt, it's safer to use memmove. It se...
2015-12-07, 595👍, 0💬

What should malloc0 do? Return a null pointer or a pointer to 0 bytes?
What should malloc0 do? Return a null pointer or a pointer to 0 bytes? The ANSI/ISO Standard says that it may do either; the behavior is implementation-defined Portable code must either take care not to call malloc(0), or be prepared for the possibility of a null return.
2015-12-04, 663👍, 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, 636👍, 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, 619👍, 0💬

Wy compiler is rejecting the simplest possible test programs ...
My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors. It's complaining about the first line of main(int argc, char **argv) { return 0; } Perhaps it is a pre-ANSI compiler, unable to accept function prototypes and the like. If you don't have access to an ANSI ...
2015-12-02, 563👍, 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, 592👍, 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, 549👍, 0💬

Does anyone have a tool for converting old-style C programs to ANSI C?
Does anyone have a tool for converting old-style C programs to ANSI C, or vice versa, or for automatically generating prototypes? Two programs, protoize and unprotoize, convert back and forth between prototyped and ``old style'' function definitions and declarations. (These programs do not handle fu...
2015-11-25, 576👍, 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, 652👍, 0💬

People seem to make a point of distinguishing between ...
People seem to make a point of distinguishing between implementation-defined, unspecified, and undefined behavior. What do these mean? First of all, all three of these represent areas in which the C Standard does not specify exactly what a particular construct, or a program which uses it, must do. T...
2015-11-23, 575👍, 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, 594👍, 0💬

I am appalled that the ANSI Standard leaves so many issues undefined ...
I'm appalled that the ANSI Standard leaves so many issues undefined. Isn't a Standard's whole job to standardize these things? It has always been a characteristic of C that certain constructs behaved in whatever way a particular compiler or a particular piece of hardware chose to implement them. Thi...
2015-11-18, 666👍, 0💬

People keep saying that the behavior of ...
People keep saying that the behavior of i = i++ is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected. A compiler may do anything it likes when faced with undefined behavior (and, within limits, with implementation-defined and unspecified behavior), includi...
2015-11-18, 632👍, 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, 646👍, 0💬

I have a simple little program that reads characters until EOF ...
I have a simple little program that reads characters until EOF, but how do I actually enter that ``EOF'' value from the keyboard? I see that EOF is defined by to be -1; am I supposed to enter -1? If you think about it, what you enter can't be -1, because ``-1'' is two characters, and getchar is read...
2015-11-16, 655👍, 0💬

Why does the simple line-copying loop while ...
Why does the simple line-copying loop while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); } copy the last line twice? p> In C, end-of-file is only indicated after an input routine has tried to read, and failed. (In other words, C's I/O is not like Pascal's.) Usually, you should just c...
2015-11-13, 659👍, 0💬

Why do all the lines end up containing copies of the last line?
I'm using fgets to read lines from a file into an array of pointers. Why do all the lines end up containing copies of the last line? You have only allocated memory for one line, linebuf. Each time you call fgets, the previous line is overwritten. fgets doesn't do any memory allocation: unless it rea...
2015-11-13, 627👍, 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, 640👍, 0💬

How can I read one character at a time, without waiting for the RETURN key
How can I read one character at a time, without waiting for the RETURN key Alas, there is no standard or portable way to do these things in C. Concepts such as screens and keyboards are not even mentioned in the Standard, which deals only with simple I/O ``streams'' of characters. Input to a compute...
2015-11-11, 777👍, 0💬

How can I print a character in a printf format string?
How can I print a '%' character in a printf format string? I tried \%, but it didn't work. Simply double the percent sign: %% . The reason it's tricky to print % signs with printf is that % is essentially printf's escape character. Whenever printf sees a %, it expects it to be followed by a characte...
2015-11-09, 626👍, 0💬

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