Files and C Input/Output

Learning outcomes

At the end of this section you will be able to Understand the concepts involved in accessing

secondary storage by computer programs Understand how to deal with disk files in C Write programs those read and write into files in a

secondary storage

Introduction

All input and output in C are from and to files. File is a sequence of bytes. (text files use \n) Open, Process, Close An open file is identified by a “File Access

Control Block”

File Access Control Blocks

struct iobuf {

char *_ptr; // Current ch. position

int *_cnt; // Remaining bytes

char *_base; // Buffer base address

char _flag; // Access control flags

char _file; } // File number #define FILE struct _iobuf /* stdio.h */ #define EOF -1 typedef struct _iobuf FILE;

FCB’s cont…

First 3 file access control blocks are automatically assigned to standard input standard output and standard error devices FILE *stdin, *stdout, *stderr

Standard input and output devices can be redefined for a program at the command line as, Process <infile >outfile

stdio.h

Many IO libray functions come in pairs (or in triplets) whose operational differs only slightly. printf(“Hello world”); // prints to stdout fprintf(fp,“Hello world”) // prints to a file identified

by fp fprintf(stdout,“Hello world”) // same as the first

fp is a pointer to a file access control block and is returned by a successful fopen() operation.

fopen()

FILE *fp; /* declares a file pointer or a stream */

char s[]=“c:\\mydir\\temp.txt”;

If ((fp=fopen(s,“wt”)) == NULL) printf(“file open error”);

Mode string “r | w | a [+] [ t | b ]” r : open for read w : create for write a : open for append/create for write + : permits both read and write t | b : in text or binary mode

fclose()

To close an opened file, if (fclose(fp)!=0) printf(“file close error”);

If necessary file’s buffer is first flushed (written back to the file) before it is closed.

fclose() is automatically called for each open file when a program terminates normally.

Character transfers to/from files

getc(fp) and fgetc((fp) reads a ch from the stream fp. putc(c,fp) and fputc(c,fp) writes c into the stream fp. ungetc(c,fp) push back the character just read to fp. All 5 returns the ch. on success and EOF on failure

Eg –

while ((*ptr++=getc(fp))!=EOF) …;

while (putc(*ptr++)) …; #define getchar() getc(stdin) #define putchar(c) putc(c,stdout) // in

stdio.h

Line IO

Line ch sequence terminating in a ‘\n’ Simplest form of a record in a C file.

fgets(s,n,fp) reads the next line (including ‘\n’) up to a maximum of n chs from stream fp and writes into memory buffer starting from s with a terminating ‘\0’.

gets(s) reads a line from stdin and placed in memory starting from s, ‘\n’ replaced by a ’\0’.

gets() and fgets() return s on success and a NULL on failure.

fputs(s,fp) writes a string beginning at s onto the stream fp without ‘\0’.

puts(s) same as fputs(s,stdout) but appends a ‘\n’. puts() fputs() returns last ch or 0 on success and

EOF on failure.

These return values permit statements such as, while(strlen(gets(s)) …; while (fgets(s,MAX_LINE,fp1) != NULL) fputs(s,fp2);

Formatted IO

The characters –123.45E-7 printed on a display or typed in a keyboard is totally different from the form it is stored in memory.

printf(f), fprintf(fp,f), sprintf(s,f) - Return the no. of bytes output on success and EOF on failure.

Eg- printf(“\nAverage = %-8.4f”,avg); scanf(f), fscanf(fp,f), sscanf(s,f) – On success

return the no. of fields scanned and stored. Return EOF on failure.

Binary IO

fread(ptr,i,n,fp) Transfers n data blocks of each of size i bytes

from stream fp into memory buffer at ptr. Returns the no.of blocks read.

fwrite(ptr,i,n,fp) Appends n data items in memory at ptr each of

size i to the stream fp. Returns no. of bytes written.

Further File Operations

unlink(“fname”) removes fname from the file system

rewind(fp) set the current file position at start of file.

fseek(fp,L,i) place the file position L bytes from i i is 0|1|2 for start | current | end L is a long integer

unlink and fseek return 0 on success and –1 on failure. rewind does not return any value.

ftell(fp) returns current byte position as a long and -1 on failure

feof() detects end-of-file marker in a file