C programming notes - School of Physics

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C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 8 of 40 19/03/2007 10:06 AM command | ./a.out standard input comes from the standard output of "command" Standard output Standard output (or STDOUT) is a stream of bytes produced by a program. Standard output goes to one of three locations depending on how you run your program. E.g., ./a.out ./a.out > file ./a.out | command standard output appears on the computer screen as text standard output is sent to the file called "file" standard output is sent to the standard input of "command" Standard error Standard error (or STDERR) is a stream of bytes produced by a program. Usually it contains error messages (e.g., 'divide by zero') that are not normally considered as part of the program output. Standard error can be sent to one of three locations. E.g., ./a.out ./a.out 2> file ./a.out 2>&1 | command standard error appears on the computer screen as text, intermingled with the standard output standard error is sent to the file called "file" standard error (and standard output) is sent to the standard input of "command" Command-line arguments Anything you type after the program name, and before any special shell characters such as '>' and '|', is made available to your program as a command-line argument. These can be a useful source of additional input to a program. E.g., ./a.out 23.45 albatross 7 In the above example, "23.45", "albatross", and "7" are three command-line arguments.We will see later how you can read them from within a C program. Exit status Another form of output that is available to all programs is an integer exit status. By convention, this is used to indicate whether the program ran successfully (in which case the exit status is zero) or if an error occurred (in which case the exit status is nonzero and its value can be decoded to determine what the error was). The exit status of a program is stored in the shell variable called "$?" (don't worry if you don't understand this at this stage). Putting it all together Here are some more complex examples of program input/output: ./a.out 2.1 < data.txt > results.txt "2.1" is an argument, standard input comes from "data.txt", standard output goes to "results.txt"
C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 9 of 40 19/03/2007 10:06 AM ./a.out > results.txt 2> error.txt there are no arguments, standard input is from the keyboard, standard output to "results.txt" and standard error to "error.txt" Command Toolkit Here are some of the basic GNU/Linux commands that you will be using: cat myfile - concatenate a file (i.e., list the file's data on standard output) less myfile - like "cat", but allows you to scroll up and down in the file using the page-up, and page-down keys. Exit with 'q'. rm myfile - remove a file (i.e., delete the file; it is gone for good) mv myfile newname - renames a file cp myfile copyOfMyFile - makes a copy of a file, with a different name cd mydir - change directory mkdir mynewdir - make a new directory rmdir mydir - remove the directory (you have to remove the files in it first) ls - list the files and directories in the current working directory pwd - print the name of the current working directory gcc myprog.c - compiles your program ./a.out - runs the program compiled as above man mytopic - displays the manual pages about "mytopic" man -k mytopic - searches the manual pages for any information about "mytopic" gedit myprog.c - starts up a simple GUI editor logout - logs out of the computer The on-line manual pages Use "man -k keyword" to search for information about "keyword". For example, suppose you want to know something about random number generators: "man -k random" on a GNU/Linux system will return output containing lines like these: ppmspread (1) - displace a portable pixmap's pixels by a random amount rand (3) - random number generator RAND_bytes (3ssl) - generate random data random (3) - random number generator random (4) - kernel random number source devices RAND_pseudo_bytes [RAND_bytes] (3ssl) - generate random data rand [sslrand] (1ssl) - generate pseudo-random bytes rand [sslrand] (3ssl) - pseudo-random number generator seed48 [drand48] (3) - generate uniformly distributed pseudo-random numbers The number in parentheses is called the section number (e.g., section 3 is reserved for C functions, section 4 for low-level kernel interfaces), and you need to specify this if there are entries in multiple volumes. So, e.g., to read about the kernel random number source devices, you would type "man 4 random".
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C programming notes - School of Physics

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