7.0 files and c input

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:\\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

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++)) …;
#definegetchar()getc(stdin)
#defineputchar(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