C-based file operations

ArticleDirectory

• I. Stream File Operations
• 2. Direct I/O File Operations

In ansi c, file operations are divided into two methods: stream file operations and I/O file operations, which are described below.

I. Stream File Operations

This method has an important structure file. The file is defined in the header file stdio. h as follows:

 1 Typedef Struct  {  2   Int Level; /*  Fill/empty level of Buffer  */  3 Unsigned flags; /* File Status flags  */  4   Char FD; /*  File descriptor  */  5 Unsigned Char Hold; /*  Ungetc char if no buffer  */  6   Int Bsize; /* Buffer size  */  7 Unsigned Char _ Far * buffer; /*  Data Transfer Buffer  */  8 Unsigned Char _ Far * CURP; /*  Current active pointer  */  9 Unsigned istemp; /* Temporary File indicator  */  10   Short Token; /*  Used for validity checking  */  11 } File; /*  This is the file object  */ 

The file structure contains the basic attributes of file operations. All file operations must be performed through the pointer of this structure. For common functions of such file operations, see the following table functions.

Fopen () Open stream

Fclose () Close the stream

Fputc () writes a character to the stream

Fgetc () reads a character from the stream

Fseek () locates the specified character in the stream

Fputs () writes a string to a stream

Fgets () reads a row or a specified character from the stream.

Fprintf () Outputs Data to the stream in the format

Fscanf () reads data from the stream in the format

Returns the true value when feof () reaches the end of a file.

Returns the value of ferror () When an error occurs.

Rewind () resets the file locator to the beginning of the file

Remove () delete an object

Fread () reads a specified number of characters from a stream

Fwrite () writes a specified number of characters to the stream.

Tmpfile () generates a temporary file stream

Tmpnam () generates a unique file name

 

The following describes these functions.

1. fopen ()

The fopen prototype is file * fopen (const char * filename, const char * mode). fopen implements three functions.

Open a stream for use

Connect a file to this stream

Returns a filr pointer to the stream.

The filename parameter points to the name of the file to be opened. mode indicates the string in the open state. The values are as follows:

String meaning

R: open a read-only file, which must exist.

R + open a readable file, which must exist.

RB + read/write open a binary file and only allow read/write data.

RT + read/write: open a text file and allow reading and writing.

W. Open and write only the file. If the file exists, the file length is 0, indicating that the file content will disappear. If the file does not exist, the file is created.

W + open the readable and writable file. If the file exists, the file length is cleared to zero, that is, the file content disappears. If the file does not exist, the file is created.

A. Open and write-only files as an attachment. If the file does not exist, the file will be created. If the file exists, the written data will be added to the end of the file, that is, the original content of the file will be retained. (EOF reserved)

A + opens readable and writable files by appending them. If the file does not exist, the file will be created. If the file exists, the written data will be added to the end of the file, that is, the original content of the file will be retained. (The original EOF is not retained)

WB only opens or creates a new binary file; only data can be written.

WB + enables read/write operations or creates a binary file, allowing read and write operations.

Wt + read/write opens or creates a text file; read/write is allowed.

At + read/write opens a text file, allowing you to read or append data at the end of the text.

Open a binary file through AB + read/write, and allow you to read or append data to the end of the file.

A file can be opened in text or binary mode. The difference between the two is that in text mode, carriage return is treated as a character '\ n ', the binary mode considers it to be two characters: 0x0d and 0x0a. If you read 0x1b in the file, the text mode considers it to be the file Terminator, that is, the binary model does not process the file, the text mode will convert the data in a certain way.

By default, the system is enabled in text mode. You can modify the value of all variables _ fmode to modify this setting. For example, _ fmode = o_text; then, the default mode is set to text mode; _ fmode = o_binary; the default enable mode is binary.

This function returns a file pointer, so it does not need to be initialized after declaring a file pointer. Instead, it uses fopen () to return a pointer and connect it to a specific file. If it succeeds or fails, null is returned.

Example:

File * FP;

If (FP = fopen ("123.456", "WB "))

Puts ("file opened successfully ");

Else

Puts ("file opening success or failure ");

2. fclose ()

Fclose () is used to close files opened with fopen (). Its prototype is int fclose (File * FP). If the file is successful, 0 is returned. If the file is failed, EOF is returned.

InProgramRemember to close the opened file at the end. Otherwise, it may cause data loss. I used to make such a mistake.

