Chapter 7 Files

By C. Shing

ITEC Dept

Radford University

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Objectives Understand how to use file utilities Understand how to use files by file pointers Understand how to use files by file descriptors Understand how to interact with operating

environment

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File A stream of bytes

Text file: user readable Binary file: machine readable, more efficient

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File Access Can access (by opening file first) by

File pointer: address of a structure FILE * defined in stdio.h Has buffer available

File descriptor: non-negative number represents file No data structure, flexible Needs to define buffer to hold more than 1 character No formatting specified

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File Utilities Create a temporary file

tmpnam (NULL) Returns a string of a temporary file name

Delete file remove (filenamestring);

Change filename rename (oldname string, newnamestring);

Example: char *tmpfile;

tmpfile=tmpnam(NULL);remove (tmpfile);

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Access File by File Pointer File pointer: declare for each file used

Pre-defined: stdin (keyboard input), stdout (output to screen),stderr (error message to screen)

Declared asFILE *filepointername;

Example:FILE *infile, *outfile;

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Access File by File Pointer - fopen Open file:

Form: fopen (“filename”, ”permission”) Returns a file pointer

Starts from beginning of the file Need to check successful when use fopen

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Access File by File Pointer – fopen (Cont.)

Open file: (Cont.) Permission:

Unix: both text and binary file r: read, for input w: write, for output

If file not exist, create it If file exists, erase file content

a: append to end of file, for updating If file not exist, create it

r+. w+: read and write

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Access File by File Pointer – fopen (Cont.) Open file: (Cont.)

Example:infile=fopen (“/usr/include/stdio.h”, “r”);

or outfile=fopen (“current_dir_file”, “w”); or outfile=fopen (argv[1], “a”);

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Access File by File Pointer – fopen (Cont.)

Open file: (Cont.) Permission: (Cont.)

MS-DOS Tex file: same as in Unix Binary file:

rb: read wb: write ab: append r+, w+: read and write

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Access File by File Pointer – fscanf, fprintf

Read/Write file: for text file Any data type

Input form:fscanf(inputfilepointer, “format”, variable_addr) Output form:fprintf(outputfilepointer, “format”,

variable_list)

Example:char character;while (fscanf(infile,”%c”,&character) != EOF)

fprintf (outfile, ”%c”, character);

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Access File by File Pointer – getc, putc Read/Write file:

Character Input form: getc (inputfilepointer) Output form: putc (character_variable, outputfilepointer)

Example:char character;while ((character=getc(infile)) != EOF)

putc (character, outfile);

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Access File by File Pointer – fgets, fputs Read/Write file:

String Input form: fgets (string, n, inputfilepointer)

Read at most n-1 characters into string from inputfile Output form: fputs (string, outputfilepointer)

Writes the string (except NULL character into outputfile

Example:

while (fgets(instring, n, infile) != EOF)

fputs (instring, outfile);

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Access File by File Pointer – fread, fwrite

Read/Write file: for binary file Input form: fread (arrayaddress, cellsize, n,

inputfilepointer)Read at most n*cellsize bytes into arrayaddress from inputfile

Output form: fwrite (arrayaddress, cellsize, n, outputfilepointer)

Writes at most n*cellsize bytes from arrayaddress into outputfile

Example:int arrayA[n];while (fread(arrayA, 4, n, infile) > 0)

fwrite (arrayA, 4, n, outfile);

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Access File by File Pointer – rewind Move pointer to the beginning of the file:

Form: rewind (filepointer);

Example:

rewind (infile);

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Access File by File Pointer – fseek Move pointer to any place of the file: fseek

Form: fseek (filepointer, offset, position); Offset: relative to position

-: to previous +: to next

Position: 0: beginning 1: current position 2: end

Example:fseek (outfile, 0, 2); // go to the end of the file

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Access File by File Pointer – ftell check the current file position: ftell

Form: ftell (filepointer);

Example:

while (ftell (infile)>0)

putc(getc(infile), outfile);

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Access File by File Pointer – feof check the end of file: feof

