This document discusses C input/output (I/O) and file handling. It covers key concepts like file access control blocks, opening and closing files with fopen() and fclose(), reading and writing characters and lines with functions like getc(), putc(), fgets(), and fputs(), formatted I/O with printf() and scanf(), binary I/O with fread() and fwrite(), and other file operations like repositioning and deleting with fseek(), rewind(), and unlink(). The standard I/O library stdio.h contains definitions and functions for file I/O in C.

1. Files and C Input/Output

2. 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

3. 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”

4. 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;

5. 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

6. 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.

7. fopen()
FILE *fp; /* declares a file pointer or a stream */
char s[]=“c:mydirtemp.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

8. 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.

9. 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

10. 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.

11.  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);

12. 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.

13. 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.

14. 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