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How can I read a directory in a C program?
How can I read a directory in a C program? See if you can use the opendir and readdir functions, which are part of the POSIX standard and are available on most Unix variants. Implementations also exist for MS-DOS, VMS, and other systems. (MS-DOS also has FINDFIRST and FINDNEXT routines which do esse... 2015-04-01, 1662👍, 0💬

How can I generate floating-point random numbers?
How can I generate floating-point random numbers? drand48 is a Unix System V routine which returns floating point random numbers (presumably with 48 bits of precision) in the half-open interval [0, 1). (Its companion seed routine is srand48; neither is in the C Standard.) It's easy to write a low-pr... 2015-07-22, 1658👍, 0💬

How can I ensure that integer arithmetic doesnt overflow?
How can I ensure that integer arithmetic doesnt overflow? The usual approach is to test the operands against the limits in the header file &lt;limits.h> before doing the operation. For example, here is a ``careful'' addition function: int chkadd(int a, int b) { if(INT_MAX - b &lt; a) { fputs... 2015-03-04, 1658👍, 0💬

Why dont C comments nest?
Why don't C comments nest? How am I supposed to comment out code containing comments? Are comments legal inside quoted strings? A: C comments don't nest mostly because PL/I's comments, which C's are borrowed from, don't either. Therefore, it is usually better to ``comment out'' large sections of cod... 2015-01-30, 1657👍, 0💬

What is alloca and why is its use discouraged?
What is alloca and why is its use discouraged? alloca allocates memory which is automatically freed when the function which called alloca returns. That is, memory allocated with alloca is local to a particular function's ``stack frame'' or context. alloca cannot be written portably, and is difficult... 2016-03-14, 1656👍, 0💬

I am still getting errors due to library functions being undefined
I'm still getting errors due to library functions being undefined, even though I'm explicitly requesting the right libraries while linking. Many linkers make one pass over the list of object files and libraries you specify, and extract from libraries only those modules which satisfy references which... 2015-07-14, 1652👍, 0💬

Is if(p), where p is a pointer, a valid and portable test?
Is if(p), where p is a pointer, a valid and portable test? It is always valid. When C requires the Boolean value of an expression, a false value is inferred when the expression compares equal to zero, and a true value otherwise. That is, whenever one writes if(expr) where ``expr'' is any expression ... 2016-02-29, 1651👍, 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, 1651👍, 0💬

Why do some versions of toupper act strangely if given an upper-case letter?
Why do some versions of toupper act strangely if given an upper-case letter? Why does some code call islower before toupper? In earlier times, toupper was a function-like preprocessor macro and was defined to work only on lower-case letters; it misbehaved if applied to digits, punctuation, or letter... 2015-08-17, 1651👍, 0💬

When I read numbers from the keyboard with scanf ...
When I read numbers from the keyboard with scanf and a "%d\n" format, like this: int n; scanf("%d\n", &amp;n); printf("you typed %d\n", n); it seems to hang until I type one extra line of input. Perhaps surprisingly, \n in a scanf format string does not mean to expect a newline, but rather to re... 2015-10-23, 1648👍, 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, 1647👍, 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, 1646👍, 0💬

Here is a neat trick for checking whether two strings are equal
Q: Here's a neat trick for checking whether two strings are equal: if(!strcmp(s1, s2)) Is this good style? It is not particularly good style, although it is a popular idiom. The test succeeds if the two strings are equal, but the use of ! (``not'') suggests that it tests for inequality. Another opti... 2015-05-15, 1642👍, 0💬

How do I read the arrow keys? What about function keys?
How do I read the arrow keys? What about function keys? Terminfo, some versions of termcap, and some versions of curses have support for these non-ASCII keys. Typically, a special key sends a multicharacter sequence (usually beginning with ESC, '\033'); parsing these can be tricky. (curses will do t... 2015-04-24, 1642👍, 0💬

How can I handle floating-point exceptions gracefully?
How can I handle floating-point exceptions gracefully? On many systems, you can define a function matherr which will be called when there are certain floating-point errors, such as errors in the math routines in &lt;math.h>. You may also be able to use signal to catch SIGFPE 2015-03-04, 1641👍, 0💬

How can I display a percentage-done indication that updates itself in place, or show one of those twirling baton progress indica
How can I display a percentage-done indication that updates itself in place, or show one of those twirling baton progress indicators? These simple things, at least, you can do fairly portably. Printing the character '\r' will usually give you a carriage return without a line feed, so that you can ov... 2015-04-27, 1638👍, 0💬

How do I get an accurate error status return from system on MS-DOS?
How do I get an accurate error status return from system on MS-DOS? You can't; COMMAND.COM doesn't tend to provide one. If you don't need COMMAND.COM's services (i.e. if you're just trying to invoke a simple program, without I/O redirection and such) try one of the spawn routines, instead. 2015-03-16, 1638👍, 0💬

How can I write data files which can be read on other machines with different word size, byte order, or floating point formats?
How can I write data files which can be read on other machines with different word size, byte order, or floating point formats? The most portable solution is to use text files (usually ASCII), written with fprintf and read with fscanf or the like. (Similar advice also applies to network protocols.) ... 2015-02-23, 1638👍, 0💬

Why does everyone say not to use gets?
Why does everyone say not to use gets? Unlike fgets(), gets() cannot be told the size of the buffer it's to read into, so it cannot be prevented from overflowing that buffer if an input line is longer than expected--and Murphy's Law says that, sooner or later, a larger-than-expected input line will ... 2015-10-16, 1637👍, 0💬

I have got some code that is trying to unpack external structures
I've got some code that's trying to unpack external structures, but it's crashing with a message about an ``unaligned access.'' What does this mean? The code looks like this: struct mystruct { char c; long int i32; int i16; } s; char buf[7], *p; fread(buf, 7, 1, fp); p = buf; s.c = *p++; s.i32 = *(l... 2015-05-20, 1637👍, 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, 1636👍, 0💬

How can I write a function that takes a format string and a variable number of arguments
How can I write a function that takes a format string and a variable number of arguments, like printf, and passes them to printf to do most of the work? Use vprintf, vfprintf, or vsprintf. These routines are like their counterparts printf, fprintf, and sprintf, except that instead of a variable-leng... 2015-06-12, 1636👍, 0💬

Why doesnt that code work?
Why doesn't the code short int s; scanf("%d", &s); work? When converting %d, scanf expects a pointer to an int. To convert to a short int, use %hd . 2015-10-28, 1632👍, 0💬

How can I generate random numbers with a normal or Gaussian distribution?
How can I generate random numbers with a normal or Gaussian distribution? There are a number of ways of doing this. 1. Exploit the Central Limit Theorem (``law of large numbers'') and add up several uniformly-distributed random numbers: #include &lt;stdlib.h> #include &lt;math.h> #define NSU... 2015-07-22, 1632👍, 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