Form: feof (filepointer);

Example:

while (!feof (infile))

putc(getc(infile), outfile);

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Access File by File Pointer – fclose Form: fclose (filepointer);

Example:

fclose (infile);

fclose (outfile);

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Access File by File Pointer (Cont.) Class Example:

Example1

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Other Utilities Using File Pointer tmpfile(): returns a file pointer

Example:

FILE *tmpfileptr;

tmpfileptr=tmpfile();

putc(getc(infile),tmpfileptr);

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Access File by File Descriptor File descriptor: nonnegative integer for each file

Reserved: 0 (stdin), 1 (stdout), 2 (stderr) The rest files starts using 3, created when use open()

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Access File by File Descriptor - open

Open file: Starts from beginning of the file Defined in unistd.h (in MS-DOS, use io.h) check successful when use open Form: open (“filename”, options,

permission_octal) Returns a file descriptor

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Access File by File Descriptor – open (Cont.)

Example:

#include <fcntl.h>

#include <unistd.h>

int infilefd;

infilefd=open(argv[1],O_CREAT, 0400);

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Access File by File Descriptor – open (Cont.)

Open file: (Cont.) Permission: defined in /usr/include/fcntl.h,

use | to collect rights O_CREAT: create if not exists O_RDONLY: read only O_WRONLY: write only O_EXCL: give error if set O_CREAT and file exists O_RDWR: read and write

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Access File by File Descriptor – open (Cont.)

Open file: (Cont.) Permission: (Cont.)

O_APPEND: file pointer at file end O_TRUNC: if file exists, truncate file to empty O_NONBLOCK: not block for named pipe

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Access File by File Descriptor – open (Cont.)

Open file: (Cont.)

Example:infilefd=open (“/usr/include/stdio.h”,

O_RDONLY); or outfilefd=open (argv[2],

O_CREAT|O_WRONLY|O_TRUNC, 0600);

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Access File by File Descriptor – read, write

Read/Write file: for both text and binary files Input form: read (inputfd, bufferaddress, size)

Read at most size bytes into buffer from inputfile,

it returns numbers of bytes read Output form: write (outputfd, bufferaddress, size)

Prints at most size bytes from buffer address into

outputfile

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Access File by File Descriptor – read, write (Cont.)

Example:

int size;

char buffer[80];

while ((size=read(infilefd, buffer, 512)) > 0)

write (outfilefd, buffer, size);

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Access File by File Descriptor – lseek Move pointer to any place of the file: lseek

Form: lseek (filefd, offset, position); Offset (long): relative to position

-: to previous +: to next

Position: defined in /usr/include/stdio.h or

/usr/include/unistd.h 0 (or SEEK_SET): beginning 1 (or SEEK_CUR): current position 2 (or SEEK_END): end

Example:lseek (outfilefd, 0, 2); // go to the end of the file

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Access File by File Descriptor – close

Form: close (filedescriptor);

Example:

close (infilefd);

close (outfilefd);

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Access File by File Descriptor (Cont.)

Class Example:

Example 2

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Access File by File Descriptor (Cont.)

Example:

sparse.c

sparse.txt

normal.txt

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Interacting with Operating Environment Call Unix command

Use system tool Form: system (“Unix commands”);

Wait program

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Interacting with Operating Environment (Cont.) Send in output from Unix environment

using pipe Open pipe

Form: popen (“Unix command”, “permission”); Return a file pointer

Close pipe Use pclose(filepointer)

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Interacting with Operating Environment - system Example:

system (“vi myfile”);

Or

char *cmdstr;

sprintf(cmdstr, “vi %s”, argv[1]);

system (cmdstr);

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Interacting with Operating Environment – popen, pclose Example:

FILE *fileptr;

fileptr=popen (“find . -name myfile.c -print

| more”, “r”);

while ((character=getc(fileptr)) != EOF)

putc(character, stdout);

pclose(fileptr);

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References Deitel & Deitel: C How to Program, 4th ed.,

Chapter 11, Prentice Hall