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How can I write a function that takes a variable number of arguments?
How can I write a function that takes a variable number of arguments? Use the facilities of the &lt;stdarg.h> header. Here is a function which concatenates an arbitrary number of strings into malloc'ed memory: #include &lt;stdlib.h> /* for malloc, NULL, size_t */ #include &lt;stdarg.h> /... 2015-06-15, 1315👍, 0💬

How can I tell if standard input or output is redirected
How can I tell if standard input or output is redirected (i.e. whether ``&lt;'' or ``&gt;'' was used on the invocation command line)? You can't tell directly, but you can usually look at a few other things to make whatever decision you need to. If you want your program to take input from std... 2015-10-01, 1312👍, 0💬

I am porting this program, and it calls a routine drand48, which my library doesnt have. What is it?
I am porting this program, and it calls a routine drand48, which my library doesnt have. What is it? 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... 2015-07-20, 1308👍, 0💬

How can I implement a delay, or time a users response, with sub-second resolution?
How can I implement a delay, or time a users response, with sub-second resolution? Unfortunately, there is no portable way. Routines you might look for on your system include clock, delay, ftime, gettimeofday, msleep, nap, napms, nanosleep, setitimer, sleep, Sleep, times, and usleep. (A function cal... 2015-03-06, 1308👍, 0💬

People always say that good style is important
People always say that good style is important, but when they go out of their way to use clear techniques and make their programs readable, they seem to end up with less efficient programs. Since efficiency is so important, isn't it necessary to sacrifice some style and readability? It's true that g... 2015-05-06, 1307👍, 0💬

How can I shut off the warning ...
How can I shut off the ``warning: possible pointer alignment problem'' message which lint gives me for each call to malloc? A modern lint shouldn't be complaining about this. Once upon a time, lint did not and could not know that malloc ``returns a pointer to space suitably aligned for storage of an... 2015-05-05, 1307👍, 0💬

My compiler isnt letting me declare a function
My compiler isnt letting me declare a function A; My compiler isn't letting me declare a function int f(...) { } i.e. accepting a variable number of arguments, but with no fixed arguments at all. A: Standard C requires at least one fixed argument, in part so that you can hand it to va_start. (In any... 2015-06-08, 1302👍, 0💬

I just typed in this program, and it is acting strangely. Can you see anything wrong with it?
I just typed in this program, and it is acting strangely. Can you see anything wrong with it? See if you can run lint first (perhaps with the -a, -c, -h, -p or other options ). Many C compilers are really only half-compilers, taking the attitude that it's not their problem if you didn't say what you... 2015-05-06, 1302👍, 0💬

What is the most efficient way to count the number of bits which are set in an integer?
What is the most efficient way to count the number of bits which are set in an integer? Many ``bit-fiddling'' problems like this one can be sped up and streamlined using lookup tables (but see question 20.13). Here is a little function which computes the number of bits in a value, 4 bits at a time: ... 2015-02-11, 1300👍, 0💬

How do I convert a string to all upper or lower case?
How do I convert a string to all upper or lower case? Some libraries have routines strupr and strlwr or strupper and strlower, but these are not Standard or portable. It's a straightforward exercise to write upper/lower-case functions in terms of the toupper and tolower macros in &lt;ctype.h>;(T... 2015-08-19, 1298👍, 0💬

How can I change their mode to binary?
I'm writing a ``filter'' for binary files, but stdin and stdout are preopened as text streams. How can I change their mode to binary? There is no standard way to do this. On Unix-like systems, there is no text/binary distinction, so there is no need to change the mode. Some MS-DOS compilers supply a... 2015-09-21, 1295👍, 0💬

What is the deal on sprintfs return value?
What's the deal on sprintf's return value? Is it an int or a char *? The Standard says that it returns an int (the number of characters written, just like printf and fprintf). Once upon a time, in some C libraries, sprintf returned the char * value of its first argument, pointing to the completed re... 2015-10-16, 1294👍, 0💬

