240-491 Adv. UNIX:io/91 Advanced UNIX v Objectives of these slides: –look in some detail at standard input and output in C 240-491 Special Topics in Comp.

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Advanced UNIXAdvanced UNIX

Objectives of these slides:Objectives of these slides:– look in some detail at standard input look in some detail at standard input

and output in Cand output in C

240-491 Special Topics in Comp. Eng. 1Semester 2, 2000-2001

9. Standard I/O in C

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OverviewOverview

1. Background1. Background

2. Formatted Output: 2. Formatted Output: printf()printf()

3. Formatted Input: 3. Formatted Input: scanf()scanf()

4. Line I/O: 4. Line I/O: gets()gets(), , puts()puts()

5. Character I/O: 5. Character I/O: getchar()getchar(), , putchar()putchar()

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1. Background1. Background

Most I/O library functions are in Most I/O library functions are in stdio.hstdio.h

Basic methods:Basic methods:– 1. Formatted I/O1. Formatted I/O– 2. Line by line2. Line by line– 3. Character by character3. Character by character

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2. Formatted Output: printf()2. Formatted Output: printf()

int i = 2, j = 7;int i = 2, j = 7;printf(“i = %d, j = %d\n”, i, j);printf(“i = %d, j = %d\n”, i, j);

floatfloat, , doubledouble printed with printed with %f%f, , %e%e, , %g%g::

double x = 3.14;double x = 3.14;printf(“%f, printf(“%f, %e%e, , %g%g\n”, x, x, x);\n”, x, x, x);

3.140000, 3.140000e+00, 3.143.140000, 3.140000e+00, 3.14

6 dp is the default

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Character StringsCharacter Strings

char name[SIZE], ch;char name[SIZE], ch; : :printf(“Hi, there, %s\n”, name);printf(“Hi, there, %s\n”, name);printf(“The letter is printf(“The letter is %c%c \n”, ch); \n”, ch);

See man ascii for some unusual chars, e.g. '\a' '\b' '\r'

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Less Used printf() Format SpecifiersLess Used printf() Format Specifiers

SpecifierSpecifier MeaningMeaning%%%% % character% character%u%u Decimal unsigned integerDecimal unsigned integer%o%o Octal integerOctal integer%x%x Hexadecimal integerHexadecimal integer%p%p Pointer (decimal or hex)Pointer (decimal or hex)%n%n Store the number of charsStore the number of chars

written so farwritten so far

continued

useful forprintingunusualchars, seeman ascii

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ExamplesExamples

int i = 125, num;int i = 125, num;int *p = &i;int *p = &i;

printf(“%d, printf(“%d, %o, %x, %X, %p%n%o, %x, %X, %p%n\n”,\n”,i, i, i, i, p, &num);i, i, i, i, p, &num);

printf(“num = %d\n”, num);printf(“num = %d\n”, num);

125, 175, 7d, 7D, 7fffbe48125, 175, 7d, 7D, 7fffbe48num = 31num = 31

upper casehex letters

the decimal isconverted

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Four Types of Format ModifiersFour Types of Format Modifiers

1. Flags1. Flags– e.g. e.g. "%"%++d"d"

2. Field Widths2. Field Widths– e.g. e.g. "%"%1010d"d"

3. Precision3. Precision– e.g. e.g. "%"%.4.4d"d"

4. Type Modifiers4. Type Modifiers– e.g. e.g. "%"%lld"d"

printf(".....", x, y, z);

format string

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2.1. Flags2.1. Flags

FlagFlag MeaningMeaning++ Prefix positive numbers with a + Prefix positive numbers with a + signsign

spacespace Prefix positive numbers with a spacePrefix positive numbers with a space

00 Pad with leading zerosPad with leading zeros

## Alternate output formAlternate output form

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ExamplesExamples

printf(“%printf(“%++d\n%d\n%++d\n”, 786, -786);d\n”, 786, -786);+786+786-786-786

printf(“% d\n% d\n”, 547, -547);printf(“% d\n% d\n”, 547, -547); 547 547-547-547

printf(“%printf(“%+0+09d\n%9d\n%009d\n”, 452, 452);9d\n”, 452, 452);+00000452+00000452000000452000000452

useful fortidy columns

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Meaning of #Meaning of # Prefix Prefix 00 is added to is added to octaloctal output ( output (%#o%#o).).

Prefix Prefix 0x0x or or 0X0X is added to is added to hexadecimalhexadecimal output output ((%#x%#x or or %#X%#X).).

