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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, 1472👍, 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, 1472👍, 0💬

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, 1470👍, 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, 1469👍, 0💬

I thought that using large model meant that I could use more than 64K of data!
What does the error message ``DGROUP data allocation exceeds 64K'' mean, and what can I do about it? I thought that using large model meant that I could use more than 64K of data! Even in large memory models, MS-DOS compilers apparently toss certain data (strings, some initialized global or static v... 2015-03-20, 1469👍, 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, 1468👍, 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, 1467👍, 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, 1465👍, 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, 1464👍, 0💬

How can I invoke another program or command and trap its output?
How can I invoke another program or command and trap its output? Unix and some other systems provide a popen function, which sets up a stdio stream on a pipe connected to the process running a command, so that the calling program can read the output (or alternatively supply the input). Using popen, ... 2015-03-16, 1464👍, 0💬

Is exit(status) truly equivalent to returning the same status from main?
Is exit(status) truly equivalent to returning the same status from main? Yes and no. The Standard says that a return from the initial call to main is equivalent to calling exit. However, a return from main cannot be expected to work if data local to main might be needed during cleanup; A few very ol... 2015-03-09, 1464👍, 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, 1462👍, 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, 1461👍, 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, 1458👍, 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, 1456👍, 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, 1455👍, 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, 1455👍, 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, 1454👍, 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, 1453👍, 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, 1453👍, 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, 1452👍, 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, 1449👍, 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, 1448👍, 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, 1446👍, 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