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My compiler is complaining that printf is undefined ...
My compiler is complaining that printf is undefined! How can this be? It's the world's most popular C function... Allegedly, there are C compilers for Microsoft Windows which do n ot support printf, on the argument that printf is for printing to old-fashioned terminals, while under Windows the right... 2015-07-03, 1645👍, 0💬

I am trying to compile this program
I'm trying to compile this program, but the compiler is complaining that ``union REGS'' is undefined, and the linker is complaining that int86 is undefined. Those have to do with MS-DOS interrupt programming. They don't exist on other systems. 2015-03-02, 1644👍, 0💬

I am getting baffling syntax errors which make no sense at all
I'm getting baffling syntax errors which make no sense at all, and it seems like large chunks of my program aren't being compiled. Check for unclosed comments, mismatched #if/#ifdef/#ifndef/#else/#endi fdirectives, and perhaps unclosed quotes; remember to check header files, too. 2015-06-01, 1642👍, 0💬

How can I make it pause before closing the program output window?
I'm compiling some test programs on a windows-based system, and the windows containing my program's output are closing so quickly after my program calls exit that I can't see the output. How can I make it pause before closing? After wondering why the author of your compiler's run-time system didn't ... 2015-04-24, 1642👍, 0💬

I had a frustrating problem which turned out to be caused by the line
I had a frustrating problem which turned out to be caused by the line printf("%d", n); where n was actually a long int. I thought that ANSI function prototypes were supposed to guard against argument type mismatches like this. When a function accepts a variable number of arguments, its prototype doe... 2015-06-15, 1637👍, 0💬

How can I get the current date or time of day in a C program?
How can I get the current date or time of day in a C program? Just use the time, ctime, localtime and/or strftime functions. Here is a simple example: #include &lt;stdio.h> #include &lt;time.h> int main() { time_t now; time(&amp;now); printf("It's %s", ctime(&amp;now)); return 0; } C... 2015-08-07, 1632👍, 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, 1627👍, 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, 1622👍, 0💬

I heard that you have to include stdio.h before calling printf. Why?
I heard that you have to include stdio.h before calling printf. Why? So that a proper prototype for printf will be in scope. A compiler may use a different calling sequence for functions which accept variable-length argument lists. (It might do so if calls using variable-length argument lists were l... 2015-06-17, 1618👍, 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, 1616👍, 0💬

How can I do graphics?
How can I do graphics? Once upon a time, Unix had a fairly nice little set of device-independent plot functions described in plot(3) and plot(5). The GNU libplot library, written by Robert Maier, maintains the same spirit and supports many modern plot devices; see http://www.gnu.org/software/pl otuti... 2015-04-17, 1616👍, 0💬

How can I send mail from within a C program?
How can I send mail from within a C program? Under Unix, open a pipe to the mail program, or perhaps /usr/lib/sendmail. 2015-04-17, 1616👍, 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, 1612👍, 0💬

What is the difference between text and binary I/O?
What is the difference between text and binary I/O? In text mode, a file is assumed to consist of lines of printable characters (perhaps including tabs). The routines in the stdio library (getc, putc, and all the rest) translate between the underlying system's end-of-line representation and the sing... 2015-09-21, 1611👍, 0💬

How can I return a sequence of random numbers which dont repeat at all?
How can I return a sequence of random numbers which dont repeat at all? What you're looking for is often called a ``random permutation'' or ``shuffle.'' One way is to initialize an array with the values to be shuffled, then randomly interchange each of the cells with another one later in the array: ... 2015-07-24, 1611👍, 0💬

I keep getting errors due to library functions being undefined, but I am including all the right header files.
I keep getting errors due to library functions being undefined, but I am including all the right header files. In the general case of calling code in an external library, using #include to pull in the right header file(s) is only half of the story; you also have to tell the linker to search the exte... 2015-07-14, 1609👍, 0💬

Dont ANSI function prototypes render lint obsolete?
Dont ANSI function prototypes render lint obsolete? Not really. First of all, prototypes work only if they are present and correct; an inadvertently incorrect prototype is worse than useless. Secondly, lint checks consistency across multiple source files, and checks data declarations as well as func... 2015-05-01, 1609👍, 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... 2015-07-20, 1608👍, 0💬

How can I find the modification date and time of a file?
How can I find the modification date and time of a file? The Unix and POSIX function is stat, which several other systems supply as well. 2015-04-13, 1608👍, 0💬

I have a program that seems to run correctly
I have a program that seems to run correctly, but it crashes as it's exiting, after the last statement in main(). What could be causing this? A:There are at least three things to look for: 1. If a semicolon in a previous declaration is missing, main might be inadvertently declared as returning a str... 2015-05-29, 1606👍, 0💬

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, 1603👍, 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 ... 2015-10-05, 1599👍, 0💬

How can I check whether a file exists? I want to warn the user if a requested input file is missing.
How can I check whether a file exists? I want to warn the user if a requested input file is missing. It's surprisingly difficult to make this determination reliably and portably. Any test you make can be invalidated if the file is created or deleted (i.e. by some other process) between the time you ... 2015-04-15, 1599👍, 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, 1595👍, 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