%#e%#e, , %#f%#f, , %#g%#g have a have a decimaldecimal point, even when point, even when there are no digits after it.there are no digits after it.

%#g%#g does not remove trailing zeros. does not remove trailing zeros.

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int c = 1427;int c = 1427;float p = 1427.0;float p = 1427.0;printf(“%printf(“%##o, %o, %##x, %x, %##X, %g, %X, %g, %##g\n”, g\n”,

c, c, c, p, p);c, c, c, p, p);

02623, 0x593, 0X593, 1427, 1427.0002623, 0x593, 0X593, 1427, 1427.00

# Examples# Examples

octal hex hex remove '.'and 0's

do notremove '.'and 0's

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2.2. Field Widths2.2. Field Widths

Specify minimum number of characters printed:Specify minimum number of characters printed:– if too few characters, right justify outputif too few characters, right justify output

– if too many, ignore field widthif too many, ignore field width

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ExampleExample

char *s = “hello”;char *s = “hello”;int x = 23;int x = 23;printf(“%printf(“%1010d%10s\n”, x, s);d%10s\n”, x, s);printf(“%printf(“%-10-10d%-10s\n”, x, s);d%-10s\n”, x, s);

23 hello 23 hello23 hello23 hello

width

- = leftjustify

10 10

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Field Width (*) in printf()Field Width (*) in printf()

Calculate field width based on the next Calculate field width based on the next printf()printf() argument: argument:

#define FIELD_WIDTH 10#define FIELD_WIDTH 10printf(“%printf(“%**d\n”, FIELD_WIDTH, val);d\n”, FIELD_WIDTH, val);

uses FIELD_WIDTH

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2.3. Precision (.) in printf()2.3. Precision (.) in printf()

SpecifierSpecifier EffectEffect%d%d Min. no. of digits Min. no. of digits

(pad with leading zeros)(pad with leading zeros)

%e, %f%e, %f No. of digits after the decimal pointNo. of digits after the decimal point

%g%g No. of significant digitsNo. of significant digits

%s%s Min. no. of characters from a stringMin. no. of characters from a string

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ExampleExample

int i = 873;int i = 873;float f = 123.94536;float f = 123.94536;char s[] = “Happy Birthday”;char s[] = “Happy Birthday”;printf(“%printf(“%.4d.4d\n%.9d\n”, i, i);\n%.9d\n”, i, i);printf(“%printf(“%.3f.3f\n%\n%.3e.3e\n%\n%.3g.3g\n”, f, f, f);\n”, f, f, f);printf(“%printf(“%.11s.11s\n”, s);\n”, s);

08730873000000873000000873123.945123.9451.293e+021.293e+02124124Happy BirthHappy Birth

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Meaning of *.*Meaning of *.*

Calculate field width Calculate field width andand precision from the precision from the next arguments of the next arguments of the printf()printf()

printf(“%printf(“%*.**.*s\n”, s\n”, field_width, precision, field_width, precision,

str);str);

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2.4. Type Modifiers2.4. Type Modifiers

Place h or l before d, o, x, or uPlace h or l before d, o, x, or u

Place L before e, f, or gPlace L before e, f, or g

LetterLetter EffectEffecthh Specify Specify shortshort typetypell Specify Specify longlong type typeL L Specify Specify long doublelong double type type

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3. Formatted Input: scanf()3. Formatted Input: scanf()

int i, x, y;int i, x, y;float val;float val;char str[100];char str[100];

scanf(“%d”, &i);scanf(“%d”, &i); /* read in integer /* read in integer */*/scanf(“%f”, &val);scanf(“%f”, &val); /* read in float */ /* read in float */scanf(“%s”, str);scanf(“%s”, str); /* read in string /* read in string */*/

scanf(“%d%d”, &x, &y); scanf(“%d%d”, &x, &y); /* read space-separated integers *//* read space-separated integers */

23 2.3hi -5 62

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NotesNotes %d%d will read a sign (e.g. will read a sign (e.g. -27-27))

%f%f will accept a float in fractional or will accept a float in fractional or exponential form (e.g. exponential form (e.g. 2.32.3, , 1.2e+021.2e+02) )

scanf()scanf() will ignore whitespace to get to will ignore whitespace to get to a numbera number– scanf("%d", &x);scanf("%d", &x);