Example: fclose (FP );

3. fputc ()

Write a character to the stream. The prototype is int fputc (int c, file * stream). If this character is returned successfully, EOF is returned for failure.

Example: fputc ('x', FP );

4. fgetc ()

Read a character from the stream. The prototype is int fputc (File * stream). If this character is returned successfully, EOF is returned if it fails.

Example: Char struct = fgetc (FP );

5. fseek ()

This function is generally used in files opened in binary mode. It is used to locate the specified position in the stream. The prototype is int fseek (File * stream, long offset, int whence ); if 0 is returned successfully, the offset parameter indicates the number of characters to be moved, and the whence parameter indicates the moving benchmark. The value is

Symbol constant value reference position

Start with seek_set 0

Seek_cur 1 Current read/write location

Seek_end 2 file tail

Example: fseek (FP, 1234l, seek_cur); // move the read/write position 1234 bytes backward from the current position (L suffix indicates a long integer)

Fseek (FP, 0l, 2); // move the read/write location to the end of the file

6. fputs ()

Write a string to the stream. The prototype is int fputs (const char * s, file * stream );

Example: fputs ("I love you", FP );

7. fgets ()

Read a row or a specified character from the stream. The prototype is char * fgets (char * s, int N, file * stream). Read n-1 characters from the stream, unless one line is read, the parameter S is used to receive strings. If the string is successful, the pointer of S is returned. Otherwise, null is returned.

For example, if the text at the current position of a file is as follows:

Love, I have

But ........

If you use

Fgets (str1, 4, file1 );

After execution, str1 = "lov" reads 4-1 = 3 characters.

Fgets (str1, 23, file1 );

Run STR = "love, I have" to read a row (excluding '\ n' at the end of the row ').

8. fprintf ()

Input to the stream in the format. Its prototype is int fprintf (File * stream, const char * Format [, argument,...]); the method is the same as printf (), but it is not written to the console, but to the stream.

Example: fprintf (FP, "% 2D % s", 4, "HAHAHA ");

9. fscanf ()

Read from the stream in the format. Its prototype is int fscanf (File * stream, const char * Format [, address,...]); the method is the same as scanf (), but it is not read from the console, but from the stream.

Example: fscanf (FP, "% d", & X, & Y );

10. feof ()

Check whether the end of the file is reached. If it is true, 0 is returned. The prototype is int feof (File * stream );

Example: If (feof (FP) printf ("ended at the end of the file ");

11. ferror ()

The prototype is int ferror (File * stream); returns the most recent stream error.Code, Clearerr () can be used to clear it. The prototype of clearerr () is void clearerr (File * stream );

Example: printf ("% d", ferror (FP ));

12. Rewind ()

Return the current read/write location to the start of the file. The prototype is void rewind (File * stream). In fact, this function is equivalent to fseek (FP, 0l, seek_set );

Example: rewind (FP );

13. Remove ()

Delete the file. The prototype is int remove (const char * filename). The parameter is the name of the file to be deleted. 0 is returned if the file is successfully deleted.

Example: Remove ("C: \ Io. sys ");

14. fread ()

Read a specified number of characters from the stream. The prototype is size_t fread (void * PTR, size_t size, size_t N, file * stream). The PTR parameter stores the read data, void * pointers can be replaced by pointers of any type, such as char * and int *. Size indicates the number of bytes in each block. N indicates the number of read blocks. If yes, returns the number of actually read parts (not the number of bytes). This function is generally used in files opened in binary mode.

Example:

Char X [4230];

File * file1 = fopen ("C: \ msdos. sys", "R ");

Fread (x, 200, 12, file1); // read 2400*12 = bytes in total

15. fwrite ()

Corresponds to fread and writes the specified data to the stream. The prototype is size_t fwrite (const void * PTR, size_t size, size_t N, file * stream ); the PTR parameter is the data pointer to be written. The Void * pointer can be replaced by any type of pointer, such as char * and int *. The size is the number of bytes per block; n is the number of blocks to be written. If the number of actually written blocks is returned (not the number of bytes), this function is generally used in files opened in binary mode.

Example:

Char X [] = "I Love You ";

Fwire (x, 6, 12, FP); // write 6*12 = 72 bytes

Write "I love" to the stream FP 12 times, a total of 72 bytes

16. tmpfile ()

The prototype is file * tmpfile (void). A temporary file is generated, opened in "W + B" mode, and a pointer to the temporary stream is returned. If the stream fails, null is returned. At the end of the program, the file will be automatically deleted.

