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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, 1586👍, 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, 1586👍, 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, 1583👍, 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, 1581👍, 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, 1578👍, 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, 1575👍, 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, 1573👍, 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, 1573👍, 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, 1571👍, 0💬

A third-party header file I just started using is defining its own TRUE and FALSE values incompatibly ...
A third-party header file I just started using is defining its own TRUE and FALSE values incompatibly with the code I've already developed. What can I do? This is indeed an annoying situation. It's a classic namespace problem;Ideally, third-party vendors would be conscientious when defining symbols ... 2016-02-26, 1570👍, 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, 1570👍, 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, 1568👍, 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, 1567👍, 0💬

But I cant use all these nonstandard, system-dependent functions, because my program has to be ANSI compatible!
But I cant use all these nonstandard, system-dependent functions, because my program has to be ANSI compatible! You're out of luck. Either you misunderstood your requirement, or it's an impossible one to meet. ANSI/ISO Standard C simply does not define ways of doing these things; it is a language st... 2015-02-27, 1566👍, 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, 1561👍, 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, 1559👍, 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, 1558👍, 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, 1553👍, 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, 1553👍, 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, 1546👍, 0💬

How can I open files with names like ...
How can I open files with names like ``file1'', ``file2'', ``file3'', etc., where the numeric part is controlled by a variable? Basically I want ``file%d'', like printf.. You want printf's close cousin sprintf, which ``prints'' to a string: char filename[FILENAME_MAX]; sprintf(filename, "file%d", i)... 2015-10-07, 1542👍, 0💬

Did C have any Year 2000 problems?
Did C have any Year 2000 problems? No, although poorly-written C programs might have. The tm_year field of struct tm holds the value of the year minus 1900; this field therefore contains the value 100 for the year 2000. Code that uses tm_year correctly (by adding or subtracting 1900 when converting ... 2015-08-03, 1540👍, 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, 1537👍, 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, 1537👍, 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