– whitespace includes new lineswhitespace includes new lines

continued

-27

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%s%s only reads non-whitespace characters only reads non-whitespace characters– scanf("%s %s", str1, str2);scanf("%s %s", str1, str2);

– skips whitespace to start of textskips whitespace to start of text– stops reading in text when it gets to a whitespacestops reading in text when it gets to a whitespace– adds ‘\0’adds ‘\0’– the string variable (e.g. the string variable (e.g. str1str1) must have enough memory) must have enough memory

continued

hello andy

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scanf("%d%d", &x, &y);scanf("%d%d", &x, &y);

same assame as scanf(“%d”, &x);scanf(“%d”, &x);scanf(“%d”, &y);scanf(“%d”, &y);

scanf()scanf() will read will read xx and and yy values from the values from the same line, or any distance apart, so long as same line, or any distance apart, so long as the values are separated by whitespace.the values are separated by whitespace.

continued

27 13

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%c%c will read any character (even will read any character (even whitespace):whitespace):scanf(“%c”, &ch);scanf(“%c”, &ch);

Other characters in the Other characters in the scanf()scanf() format format string are used for input matching:string are used for input matching:scanf(“%d-%d %d”, &x, &y, &z);scanf(“%d-%d %d”, &x, &y, &z);

continued

27-13 5

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scanf()scanf() leaves non-matching characters leaves non-matching characters on the input streamon the input stream

scanf("%d", &x);scanf("%d", &x);scanf("%s", str);scanf("%s", str);

scanf()scanf() returnsreturns the number of bound the number of bound variables, or variables, or EOFEOF at end-of-file. at end-of-file.

int num;int num;num = scanf("%d %d", &x, &y);num = scanf("%d %d", &x, &y);

hello

27 13

continued

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scanf()scanf() uses line buffering uses line buffering– when when scanf()scanf() is reading from the keyboard, it is reading from the keyboard, it

only gets a line to process when a only gets a line to process when a RETURNRETURN is is pressedpressed

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3.1. Assignment Supression (*)3.1. Assignment Supression (*)

int month, day, year;int month, day, year;

printf(“Enter date as mm-dd-yy: “);printf(“Enter date as mm-dd-yy: “);scanf(“%dscanf(“%d%*c%*c%d%d%*c%*c%d”, &month, %d”, &month,

&day, &year);&day, &year);printf(“month = %d day = %d year = %d\n”, printf(“month = %d day = %d year = %d\n”,

month, day, year);month, day, year);

Enter date as mm-dd-yy: Enter date as mm-dd-yy: 11-18_9711-18_97month = 11 day = 18 year = 97month = 11 day = 18 year = 97

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3.2. Field Widths3.2. Field Widths

A field width specifies the maximum number of A field width specifies the maximum number of input characters to process (ignoring whitespace input characters to process (ignoring whitespace before the start).before the start).

int x, y;int x, y;scanf(“%scanf(“%2d2d%d”, &x, &y);%d”, &x, &y);printf(“Integers are: %d, %d\n”, x, y);printf(“Integers are: %d, %d\n”, x, y);

123456123456Integers are: 12, 3456Integers are: 12, 3456

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3.3. Scan Sets [...]3.3. Scan Sets [...]

Specify characters that Specify characters that cancan be read. be read.

char z[9];char z[9];printf(“Enter String: “);printf(“Enter String: “);scanf(“scanf(“%[aeiou]%[aeiou]”, z);”, z);printf(“The input was \”%s\”\n”, z);printf(“The input was \”%s\”\n”, z);

Enter String: Enter String: ooeeooahahooeeooahahThe input was “ooeeooa”The input was “ooeeooa”

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Inverted Scan Sets [^...]Inverted Scan Sets [^...]

Specify characters Specify characters notnot to be read. to be read.

char z[9];char z[9];printf(“Enter String: “);printf(“Enter String: “);scanf(“scanf(“%[^aeiou]%[^aeiou]”, z);”, z);printf(“The input was \”%s\”\n”, z);printf(“The input was \”%s\”\n”, z);

Enter String: Enter String: PhreakPhreakThe input was “Phr”The input was “Phr”

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3.4. Handling Incorrect Input3.4. Handling Incorrect Input

Things to remember:Things to remember:

– scanf()scanf() returns when it sees a character returns when it sees a character that it does not want. It leaves the that it does not want. It leaves the character on the input stream;character on the input stream;

