Basic I/O. COSC Software Tools. Streams. Standard I/O. Standard I/O. Formatted Output

Transcription

1 Basic I/O COSC Software Tools C - Input/Output (K+R Ch. 7) We know how to do some basic input and output: getchar - reading characters putchar - writing characters printf - formatted output Input and output are sequences of bytes Commonly called streams This word is also used for in C e.g. stdin is an input stream Standard I/O There are a number of standard input and output streams: stdin - Standard Input default: keyboard stdout - Standard Output default: screen stderr - Standard Error default: screen Standard I/O stdin - input for program stdout - output of program stderr - error messages can override defaults: wc < m y f i l e file called myfile is used as stdin instead of the keyboard Formatted Output printf() formats the values of variables and writes them on stdout printf() can do a lot more than what we have discussed so far: %c = char, %d = int, %f = double, %% = %, %s = string 1

2 Formatted Output Some Examples % h d format (d = decimal int) type modifier (h = short, l = long) precision (varies) width - how big the field is justify to left (default is right) printf( %5.2f\n, 3.1); (width 5 chars, 2 digits after decimal, double) printf( %-4.2d\n, 2) 0 2 (width 4 chars, left justified, min. 2 digits, int) Important Given a string to print: ch a r * s = % H e l l o T h e r e %! ; p r i n t f ( s ) ; -- won t work p r i n t f ( % s, s ) ; -- safe Another way without printf: p u t s ( s ) ; -- writes the string to stdout Formatted Output Lookup and learn the details of the various escapes we haven t discussed: %o, %x, %X, %u, %e, %E, %g, %G, %p There are printf() variants: sprintf( c h a r * str, c h a r * fm t, ) ; Behaves the same as printf() except puts the result in str rather than writing on stdout How do printf() arguments work? p r i n t f ( % d, 2 ) ; -- printf(char *,int); p r i n t f ( % f % s, 5. 0, h e l l o ) ; -- printf(char *, double, char *); printf() has a variable number (and type) of arguments i n t p r i n t f ( ch a r * f o r m a t, ) ; the means that any number of arguments can follow Note that the first argument is always the format string 2

3 stdarg.h defines the type va_list and macros for accessing these arguments: An Example v o i d e r r m s g ( ch a r * f m t, ) { v a _ l i s t a p ; v a _ l i s t a p ; v a _ s t a r t ( a p, var) ; - variable arguments start after argument named var v a _ a r g ( a p, type) ; - get next argument and treat it as type type v a _ e n d ( a p ) ; - clean up } v a _ s t a r t ( a p, f m t ) ; p r i n t f ( e r r o r : ) ; v p r i n t f ( f m t, a p ) ; p r i n t f ( \ n ) ; v a _ e n d ( a p ) ; There are serious limitations You need to know the type of the argument you are extracting: v a _ a r g ( a p, d o u b l e ) ; - not a conversion! e.g. this tells the compiler that the next argument should be a double if it isn t trouble! printf() has an opposite: scanf() scanf() reads values as strings and stores them in variables: i n t i ; s ca n f ( % d, & i ) ; characters are read from standard input (like getchar) note that a pointer is used as an argument to scanf e.g. if the following characters are input: 5 2 then the value 52 will be stored in i like printf(), scanf() can read multiple values at once i n t i, j ; d o u b l e d ; s ca n f ( % d % f % d, & i, & d, & j ) ; given the input:

4 Beware: scanf() largely ignores whitespace whitespace includes spaces, tabs, and newlines! scanf() s interactions can be subtle avoid complex scanf() expressions like printf(), scanf() has a string counterpart ssc a nf( c h a r * str, c h a r * fm t, ) ; - Another Note You were forbidden from using scanf() in assignment 1 To know why, examine the manual page [scanf(3c)] or the text and see what: i n t k ; s ca n f ( % i, & k ) ; would do. (more abstractly) So we can do stuff with stdin and stdout what about stderr? FILE *f; - f is a pointer to a stream stdio.h defines constants: stdin, stdio, stderr which are all of type FILE * Do not try to dereference these pointers Treat FILE as an opaque type printf() and scanf() have variants which operate on any stream: int fprintf(f I L E * f, char * fm t, ) ; int fs canf(f I L E * f, char * fm t, ) ; so printf( hello, ); == fprintf(stdout, hello, ); To write something to stderr, fprintf(stderr, error!\n ); Note that stdin is read-only and stdout and stderr are write-only, so fprintf(stdin, this w ill not w ork ); will fail but will not crash your program Why Does printf() Return Int? printf() returns the number of characters it wrote or a negative number if an error occurred so fprintf(stdin, hello? ); would return a negative number 4

5 There are similar functions for getchar and putchar: int getc (FILE *stream); int putc (int c, FILE *stream); so getc har() == getc (stdin) - An Exercise If you look at the text (and the manual page) you will find that the following functions are defined: int fgetc (FILE *); int fputc (int c, FILE *); They are similar to getc() and putc() but not identical - why? Line-based I/O We have functions for dealing with lines: c har *fgets(c har *s, int n, FILE *f) Reads in a line from f and stores it s until n-1 characters are read, or until a newline character is read ( \n ) If \n is read, it is also stored in the string Note: we are not guaranteed a full line! Line-based I/O fgets() returns NULL on an error or EOF, otherwise it returns s c har str[ 100] ; w hile(fgets(str,100,stdin)) { } Status of int feof(file *f) - returns non-zero if EOF has been reached on f, 0 otherwise int ferror(file *f) - returns nonzero if an error has occurred on f, 0 otherwise Files We are not restricted to just stdin, stdout and stderr FILE *fopen(c har *file, c har *mode); Creates a new stream by opening the file called file If it fails, returns NULL 5

6 Stream Modes mode tells us whether we are reading or writing (it s a string) r - read-only file must exist w - write-only file is created if necessary, contents are destroyed a - append (write-only) file is created if necessary, contents preserved (write at the end of the file) Stream Modes There are also update modes: r+ - like r but we can also write w + - like w but we can also read a+ - like a but we can also read Some systems (not Unix) distinguish between text and binary files Default is to open in text mode Add b for binary mode (e.g. rb ) Files When we are done with a file, we can get rid of the stream with: int fc lose(file *f); When a program exits all open files are automatically closed An Example FILE *f; c har str[ 100] ; f = fopen( myfile, r ); w hile(fgets(str,100,f)) { printf( line: %s,str); } fc lose(f); Binary I/O are made up bytes They may not be text: int fread(void *ptr, int siz e, int n, F I L E *f); Reads n elements, each size bytes long and puts them at the address specified by ptr e.g. to read n bytes Binary I/O There is a corresponding function for writing out binary data int fw rite(void *ptr, int siz e, int n, F I L E *f); Note - these are binary representations. Try: int x = 0x 57 6 f7 7 0a; fw rite(& x,siz eof(x ),1,stdout); fread(ptr,1,n,f); 6

7 Buffering - A Note What happens when you call: g etc (stdin) Asking the operating system for one character is inefficient A number of characters are read (if possible) and stored in memory This is called buffering Buffering - A Note The same thing happens when writing: putc ( x,stdout); Again, asking the operating system to write one character is inefficient Many characters are buffered and then given to the operating system at once Buffering - A Note By default, stdin and stdout are linebuffered (that is, data is read/written in lines) and stderr is unbuffered Look at /cs/course/2031/buffer.c for an example to demonstrate this Manual Pages All of the functions discussed here have manual pages you may want to read: fopen(3c), fclose(3c), printf(3c), scanf(3c), fgetc(3c), fread(3c), stdio(3c) 7