How can I insert or delete a line (or record) in the middle of a file?
How can I insert or delete a line (or record) in the middle of a file? In general, there is no way to do this. The usual solution is simply to rewrite the file. When you find yourself needing to insert data into an existing file, here are a few alternatives you can try: * Rearrange the data file so ... 2015-04-13, 1290👍, 0💬

How can I find out how much free space is available on disk?
How can I find out how much free space is available on disk? There is no portable way. Under some versions of Unix you can call statfs. Under MS-DOS, use interrupt 0x21 subfunction 0x36, or perhaps a routine such as diskfree. Another possibility is to use popen to invoke and read the output of a ``d... 2015-04-03, 1288👍, 0💬

How can I sort more data than will fit in memory?
How can I sort more data than will fit in memory? You want an ``external sort,'' which you can read about in Knuth, Volume 3. The basic idea is to sort the data in chunks (as much as will fit in memory at one time), write each sorted chunk to a temporary file, and then merge the files. Your operatin... 2015-08-07, 1282👍, 0💬

My floating-point calculations are acting strangely and giving me different answers on different machines.
My floating-point calculations are acting strangely and giving me different answers on different machines. If the problem isn't that simple, recall that digital computers usually use floating-point formats which provide a close but by no means exact simulation of real number arithmetic. Among other ... 2015-06-29, 1280👍, 0💬

How can I find out if there are characters available for reading?
How can I find out if there are characters available for reading (and if so, how many)? Alternatively, how can I do a read that will not block if there are no characters available? These, too, are entirely operating-system-specific. Some versions of curses have a nodelay function. Depending on your ... 2015-04-29, 1280👍, 0💬

How can I read data from data files with particular formats?
How can I read data from data files with particular formats? How can I read ten floats without having to use a jawbreaker scanf format like "%f %f %f %f %f %f %f %f %f %f"? How can I read an arbitrary number of fields from a line into an array? In general, there are three main ways of parsing data l... 2015-10-26, 1278👍, 0💬

I am trying to update a file in place ...
I'm trying to update a file in place, by using fopen mode "r+", reading a certain string, and writing back a modified string, but it's not working. Be sure to call fseek before you write, both to seek back to the beginning of the string you're trying to overwrite, and because an fseek or fflush is a... 2015-10-07, 1277👍, 0💬

I need a random true/false value ...
I need a random true/false value, so I'm just taking rand() % 2, but it's alternating 0, 1, 0, 1, 0... Poor pseudorandom number generators (such as the ones unfortunately supplied with some systems) are not very random in the low-order bits. (In fact, for a pure linear congruential random number gen... 2015-07-24, 1275👍, 0💬

How can I write code to conform to these old, binary data file formats?
How can I write code to conform to these old, binary data file formats? It's hard, because of word size and byte order differences, floating-point formats, and structure padding. To get the control you need over these particulars, you may have to read and write things a byte at a time, shuffling and... 2015-09-18, 1273👍, 0💬

How can I write a function analogous to scanf
How can I write a function analogous to scanf, i.e. that accepts similar arguments, and calls scanf to do most of the work? C99 (but not any earlier C Standard) supports vscanf, vfscanf, and vsscanf. 2015-06-12, 1266👍, 0💬

I am trying to port this old program. Why do I get undefined external errors for some library functions?
I am trying to port this old program. Why do I get undefined external errors for some library functions? Some old or semistandard functions have been renamed or replaced over the years; if you need:/you should instead: index use strchr. rindex use strrchr. bcopy use memmove, after interchanging the ... 2015-07-16, 1260👍, 0💬

I know that the library function localtime will convert ...
I know that the library function localtime will convert a time_t into a broken-down struct tm, and that ctime will convert a time_t to a printable string. How can I perform the inverse operations of converting a struct tm or a string into a time_t? ANSI C specifies a library function, mktime, which ... 2015-08-05, 1259👍, 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