– scanf()scanf() returns the number of returns the number of variables it has boundvariables it has bound

continued

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/* Try to read two integers *//* Try to read two integers */#include <stdio.h>#include <stdio.h>

int main()int main(){ int i, j, count;{ int i, j, count; do { do { printf(“Enter two integers: “); printf(“Enter two integers: “); count = scanf(“%d %d, &i, &j)count = scanf(“%d %d, &i, &j);; if (count < 2) { if (count < 2) { /* somthing wrong *//* somthing wrong */ scanf(“ scanf(“%*[^\n]%*[^\n]”);”); /* skip to end of line *//* skip to end of line */ printf(“Incorrect. Try again: “); printf(“Incorrect. Try again: “); } } } while (count < 2); } while (count < 2); /* until valid input *//* until valid input */ : :}}

* supresses assign;[^\n] matches uptonewline (whitespace)

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4. Line I/O4. Line I/O

int puts(char *line);int puts(char *line);

– e.g. e.g. puts(“Hello World”);puts(“Hello World”);

– Adds a newline to its output.Adds a newline to its output.

– If there is an error it returns If there is an error it returns EOFEOF; ; If okay it returns a non-negative integer.If okay it returns a non-negative integer.

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char *gets(char *line);char *gets(char *line);

– e.g: e.g: gets(str);gets(str);

– Reads to a newline in Reads to a newline in stdinstdin; ; replaces newlinereplaces newline by by ‘\0’‘\0’ in input argument in input argument

– gets()gets() returns a returns a NULLNULL when when EOFEOF or an error occurs. or an error occurs.

– Main problemMain problem: : gets()gets() may read in a bigger string may read in a bigger string than can be stored in the variable; than can be stored in the variable; use use fgets()fgets() instead. instead.

Bad style:avoid gets()

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char *fgets(char *line, int n,char *fgets(char *line, int n,FILE *stream);FILE *stream);

– e.g: e.g: fgets(str, 80, stdin);fgets(str, 80, stdin);

– Reads at most Reads at most n-1n-1 chars into chars into lineline, stopping if a newline is , stopping if a newline is encountered;encountered;

– the the newline is includednewline is included in in lineline, and a , and a ‘\0’‘\0’ is added is added

– fgets()fgets() returns a returns a NULLNULL when when EOFEOF or an error occurs. or an error occurs.

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sscanf()sscanf()

int sscanf(char *s, int sscanf(char *s, const char *format, ...);const char *format, ...);

Same as Same as scanf()scanf() but its input comes from but its input comes from the string argument (the string argument (ss) not from ) not from stdinstdin..

Very useful for separatingreading from processingin code.

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Read Read thenthen Process Process

#include <stdio.h>#include <stdio.h>int main()int main(){ char str[120]; /* magic number: poor style! */{ char str[120]; /* magic number: poor style! */ int i, j; int i, j; while (fgets(str,120,stdin) != NULL) { /* read */ while (fgets(str,120,stdin) != NULL) { /* read */

/* process *//* process */ if ( if (sscanf(str, “%d %d”, &i, &j) == 2sscanf(str, “%d %d”, &i, &j) == 2) ) printf(“i = %d j = %d\n”, i, j); printf(“i = %d j = %d\n”, i, j); else if ( else if (sscanf(str, “%d”, &i) == 1sscanf(str, “%d”, &i) == 1)) printf(“i = %d j = ??\n”, i); printf(“i = %d j = ??\n”, i); else else printf(“Could not understand anything!\n”); printf(“Could not understand anything!\n”); : :}}

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5. Character I/O5. Character I/O

int putchar(int c);int putchar(int c);

– e.g.:e.g.: putchar(‘\n’);putchar(‘\n’);

putchar(32);putchar(32);

int getchar(void);int getchar(void);

– e.g.:e.g.: intint ch; ch;ch = getchar();ch = getchar();

while (while (((ch = getchar()ch = getchar())) != EOF) != EOF)......

Remember the brackets.

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CannotCannot “Press any key” “Press any key”

#include <stdio.h>#include <stdio.h>int main()int main(){ char ch; { char ch;

printf(“Press any key to go on...”); printf(“Press any key to go on...”); ch = getchar(); /* type conversion */ ch = getchar(); /* type conversion */ printf(“You pressed ‘%c’\n”, ch); printf(“You pressed ‘%c’\n”, ch); : :}}

ProblemProblem: UNIX line buffering: UNIX line buffering Quick fixQuick fix: say : say “Press RETURN”“Press RETURN”