C programming notes - School of Physics

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C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 6 of 40 19/03/2007 10:06 AM Note that there are some redundant bit patterns in the representation of floating-point numbers: e.g., 0.0 with any exponent is still zero. One of these bit patterns is used as a flag to indicate that a calculation produced an illegal result. This bit pattern is called "NaN" which stands for "not a number". E.g, if you take the square root of a negative number, you will receive the result NaN. Any operation on a NaN will continue to produce a NaN (e.g., if you subtract two NaNs you will still have a NaN), this is useful to propagate such error conditions in your programs so that you are aware when they affect the final result of a calculation. GNU/Linux Fundamentals The operating system The operating system of a computer is, somewhat loosely speaking, the program that is always operating in the background on the computer. It handles the interaction with the hardware, and running other programs for the user(s) of the computer. The operating system that we will be using is called GNU/Linux. Other examples of operating systems are Windows XP, OpenBSD, and FreeBSD. The shell The shell is an interactive program that allows the user to interface with the operating system. This is the program that provides the command-line prompt (it also helps you run programs, allows you to use the arrow keys on your keyboard to edit the command-line, and a bunch of other useful things). The shell that we will be using is called bash, it is the standard shell that is used with GNU/Linux. Other examples of shells are ksh, tcsh, and zsh. Files A file consists of two components: File data The data in a file is simply a list of bytes. The list may be empty (i.e., the file may have no data). The list of bytes might be used for a variety of different purposes, for example, it may be a C program, an e-mail message, a photograph, a song, a movie. In principle there is no way to distinguish the purpose of a file's data by looking at it. In practice, you can usually make a pretty good guess. To list the data in a file, you can, e.g., use "cat myfile". File metadata The metadata associated with a file consists of miscellaneous additional information that the operating system keeps with the file. For example: the name of the file, the number of bytes in the file, the time at which the file was created, which user owns the file, and the file permissions (i.e., the list of users who read and/or write the file, and whether the file is executable (i.e., is a program that can be run)). The term "metadata" means "data about data". To list the metadata associated with a file, you can, e.g., use "stat myfile". File names and file extensions When you create a file, you give it a name. The name is a list of bytes. In principle the name could be bizarre, such as " _1 `", but in practice you should choose simple, lowercase, names without spaces, e.g., "thesis.txt", "photo.jpg", "assignment0001.c". Note the convention of suffixing a file name with an extension that gives a clue as what the file is being used for. So, C programs invariably end with the ".c" extension. The GNU/Linux filesystem Directories (folders) To assist with organising groups of files, GNU/Linux has the concept of a directory (or folder). A directory is actually just a file (this adheres to the UNIX philosophy of making things simple and logical). A directory contains a list of the file names that are "in" the directory. Directories can contain directories. This leads to a hierarchical filesystem, where, in order to fully specify the location of a file, you have to specify the list of directories to follow to reach the file. By convention a slash character '/' is used to separate the directory names when giving the fully qualified filename. "/" itself is known as the root directory - there are no directories above the root directory in the filesystem. "/thesis.txt" would
C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 7 of 40 19/03/2007 10:06 AM then refer to file "thesis.txt" in the root directory. "/home/fred/thesis/chapter4.txt" would be the file "chapter4.txt" in the directory "thesis", in the directory "fred", in the directory "home", in the root directory. The Windows operating system uses letters such as "C:", "D:", etc to specify different filesystems (which might be in on different hard disks). GNU/Linux places these filesystems within the "/" hierarchy, i.e., the floppy disk might be accessible as "/mnt/floppy", the CD-ROM might be "/mnt/cdrom". The current working directory GNU/Linux has the concept of the current working directory. You can specify a file in the current working directory by just giving the file name. For convenience, "./" refers to the current working directory, and "../" refers to the directory above the current working directory. The fully-qualified filename of a file in the current working directory can be written as, e.g., "./thesis.txt". Programs To get a computer to do something for you, you create and run a program. You will be doing this many times during this course. To create a program, you type in the program's source code, and then use, e.g, the C compiler (another program, which someone else wrote) to turn the source code into a program. The program itself is just a file. The file's metadata tells the operating system that the file is executable (i.e., that it is a program able to be run by the computer). To run the program, you just type its fully qualified filename. GNU/Linux commands GNU/Linux provides you with many hundreds of programs, often called commands, but they are really exactly like the programs that you can create yourself (actually, some of the commands are a subset of the bash shell, but this is a detail). The commands range from listing the contents of a directory ("ls"), listing the contents of a file ("cat"), the C compiler ("gcc"), to the GNU image manipulation program ("gimp"). To run a program, you simply type its filename. In the case of your own programs, you should use the fully qualified filename (e.g., "./a.out"). In the case of programs provided by GNU/Linux, the shell has a list of default directories where it searches for programs, so you normally only need to type the filename (without any directories). E.g., the "ls" program to list directories can be run by simply typing "ls", or "/bin/ls", or, if your current working directory happened to be "/bin", you could type "./ls". Input and Output In general, for a program to do useful work, it needs to be provided with input of some sort, and to produce output. Input to a program can take many forms: e.g., typing at the keyboard, a file containing data, movements of a mouse, or something more exotic such as audio input from a sound card, or video from a camera. Output from a program can also take many forms, e.g., text appearing on your computer screen, a graph on your screen, a file of results, or something more exotic such as audio output, or movement of a robotic arm. GNU/Linux provides the following input and output methods for all programs (I am covering all of the possibilities here, but you are only expected to really understand the first two at this stage): Standard input Standard input (or STDIN) is a stream of bytes provided to the program. The program can read from the stream, and can determine when there is no more data. Standard input comes from one of three sources depending on how you run your program. For example, suppose that your program is called "a.out" (the default name assumed by the C compiler), then ./a.out ./a.out < file standard input comes from the keyboard standard input comes from a file called "file"
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C programming notes - School of Physics

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