Standard Input and Output

Overview Data communication with a C program and the outside world is performed through files Files are a non-volatile way to store data by means of such media as tape, CD-ROM, floppy disk, ZIP disk, hard drive, etc. C (just like the UNIX operating system) considers all process communication media to be files –An ordinary file on a disk is considered to be a file –So is the keyboard, the screen, parallel ports, and serial ports

Communication through Files Programs access files through basic file operations –Open a file –Read data from a file –Write data to a file –Close a file Text files store all data types as character bytes; the way a program reads the data (either text or binary mode) determines how the data is interpreted –Example: 12 z rti room –character?, character string?, integer?, floating point number?

stdin, stdout, stderr Three files are automatically opened each time a C program is run and automatically closed when a C program ends: stdin, stdout, and stderr stdin –Standard input, default source is the keyboard stdout –Standard output, default destination is the screen stderr –Standard error, default destination is the screen These three variable names are referred to as file descriptors; each is a handle (or pointer) to a record (i.e., struct) describing the actual file

C Program Entrances and Exits stdout (Screen) stdin (Keyboard) stderr (Screen) Constants and variables Operations and functions main XYZ ABCD Instructions and operating procedures

Files as Pipes for Data Program to compute average stdinstdout Average is Files act as pipes to bring a stream of data into the program and send a stream of data out of the program The program reads data from stdin and writes data to stdout as if they were ordinary files

One Byte At a Time Data is read or written one byte at a time from a file until the end of the file is reached or until the file is closed. The file system uses a pointer to keep track of the next byte to read or to write Smith Jack Manager 15 Hanson Susan Operator 7 Jones Nancy Administrator 10 Doe Carl Technician 12 In this file, the program has read the data as far as the letter 'J'. The next character to be read is 'a'

Functions Using stdin and stdout-1 Some standard C functions implicitly use stdin and stdout –scanf("%d", &aNumber); –aSymbol = getchar(); –gets(theBuffer); // High security risk –printf("Average: %5.2f", theAverage); –putchar(aCharacter); –puts(aPhrase);

Functions Using stdin and stdout-2 Other functions need to have stdin and stdout specified as the file descriptor –fscanf(stdin, "%d", &aNumber); –aSymbol = fgetc(stdin); –fgets(theBuffer, sizeof(theBuffer), stdin); –fprintf(stdout, "Average: %5.2f", theAverage); –fputc(aCharacter, stdout); –puts(aPhrase, stdout);

Input and Output Redirection Because a C program considers stdin and stdout to be ordinary files, a user can redirect the input and output to another source or destination That source or destination can be an I/O device or an ordinary file This redirection can be done when a program is run from the DOS or UNIX command line The '<' sign redirects input from a file The '>' sign redirects output to a new file The ">>" sign redirects output and appends the data to a current file or creates the file if it doesn't yet exist

Example of I/O Redirection Input comes from a file, output goes to the screen –C:\myprogram <numbers.dat Input comes from the keyboard, output goes to a file –C:\myprogram >results.txt Input comes from a file; output goes to a file –C:\myprogram findings.dat Input comes from the keyboard and the output is appended to the contents in a file –C:\myprogram >>findings.dat

Some Redirection to Try Out in MS-DOS Example #1 –C:\dir >dirwords.txt –C:\type dirwords.txt Example #2 –C:\dir >dir.dat –C:\find ".c" <dir.dat –C:\find ".c" findings.txt –C:\find ".exe" >findings.txt –C:\type findings.txt

printf Function The printf function writes formatted output to stdout –Ex. printf("Average: %5.2f \n", theAverage); The first argument is a character string containing ordinary text, escape characters, and format specifiers Syntax for format specifier: %. Each escape character is a combination of a '\' and a character, which represents a nonviewable ASCII character. An example is \n for newline The order, number, and type of the format specifiers corresponds to the order, number, and type of the arguments in the printf call

printf Examples No format specifiers, one escape character, no other arguments –printf("Name Age Address Distance\n"); One format specifier, two arguments –printf("Results is %d\n", theResult); Four format specifiers, five arguments –printf("%c %d %f %s", aCharacter, anInteger, aFloat, aString); Same as above, but with field width and precision specified –printf("%4c %3d %5.3f %8s", aCharacter, anInteger, aFloat, aString);

scanf Function The scanf function reads formatted input from stdin –scanf("%d", &aNumber); The first argument is a character string containing one or more conversion specifiers Syntax for format specifier: % The order, number, and type of the conversion specifiers corresponds to the order, number, and type of the other arguments in the scanf call Each argument value must by the address of the variable, NOT the variable itself, where the value should be stored To designate the address for a character, integer, structure, or floating point variable, precede the variable name by the & operator, which means "address of“ Array names are a special case when passed as an argument

scanf Examples One format specifier, two arguments –scanf("%d", &theResult); Three format specifiers, four arguments –scanf("%d%f%s", &anInteger, &aFloat, aString); Same as above, but in a different order –scanf("%s %f %d", aString, &aFloat, &anInteger);