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Each time I run my program, I get the same sequence of numbers back from rand
Each time I run my program, I get the same sequence of numbers back from rand It's a characteristic of most pseudo-random number generators (and a defined property of the C library rand) that they always start with the same number and go through the same sequence. (Among other things, a bit of predi...
2015-07-29, 1182👍, 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, 1182👍, 0💬

How can I increase the allowable number of simultaneously open files?
I'm getting an error, ``Too many open files''. How can I increase the allowable number of simultaneously open files? A: There are typically at least two resource limitations on the number of simultaneously open files: the number of low-level ``file descriptors'' or ``file handles'' available in the ...
2015-04-03, 1181👍, 0💬

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, 1180👍, 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, 1180👍, 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, 1180👍, 0💬

Why isnt there a numbered, multi-level break statement to break out
Why isn't there a numbered, multi-level break statement to break out of several loops at once? What am I supposed to use instead, a goto? A:First, remember why it is that break and continue exist at all--they are, in effect, ``structured gotos'' used in preference to goto (and accepted as alternativ...
2015-01-30, 1180👍, 0💬

How can I specify a variable width in a scanf format string?
How can I specify a variable width in a scanf format string? You can't; an asterisk in a scanf format string means to suppress assignment. You may be able to use ANSI stringizing and string concatenation to construct a constant format specifier based on a preprocessor macro containing the desired wi...
2015-10-26, 1179👍, 0💬

How can I suppress the dreaded MS-DOS Abort, Retry, Ignore? message?
How can I suppress the dreaded MS-DOS Abort, Retry, Ignore? message? Among other things, you need to intercept the DOS Critical Error Interrupt, interrupt 24H. See the comp.os.msdos.programmer
2015-04-06, 1179👍, 0💬

Having dynamically allocated an array , can I change its size?
Having dynamically allocated an array , can I change its size? Yes. This is exactly what realloc is for. Given a region of malloced memory its size can be changed using code like: dynarray = realloc(dynarray, 20 * sizeof(int)); Note that realloc may not always be able to enlarge memory regions in-pl...
2016-03-21, 1177👍, 0💬

Why doesnt strcat work? ....
Why doesn't strcat(string, '!'); work? There is a very real difference between characters and strings, and strcat concatenates strings. A character constant like '!' represents a single character. A string literal between double quotes usually represents multiple characters. A string literal like "!...
2016-03-14, 1177👍, 0💬

I am including the right header file for the library function I am using, but the linker keeps saying its undefined.
I am including the right header file for the library function I am using, but the linker keeps saying its undefined. 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 th...
2016-02-10, 1177👍, 0💬

Is there anything like an ifdef for typedefs?
Is there anything like an ifdef for typedefs? Unfortunately, no. (There can't be, because types and typedefs haven't been parsed at preprocessing time.) You may have to keep sets of preprocessor macros (e.g. MY_TYPE_DEFINED) recording whether certain typedefs have been declared.
2016-02-01, 1177👍, 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, 1177👍, 0💬

How can I call system when parameters (filenames, etc.) of the executed command arent known until run time?
How can I call system when parameters (filenames, etc.) of the executed command arent known until run time? Just use sprintf (or perhaps strcpy and strcat) to build the command string in a buffer, then call system with that buffer. (Make sure the buffer is allocated with enough space; Here is a cont...
2015-03-18, 1177👍, 0💬

I a'm getting strange syntax errors inside lines I have ifdeffed out.
I a'm getting strange syntax errors inside lines I have ifdeffed out. Under ANSI C, the text inside a ``turned off'' #if, #ifdef, or #ifndef must still consist of ``valid preprocessing tokens.'' This means that the characters " and ' must each be paired just as in real C code, and the pairs mustn't ...
2016-01-29, 1176👍, 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, 1176👍, 0💬

Can I declare main as void, to shut off these annoying ....
Can I declare main as void, to shut off these annoying ``main returns no value'' messages? No. main must be declared as returning an int, and as taking either zero or two arguments, of the appropriate types. If you're calling exit() but still getting warnings, you may have to insert a redundant retu...
2015-12-20, 1175👍, 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, 1175👍, 0💬

How do I copy files?
How do I copy files? Either use system() to invoke your operating system's copy utility, or open the source and destination files (using fopen or some lower-level file-opening system call), read characters or blocks of characters from the source file, and write them to the destination file. Here is ...
2015-04-08, 1175👍, 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, 1175👍, 0💬

How can I automatically locate a programs configuration files in the same directory as the executable?
How can I automatically locate a programs configuration files in the same directory as the executable? It's hard, in general; Even if you can figure out a workable way to do it, you might want to consider making the program's auxiliary (library) directory configurable, perhaps with an environment va...
2015-03-13, 1174👍, 0💬

People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed
People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed, but my compiler can't even replace i/=2 with a shift. A: Was i signed or unsigned? If it was signed, a shift is not equivalent (hint: think about the result if i is negative and odd), s...
2015-02-06, 1174👍, 0💬

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, 1173👍, 0💬

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