lecture starting k&r chapter 7 and appendix b also, unix – various chapters in glass

Lecture

• Starting K&R Chapter 7 and Appendix B

• Also, UNIX – Various chapters in Glass

Standard Library Functions• Standard library functions in K&R Appendix B

• On line, you can get documentation about them under the corresponding header files:

http://www.digitalmars.com/rtl/stdio.html

• On UNIX, use grep to find them in the .h files, e.g. % grep strcmp /usr/include/*

• You can get details of them using the man command, e.g. % man printf

UNIX grep, Glass Pg 109+

• UNIX command “grep” finds matches for a specified string in identified filenames

• If no filenames present, grep searches stdin

• Some options in grep:-i ignore case (“TEXT” same as “text”)-n adds line numbers to display-v gives only lines that don’t match-w only matches complete words

UNIX “Pipes”, Glass Pg 175+

• Remember redirection for sysin and sysout?% command <filename1 >filename2

• In general, you can invoke a program and pass sysout from that program as sysin to another program via UNIX pipe – symbol |% command1 arguments | command2 arguments

% grep pattern filename(s) | more

Standard Input /Output, 7.1

• C code for reading stdinchar getchar (void) gets a character from stdin

• C code for writing to stdoutvoid putchar(char) puts a character to stdout

• stdin and stdout can be redirected or piped% ./tail <tail.in | more

% cat filename1 | ./tail >filename2

Formatted Output, printf, K&R 7.2

• Formats and prints out internal values.int printf(char *format, arg1, arg2, . . .);

• printf has a variable length argument list (as many arguments after the first one as % conversions in the format string)

• We will learn how to do this shortly• Return from printf is number of characters printed• Haven't used this up till now, but it may be useful if

there is some error or limit truncation

Formatted Output, printf• Between the % and the conversion character, there

are a number of other characters which may exist. In the order they must be placed, they are:

- (minus sign) left adjust printing of argumentm (number m) minimum field width. (dot) separates min field width & precisionp (integer p) precision: max chars for string

min digits for inth or l (letters) h for short int, l for long int

• ORDER of options for %d is: %[-][m][.][p][h|l]d, Note: No embedded spaces allowed!

Formatted Output, printf• Figure out what these would do:

%10d, %-10d, %hd, %e, %10ld, %10.p

• Experiment for 10 minutes with a program, using different formats

• Learn string precision given on pg. 154

• Also, to print at most max characters from string s (max is int type var or const), use * after % and include the int max as an argument before s:

printf("%.*s", max, s);

Formatted Output, printf• Can print string literal as format string with no “%s”

printf("hello, world!\n");

• Could also print a string variable as format string:char s[ ] = "hello, world";printf(s);

• If string s has a % character in it, this is unsafe!

• printf will look for another argument after format string s. Better to write out a variable string s as:

printf("%s", s);

Formatted Output, sprintf

• See sprintf in K&R Appendix B, pg 245• Function sprintf works same as printf, but it writes to

a specified string, e.g. char array[ ], with trailing ‘\0’

int sprintf(char *string, char *format, arg1, arg2, …);• Note: int return value does not include trailing ‘\0’ • Recall how we wrote itoa() and itox() functions• No functions like this in C library! • Use sprintf() to print int into a string using %d or %x

Formatted Input, scanf

• This is the opposite of printf. Reads in variables from stdin using conversion format string. See pg. 246 (and prior pg 245 which explains everything).

int scanf(char *format, …);

• Return value from scanf( ) is number of successfully scanned tokens

• Not successful if scanf can't parse any value brought in from stdin according to the specified format


• Must call scanf with a POINTER to each variable so that values can be set by scanf which is a function!

int age, weight, cnt;

char lname[100];

while(some condition) {

printf("Input your last name, age, and weight");

cnt = scanf("%s %d %d", lname, &age, &weight);

}

• Note: lname is an array and is already a pointer


• Scanf is useful to allow you to read in int or double value AS A NUMBER (instead of a character string leaving you to do your own conversion in your code)

• scanf() always see a character sequence in stdin: it just does its own conversion to int or double


• However, scanf is FLAWED, because it ignores '\n' characters.

