Transcript Text and Binary File Processing 程式設計 潘仁義 CCU COMM

Text and Binary File Processing
程式設計
潘仁義
CCU COMM
C File I/O Overview(1/3)
User Program
C Library
OS File System
File
C File I/O Overview(3/3)
開啟檔案
FILE *fopen( const char *filename, const char *mode);
存取檔案
int fgetc( FILE *fp );
int fputc( int c, FILE *fp );
int fprintf( FILE *fp, const char *format, … );
int fscanf( FILE *fp, const char *format, … );
size_t fread(void *ptr, size_t size, size_t n, FILE *fp );
size_t fwrite(const void *ptr, size_t , size_t n, FILE *fp );
char *fgets( char *s, int n FILE *fp );
int fputs( const char *s, FILE *fp);
int fseek( FILE *fp, long offset, int whence);
long ftell( FILE *fp );
Int feof(FILE *fp);
…
關閉檔案
int fclose( FILE *fp);
- stdio.h 與檔案相關之函式-
12.1 Input and Output Files, Review
text file
a named collection of characters saved in secondary storage
input (output) stream
continuous stream of character codes representing textual input (or
output) data
(FILE *) stdin
system file pointer for keyboard’s input stream
(FILE *) stdout, stderr
system file pointers for screen’s output stream
TABLE 12.2
Placeholders for printf Format Strings
Placeholder
Used for Output of
%c
a single character
%s
a string
%d
an integer (in base 10)
%o
an integer (in base 8)
%x
an integer (in base 16)
%f
a floating-point number
%e
a floating-point number in scientific notation
%E
a floating-point number in scientific notation
%%
a single % sign
TABLE 12.1
Meanings of Common Escape Sequences
Escape Sequence
Meaning
‘\n’
new line
‘\t’
tab
‘\f’
form feed (new page)
‘\r’
return (go back to column 1 of current
output line)
‘\b’
backspace
語言
TABLE 12.4
Comparison of I/O with Standard Files and
I/O with User-Defined File Pointers
Line
Functions That Access stdin and
stdout
Functions That Can Access Any
Text File
1
scanf(“%d”, &num);
fscanf(infilep,“%d”, &num);
2
printf(“Number=%d\n”,num);
fprintf(outfilep,
“Number=%d\n”,num);
3
ch=getchar();
ch=getc(infilep);
4
putchar(ch);
putc(ch,outfilep);
Figure 12.1 Program to Make a
Backup Copy of a Text File
Figure 12.1 Program to Make a
Backup Copy of a Text File (cont’d)
可以換成
feof( inp)
嗎?
不行,還是
有點不一
樣
Figure 12.2 Input and Output Streams
for File Backup Program
File Open Mode
Mode
r
w
a
r+
w+
a+
rb
wb
ab
rb+
wb+
ab+
Fig. 11.6
Description
Open a file for reading.
Create a file for writing. If the file already exists, discard the current contents.
Append; open or create a file for writing at end of file.
Open a file for update (reading and writing).
Create a file for update. If the file already exists, discard the current contents.
Append; open or create a file for update; writing is done at the end of the file.
Open a file for reading in binary mode.
Create a file for writing in binary mode. If the file already exists, discard the
current contents.
Append; open or create a file for writing at end of file in binary mode.
Open a file for update (reading and writing) in binary mode.
Create a file for update in binary mode. If the file already exists, discard the
current contents.
Append; open or create a file for update in binary mode; writing is done at the
end of the file.
File open modes.
12.2 Binary Files
Formatted Text files
contain variable length records
must be accessed sequentially, processing all records from the start of file
to access a particular record
Binary Files (random access file)
a file containing binary numbers that are the computer’s internal
representation of each file component
contain fixed length records
can be accessed directly, directly accessing the record that is required
Binary files are appropriate for online transaction processing systems,
e.g. airline reservation, order processing, banking systems,
sizeof
operator that finds the number of bytes used for storage of a data type
The Data Hierarchy
Bit - smallest data item
Value of 0 or 1
Byte – 8 bits
Used to store a character
Decimal digits, letters, and special symbols
Field - group of characters conveying meaning
Example: your name
Record – group of related fields
Represented a struct or a class
Example: In a payroll system, a record for a particular employee
that contained his/her identification number, name, address, etc.
