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What is hashing?
What is hashing? Hashing is the process of mapping strings to integers, usually in a relatively small range. A ``hash function'' maps a string (or some other data structure) to a bounded number (the ``hash bucket'') which can more easily be used as an index in an array, or for performing repeated co... 2015-01-07, 1642👍, 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, 1641👍, 0💬

What if a built-in logical or relational operator `returns something other than 1? ....
Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered ``true'' in C? What if a built-in logical or relational operator ``returns'' something other than 1? It is true (sic) that any nonzero value is considered true in C, but this applies only ``on input'', i.e. where a Boolean... 2016-02-29, 1640👍, 0💬

Should I use symbolic names like TRUE and FALSE for Boolean constants, or plain 1 and 0?
Should I use symbolic names like TRUE and FALSE for Boolean constants, or plain 1 and 0? It's your choice. Preprocessor macros like TRUE and FALSE (and, of course, NULL) are used for code readability, not because the underlying values might ever change. It's a matter of style, not correctness, wheth... 2016-02-29, 1640👍, 0💬

Are the outer parentheses in return statements really optional?
Are the outer parentheses in return statements really optional? Yes. Long ago, in the early days of C, they were required, and just enough people learned C then, and wrote code which is still in circulation, that the notion that they might still be required is widespread. (As it happens, parentheses... 2015-02-02, 1640👍, 0💬

I need some code to do regular expression and wildcard matching.
I need some code to do regular expression and wildcard matching. Make sure you recognize the difference between: * Classic regular expressions, variants of which are used in such Unix utilities as ed and grep. In regular expressions, a dot . usually matches any single character, and the sequence .* ... 2015-08-12, 1632👍, 0💬

What do Segmentation violation, `Bus error, and General protection fault mean? What is a core dump?
What do Segmentation violation, `Bus error, and General protection fault mean? What is a core dump? These symptoms (and any similar messages having to do with memory access violations or protection faults) generally mean that your program tried to access memory it shouldn't have, invariably as a res... 2015-05-20, 1632👍, 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, 1631👍, 0💬

Suggesting that there can be 62 seconds in a minute?
Suggesting that there can be 62 seconds in a minute? Q: Why can tm_sec in the tm structure range from 0 to 61, suggesting that there can be 62 seconds in a minute? A: That's actually a buglet in the Standard. There can be 61 seconds in a minute during a leap second. It's possible for there to be two... 2015-01-05, 1629👍, 0💬

Why does everyone say not to use scanf? What should I use instead?
Why does everyone say not to use scanf? What should I use instead? scanf has a number of problems, its %s format has the same problem that gets() has --it's hard to guarantee that the receiving buffer won't overflow. More generally, scanf is designed for relatively structured, formatted input (its n... 2015-10-21, 1628👍, 0💬

How can I write a function which takes a variable number of arguments
How can I write a function which takes a variable number of arguments and passes them to some other function (which takes a variable number of arguments)? In general, you cannot. Ideally, you should provide a version of that other function which accepts a va_list pointer. Suppose you want to write a... 2015-06-05, 1628👍, 0💬

Can I use base-2 constants (something like 0b101010)? Is there a printf format for binary?
Can I use base-2 constants (something like 0b101010)? Is there a printf format for binary? No, on both counts, although there are various preprocessor tricks you can try. You can convert base-2 string representations to integers with strtol. If you need to print numbers out in base 2, . 2015-02-13, 1626👍, 0💬

Does C have circular shift operators?
Does C have circular shift operators? No. (Part of the reason why is that the sizes of C's types aren't precisely defined----but a circular shift makes most sense when applied to a word of a particular known size.) You can implement a circular shift using two regular shifts and a bitwise OR: (x &... 2015-01-28, 1626👍, 0💬

What is the right way to use errno?
What is the right way to use errno? In general, you should detect errors by checking return values, and use errno only to distinguish among the various causes of an error, such as ``File not found'' or ``Permission denied''. (Typically, you use perror or strerror to print these discriminating error ... 2015-02-23, 1624👍, 0💬

How can I call system when parameters (filenames, etc.) of the executed command arent known until run time?
How can I call system when parameters (filenames, etc.) of the executed command arent known until run time? Just use sprintf (or perhaps strcpy and strcat) to build the command string in a buffer, then call system with that buffer. (Make sure the buffer is allocated with enough space; Here is a cont... 2015-03-18, 1622👍, 0💬

Can you mix old-style and new-style function syntax?
Can you mix old-style and new-style function syntax? Doing so is legal (and can be useful for backwards compatibility), but requires a certain amount of care . Modern practice, however, is to use the prototyped form in both declarations and definitions. (The old-style syntax is marked as obsolescent... 2016-01-13, 1618👍, 0💬

I am having trouble with a Turbo C program which crashes
I'm having trouble with a Turbo C program which crashes and says something like ``floating point formats not linked.'' Some compilers for small machines, including Turbo C (and Ritchie's original PDP-11 compiler), leave out certain floating point support if it looks like it will not be needed. In pa... 2015-06-19, 1618👍, 0💬

How can I add N days to a date? How can I find the difference between two dates?
How can I add N days to a date? How can I find the difference between two dates? The ANSI/ISO Standard C mktime and difftime functions provide some (limited) support for both problems. mktime accepts non-normalized dates, so it is straightforward to take a filled-in struct tm, add or subtract from t... 2015-08-05, 1617👍, 0💬

I inherited some code which contains far too many ...
I inherited some code which contains far too many #ifdef's for my taste. How can I preprocess the code to leave only one conditional compilation set, without running it through the preprocessor and expanding all of the #include's and #define's as well? There are programs floating around called unifd... 2016-01-29, 1616👍, 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, 1616👍, 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, 1615👍, 0💬

How can I find the day of the week given the date?
How can I find the day of the week given the date? Here are three methods: 1. Use mktime or localtime # . Here is a code fragment which computes the day of the week for February 29, 2000: #include &lt;stdio.h> #include &lt;time.h> char *wday[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "... 2015-01-07, 1615👍, 0💬

There seem to be a few missing operators ..
There seem to be a few missing operators, like ^^, &amp;&amp;=, and -&gt;=. A:A logical exclusive-or operator (hypothetically ``^^'') would be nice, but it couldn't possibly have short-circuiting behavior analogous to &amp;&amp; and || Similarly, it's not clear how short-circuiti... 2015-01-28, 1614👍, 0💬

What is the difference between ...
What's the difference between const MAXSIZE = 100; and #define MAXSIZE 100 A preprocessor #define gives you a true compile-time constant. In C, const gives you a run-time object which you're not supposed to try to modify; ``const'' really means ``readonly''. 2016-02-18, 1613👍, 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