• Can get very confusing if the user enters too few arguments on an input line being parsed by scanf

• (Prompt) Input your last name, age, and weight:• (User input) Clinton 52• User gets no response after carriage return. • User retries, remembers to enter weight this time• (User input) Clinton 52 200


• scanf sees Clinton 52 Clinton since the user entered carriage return is seen as white space

• scanf thinks weight has bad value and returns 2 as number of successfully scanned tokens

• scanf has gotten out of synch with input process

• Re-entered 52 will be seen as a last name and 200 as an age in next prompt/scanf loop


• Use scanf only for programs needing only ONE input item, usually "quick and dirty" programs with no input checking.

• Can't code defensively with scanf( ): can't count number of tokens parsed ON A LINE - scanf doesn't care about input lines

• Best approach is to read a line into an array s[ ] and use "sscanf( )" to parse the arguments in line

• This also allows you to try to interpret things in more than one way

Formatted Input, sscanf• See sscanf in K&R Appendix B, pg 246

• Function sscanf works same as scanf, but it reads from a specified string, e.g. char array[ ], with a trailing ‘\0’int sscanf(char *string, char *format, &arg1, &arg2, …);

• Recall how we wrote function atoi, axtoi to convert a decimal or hex character string s to an integer i?– Use sscanf(s, "%d", &i) for atoi

– Use sscanf(s, "%x", &i) for axtoi

Formatted Input, scanf/sscanf

• Note: with both scanf and sscanf, if you put specific characters in the format string, the functions must see exactly those specific characters in the user inputcnt = sscanf(s, "%d/%d/%d", &month, &day, &year);

• Expects input to look exactly like this: 07/23/96

• If not, cnt value returned by sscanf is less than 3

Variable Length Argument Lists

• Both printf and scanf have an argument (the format string) that defines the number and type of the remaining arguments in the list

• C does not support multiple declarations of the same function each with different lists

• How is it supported in C?• Look at stack frame after a function call!

Typical Stack frameDecreasing Addresses Stack

PointerBefore CallA

rg1

Return Datae.g.PC, otherRegisters, etc

Function’sAutomaticVariables

Stack PointerAfter Call

Code provides the locationof the last fixed argument incall sequence to va_start

From fixed arguments, the code must determine the number ofadditional arguments to access via offsets from stack pointer and va_arg can work its way back up the stack to get each argument

Arg2

Arg3

Address0xffffffff

Variable Length Argument Lists• Use va_list data type and va_ macro package

inside function with a variable length argument list to get args

• va_start, va_arg, va_end macros are defined in /usr/include/stdarg.h

void foo (int n, …) /* note ellipsis … */

{

va_list ap; /* variable name ap */

va_start(ap, n); /* n is last named arg */• Now ap points just before first unnamed arg

Variable Length Argument Lists

• Each call to va_arg( ) advances pointer ap by one argument and returns value by type:

ival = va_arg(ap, int);

fval = va_arg(ap, float);

sval = va_arg(ap, char *);• Function must clean up before returning:

va_end(ap);

}

More examples

• Go over the minprintf(char *fmt, …) in K&R p.156

/* minprintf program in K&H P.156 */#include <stdio.h>#include <stdarg.h>void minprintf(char *fmt,...);

int main(){ int count =5; char * ptr= "life"; minprintf("%s...%d times\n", ptr, count); return 0;}

minprintf Program Listing (page 1)

void minprintf(char *fmt,...) { va_list ap; char *p, *sval; int ival; double dval;

va_start(ap, fmt); for (p = fmt; *p; p++){ if (*p != '%'){ putchar(*p); continue; }


switch(*++p){ case 'd': ival=va_arg(ap, int); printf("%d", ival); break;

case 'f': dval=va_arg(ap,double); printf("%f", dval); break;

case 's': for (sval = va_arg(ap, char*); *sval; sval++) putchar(*sval); break;

default: putchar(*p); break; } } va_end(ap); }


lecture starting k&r chapter 7 and appendix b also, unix – various chapters in glass

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