Example: file * fp = tmpfile ();

17. tmpnam ();

Its prototype is char * tmpnam (char * s). A unique file name is generated. In fact, tmpfile () calls this function. The parameter S is used to save the obtained file name, returns the pointer. If the pointer fails, null is returned.

For example, tmpnam (str1 );

 

2. Direct I/O File Operations

This is another file operation provided by C. It processes the file by directly saving/retrieving the file, and the streaming file operation is carried out through the buffer; streaming file operations are performed around a file pointer, and such file operations are performed around a file's "handle". What is a handle? It is an integer that uniquely identifies a file (in windows, the handle concept is extended to the identifier of all device resources. Common functions for such file operations are listed in the following table. These functions and their symbols are defined in Io. h and fcntl. H, and corresponding header files must be added for use.

Function Description

Open () open a file and return its handle

Close () close a handle

Lseek () locates at the specified position of the file

Read () block Read File

Write () block Write File

Whether the EOF () test file is complete

Filelength () gets the file length

Rename () rename a file

Chsize () changes the file length

The following describes these functions:

1. open ()

Open a file and return its handle. If it fails, a value smaller than 0 will be returned. The prototype is int open (const char * path, int access [, unsigned mode]); the path parameter is the name of the file to be opened, the access mode is open, and the mode is optional. This parameter indicates the properties of a file. It is mainly used in UNIX systems and has no significance in DOS/windows. The file opening mode is shown in the following table.

Symbol Meaning symbol meaning

O_rdonly read-only mode o_wronly write-only mode o_rdwr read/write mode

O_ndelay is used to append o_creat to o_creat in the Unix system. If the file does not exist, it is created.

O_trunc cut the file length to 0 o_excl and o_creat. If the file has an error, the o_binary binary method is returned.

O_text text

You can use the "|" operator to connect to multiple requirements. For example, o_append | o_text indicates opening a file in text mode and append mode.

Example: int handle = open ("C: \ msdos. sys", o_binary | o_creat | o_write)

2. Close ()

Closes a handle. The prototype is int close (INT handle). If the handle is successful, 0 is returned.

Example: Close (handle)

3. lseek ()

Locate to the specified position. The prototype is long lseek (INT handle, long offset, int fromwhere). The offset parameter is the moving amount, and fromwhere is the moving reference position, the value is the same as fseek (). seek_set: First part of the file; seek_cur: Current Position of the file; seek_end: End of the file. This function returns the new file access location after execution.

Example:

Lseek (handle,-1234l, seek_cur); // moves the access location 1234 bytes forward from the current location.

X = lseek (hnd1, 0l, seek_end); // move the access location to the end of the file. x = The End of the file, that is, the length of the file.

4. Read ()

Read a block from the file. The prototype is int read (INT handle, void * Buf, unsigned Len). The parameter Buf stores the read data, and Len is the read byte. The function returns the bytes actually read.

Example: Char X [200]; read (hnd1, X, 200 );

5. Write ()

Write a piece of data to the file. The prototype is int write (INT handle, void * Buf, unsigned Len). The parameter meaning is the same as read (), and the actual written bytes are returned.

Example: Char X [] = "I Love You"; write (handle, X, strlen (x ));

7. EOF ()

Similar to feof (), if the test file ends, 1 is returned; otherwise, 0 is returned; prototype: int EOF (INT handle );

Example: While (! EOF (handle1 )){......};

8. filelength ()

The length of the returned file. The prototype is long filelength (INT handle). It is equivalent to lseek (handle, 0l, seek_end)

Example: long x = filelength (handle );

9. Rename ()

Rename the file. The prototype is int Rename (const char * oldname, const char * newname). The parameter oldname is the old file name and newname is the new file name. 0 is returned successfully.

For example, rename ("C: \ config. sys", "C: \ config. w40 ");

10. chsize ();

Change the file length. The prototype is int chsize (INT handle, Long SIZE). The parameter size indicates the new length of the file. 0 is returned for success; otherwise,-1 is returned, if the specified length is smaller than the file length, the file is truncated. If the specified length is greater than the file length, add '\ 0' after the file '.

Example: chsize (handle, 0x12345 );