Binghamton

University

CS-211

Fall 2017

Input and OutputStreams and Stream IO

1

Binghamton

University

CS-211

Fall 2017

Standard Input and Output

• Need: #include <stdio.h>

• Large list of input and output functions to:• Read and write from a “stream”

• Open a file and make a stream

• Close a file and remove the stream

• Create, Rename, delete, or move files

• Streams…. directional list of data (bytes)• input streams… provide data to program

• output streams… program provides data

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Binghamton

University

CS-211

Fall 2017

Unix “standard” streams : ./command

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Commandstdin

stdout

stderr

Binghamton

University

CS-211

Fall 2017

Simple IO functions

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stdin

stdout

stderr

Read a single character from stdin, write to stdoutNo opens/closes required

x=getchar();

putchar(y);

Binghamton

University

CS-211

Fall 2017

Shared Monitor

• stdout and stderr both go to the monitor by default

• If a program writes to both stderr and stdout, sometimes the output gets scrambled

>./timesTable 2

| 0 | 1 |

----|----|----|

0 | 0 |DBG: About to write the first row, second column

0 |

1 | 0 | 1 |

----|----|----|

Binghamton

University

CS-211

Fall 2017

Redirection

• UNIX provides a way to change the allocation of standard streams

• When you execute a command, UNIX looks for command line suffixes:• “<filename” tells UNIX to read filename file as stdin• “>filename” tells UNIX to write filename file as stdout• “2>filename” tells UNIX to write filename file as stderr

• Suffixes come after command name and arguments: cat x.txt >y.txt

• Cannot connect same file to stdin and stdout

• Output files are created if not already there

• Output redirected files are re-written from the beginning• Replaces whatever used to be in that file

Binghamton

University

CS-211

Fall 2017

Redirection: ./command <in.txt >out.txt

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Commandstdin

stdout

stderr

in.txt

out.txt

Binghamton

University

CS-211

Fall 2017

Pipes (|)

• Connects two UNIX commands• Connects the stdout of the first command to the stdin of the second

• Example: cat chapter*.txt | wc –w• “cat chapter*.txt” writes chapter1.txt, chapter2.txt, … to stdout

• “wc –w” counts the number of words from stdin

• If first command stalls, second command waits

• If either the first or second command ends abnormally, you get a “broken pipe” message

• Can combine pipes and redirection: cat chap*.txt | wc –w >sum.txt

Binghamton

University

CS-211

Fall 2017

Pipes: ./command1 | ./command2

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Command1stdin

stdout

stderr

Command2

stdin stdout

stderr

Binghamton

University

CS-211

Fall 2017

Example: stripBlanks

#include <stdio.h>

int main(int argc, char **argv) {

char x;

while(EOF!=(x=getchar())) {

if (x != ' ') putchar(x);

}

return 0;

}

>cat xmp.txt

This is an example.

It has blanks

>./stripBlanks <xmp.txt

Thisisanexample.

Ithasblanks

Binghamton

University

CS-211

Fall 2017

Stream Names

• Each stream has a name

• The three standard streams are named : stdin, stdout, and stderr• These are opened for you when you start your program

• These are closed for you when your program ends

• You can create your own named streams• You must open the stream before using it

• You should close the stream when you are done with it

• Named streams have a direction (input or output streams)

Binghamton

University

CS-211

Fall 2017

Using Named Streams

• Almost all i/o functions have sister functions that use named streams

I/O Function Default Stream Sister Function Equivalent

getchar() stdin getch(stdin)

putchar(x) stdout putch(stdout,x)

printf(…) stdout fprintf(stdout,…)

scanf(…) stdin fscanf(stdin,…)

Binghamton

University

CS-211

Fall 2017

Example Named Stream

• fopen(filename,mode) – returns FILE* which can be used as a stream• filename is fully qualified, or relative to the current directory• mode: “r”=read, “w”=write, “a”=write append• flclose files that you open

#include <stdio.h>

FILE *myin=fopen(“example.txt”,”r”);

if (myin==NULL) { /* open failed! */ }

char x;

while( -1 != ( x=getc(myin) ) ) { /* Work on x … */ }

fclose(myin);13

Binghamton

University

CS-211

Fall 2017

Print Formatted (printf)

• Function with prototype: int printf(char * str,…);

• str : format string (list of characters)

• … : variable number of extra arguments

• printf “formats” the format string, using the extra arguments as input into a resulting “formatted” string

• Then, writes the formatted string to stdout

• Returns number of characters printed

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Binghamton

University

CS-211

Fall 2017

Printf Formatting

• Every “%” in formatted string starts a special “format specifier”

• Format specifiers end with a format specifier “type” (next slide)

• Format specifiers are replaced with zero or more characters before printing

• Format specifiers usually “consume” one of the extra arguments

• Need an extra argument for each format specifier that requires a value

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Binghamton

University

CS-211

Fall 2017

Format Specifier Types

Specifier Meaning

%% Replace with a single %

%c Replace with next single character argument

%i or %d Replace with next integer argument converted to ASCII

%x Replace with next integer argument converted to hexadecimal ASCII

%f Replace with next floating point argument converted to ASCII

%s Replace with next string argument (list of characters)

%p Replace with address in hexadecimal

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printf(“Format c=%c, x=%d (or %x), f=%f s=%s\n”,’z’,12,12,3.14,”that’s all folks”);

Format c=z, x=12 (or 0C), f=3.1400 s=that’s all folks

Binghamton

University

CS-211

Fall 2017

Format Specifier Modifiers

• Format Specifier: %flag width .precision type

• flag : optional – “-” (left justified, default is right justified)

• width : optional – minimum size of replacement

• .precision : optional – number of decimal places for %f before rounding

printf(“Format <%5i> <%-5.2f> <%3c> <%-3c>\n”,

10,3.156,’a’,’b’);

Format < 10> <3.16 > < a> <b >

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Binghamton

University

CS-211

Fall 2017

format string functions

• printf – write output to stdout

• fprintf – write output to a named stream (stdout, stderr, etc.)

• sprintf – write output to string

• scanf – read from stdin and update memory that arguments point to

• fscanf – read from specified stream, and update memory…

• sscanf – read from a string and update memory…

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Binghamton

University

CS-211

Fall 2017

Stream Buffers

• In order to improve program efficiency, streams are often “buffered”

• A stream buffer is a block of memory that accumulates stream data until there is enough data to be efficiently read or written

• Stream buffers mean that the program interacts with the buffer (fast memory) many times before having to interact with real (slow) IO

• Sometimes, (especially when debugging) buffers are a problem• Debug message gets stuck in buffer and not printed when program dies

• Remove buffer by setting its size to zero: setbuf(stdout,0);

Binghamton

University

CS-211

Fall 2017

Stream Buffer Example

for(i=0; i<totsize; i++) {putchar(out[i]);

}

Binghamton

University

CS-211

Fall 2017

Resources

• The C Programming Language, Chapter 7

• Wikipedia: printf format string (https://en.wikipedia.org/wiki/Printf_format_string)

• C Standard Library Reference – stdio.h (https://www-s.acm.illinois.edu/webmonkeys/book/c_guide/2.12.html)

• C FAQ Stdio (http://c-faq.com/stdio/index.html)

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