File – group of related records
Example: payroll file
Database – group of related files
In a Random Access File …
Data
Data unformatted (stored as "raw bytes")
in random access files
All data of the same type (ints, for
example) use the same memory
All records of the same type have a
fixed length
Data not human readable
Random Access
Access individual records without searching through
other records
Instant access to records in a file
Data can be inserted without destroying other data
Data previously stored can be updated or deleted
without overwriting.
Implemented using fixed length records
Sequential files do not have fixed length records
0
100
200
300
400
500
}
byte offsets
}
}
}
}
}
}
100
100
100
100
100
100
bytes
bytes
bytes
bytes
bytes
bytes
Random Access a File -- fread ()
fread --Transfer bytes from a file to a location
in memory
Function fread requires four arguments
ret = fread(buffer, size, num, myptr);
How to
distinguish error
buffer: Address of first memory cell to fill
and EOF?
size: Size of one value
feof ( ), ferror( )
num: Maximum number of elements to copy from the file into
memory
myptr: File pointer to a binary file opened in mode “rb” using
function fopen
the number of objects read
Random Access a File – fwrite()
fwrite - Transfer bytes from a location in
memory to a file
fwrite( &number, sizeof( int ), 1,
myPtr );
&number - Location to transfer bytes from
sizeof( int ) - Number of bytes to transfer
1 - For arrays, number of elements to transfer
In this case, "one element" of an array is being
transferred
myPtr - File to transfer to or from
Random Access a File – fwrite() (II)
Writing structs
fwrite( &myObject, sizeof (struct myStruct), 1, myPtr );
sizeof - Returns size in bytes of object in parentheses
To write several array elements
Pointer to array as first argument
Number of elements to write as third argument
Figure 12.3 Creating a Binary File of
Integers
Access Data Randomly in a Random Access File
fseek
Sets file position pointer to a specific position
fseek( myPtr, offset, symbolic_constant);
myPtr - pointer to file
offset - file position pointer (0 is first location)
symbolic_constant - specifies where in file we are reading from
SEEK_SET - seek starts at beginning of file
SEEK_CUR - seek starts at current location in file
SEEK_END - seek starts at end of file
ftell
Return the current position in a stream
ftell( myptr)
myPtr - pointer to file
目前存取位置
位移後
file
origin: SEEK_SET
SEEK_CUR
offset
SEEK_END
12.3 SEARCHING A DATABASE
database
a vast electronic file of information that can be quickly searched
using subject headings or keywords
本節請自修, 寫作業時可參考
12.4 COMMON PROGRAMMING ERRORS
(1/2)
Remember to declare a file pointer variable
(type FILE *) for each file you want to process
fscanf, fprintf, getc and putc must be used for
text I/O only
fread and fwrite are applied exclusively to
binary files
getc, fscanf and fprintf take the file pointer as
their first argument
getc, putc, fread and fwrite take the file
pointer as their last argument
12.4 COMMON PROGRAMMING
ERRORS (2/2)
Opening a file for output by calling fopen with
a second argument of “w” or “wb” typically
results in a loss of any existing file whose
name matches the first argument
Binary files cannot be created, viewed, or
modified using an editor or word processor
program
Depending on OS, a read cannot follow a
write, a write cannot follow a read, except
using a fseek(fp, 0, SEEK_CUR) between
them
Ch.12 之番外篇
懶惰的C語言
讀取失敗了, 再來看原因是什麼 (別的語言不一定要等到讀失敗)
getc( )的EOF, fread的傳回0
用feof( ), ferror( )可分辨是 檔案結束 或是 讀取錯誤
想知道檔案的大小嗎?
fseek( ) + ftell( )
stat(), fstat( ), struct stat {... st_size;…};
+模式的陷阱
交互讀寫可能會有奇怪的現象
man fopen
Reads and writes may be intermixed on read/write streams in any order,
and do not require an intermediate seek as in previous versions of stdio.
This is not portable to other systems, however; ANSI C requires that a
file positioning function intervene between output and input, unless
an input operation encounters end-of-file.