Contents
Learning Diary
Preface
1. Basic knowledge about C files 1.
What is a file
2. File name
3. Classification of files 4. File
buffer 5. File
type pointer 2.
Open and close files
1. Open and close files
2. Use the fopen function to open the data file
3. Use the fclose function to close the data file
3. Read and write data files sequentially
1. How to
read and write characters to the file The function of reading and writing a character
Example 1
Example 2
The function of reading and writing a string
2. How to File read and write a string
Example 3
3. Read and write text files in a formatted way
4. Read and write a set of data to a file in binary mode 5.
Read attention 4.
Random read and write data files
1. File location mark and its Positioning
1. File position mark
2. Positioning of file position marks
Example
5. Error detection of file reading and writing
1. ferror function
2. clearerr function
Note
1. Basic knowledge about C files
1. What is a file
There are different types of files. In programming, two types of files are mainly used:
(1) Program files. Including source program files (suffix .c), object files (suffix .obj), executable files (suffix .exe), etc. The content of such a file is program code.
(2) Data files. The content of the file is not a program, but data for reading and writing when the program is running, such as data output to disk (or other external devices) during the running of the program, or data read in during the running of the program. Such as a batch of student achievement data, data on goods transactions, etc.
In order to simplify the user's operation of the input and output devices, so that the user does not have to distinguish between various input and output devices, the operating system treats all devices as files. From the perspective of the operating system, each input and output device connected to the host is regarded as a file. For example, the terminal keyboard is the input file, and the display screen and printer are the output files.
A file generally refers to a collection of data stored on an external medium.
The operating system manages data in units of files. Input and output is the process of data transmission, and data flows from one place to another like flowing water, so the input and output are often called stream (stream), that is, data flow. A flow represents the flow of information from a source to a destination. During input operations, data flows from the file to the computer's memory, and during output operations, data flows from the computer to the file (eg, printer, disk file).
The C language regards a file as a sequence of characters (or bytes), that is, it consists of a character (or byte) sequence of data. An input and output stream is a stream of characters or bytes (the content is binary data). The data file of C is composed of a series of characters (or bytes), regardless of the boundary of the line, the separator will not be automatically added between two lines of data, and the access to the file is in units of characters (bytes). The start and end of the input and output data streams are only controlled by the program and not by physical symbols (such as carriage return and line feed), which increases the flexibility of processing. Such files are called streaming files.
2. File name
A file must have a unique file identifier for user identification and reference.
The file identification includes 3 parts: (1) file path; (2) file name trunk; (3) file suffix.
D:\CC\temp\file1.dat
↓ ↓ ↓
Path trunk name suffix
The file path indicates the location of the file in the external storage device.
The naming rules for filename stems follow the naming rules for identifiers.
The file suffix is used to indicate the nature of the file.
For convenience, the file identifier is often referred to as the file name, but it should be understood that the so-called file name at this time actually includes the above three parts, not just the main file name.
3. Classification of files
According to the organizational form of data, data files can be divided into ASCII files and binary files. Data is stored in binary form in the memory. If it is output to the external memory without conversion, it is a binary file. It can be considered as an image of the data stored in the memory, so it is also called an image file. If it is required to store in the form of ASCII code on the external storage, it needs to be converted before storage. ASCII files are also called text files, and each byte stores the ASCII code of a character.
4. File buffer
The ANSI C standard uses a "buffer file system" to process data files. The so-called buffer file system means that the system automatically creates a file buffer in the memory area for each file being used in the program. Data output from memory to disk must be sent to the buffer in memory first, and then sent to disk together after the buffer is filled. If data is read from the disk to the computer, a batch of data is input from the disk file to the memory buffer (full buffer), and then the data is sent from the buffer to the program data area one by one (to the program variable). This is done to save access time and improve efficiency. The size of the buffer is determined by each specific C compilation system.
Explanation: Each file has only one buffer in memory. When outputting data to the file, it is used as the output buffer, and when inputting data from the file, it is used as the input buffer.
5. File type pointer
In the cache file system, the key concept is "file type pointer", referred to as "file pointer". Each used file opens up a corresponding file information area in the memory, which is used to store the relevant information of the file (such as the name of the file, the status of the file and the current location of the file, etc.). This information is stored in a structure variable. The structure type is declared by the system and named FILE.
typedef struct
{
short level; //缓冲区“满”或“空”的程度
unsigned flags; //文件状态标志
char fd; //文件描述符
unsigned char hold; //如缓冲区无内容不读取字符
short bsize; //缓冲区的大小
unsigned char*buffer; //数据缓冲区的位置
unsigned char*curp; //文件位置标记指针当前的指向
unsigned istemp; //临时文件指示器
short token; //用于有效性检查
}FILE;
FILE *fp;
//定义一个指向FILE类型数据的指针变量
**You can make fp point to the file information area of a certain file (it is a structure variable), and the file can be accessed through the information in the file information area. That is to say, the file associated with it can be found through the file pointer variable. If there are n files, n pointer variables should be set to point to n FILE type variables to realize access to n files. For convenience, this kind of pointer variable pointing to the file information area is usually referred to as a pointer variable pointing to the file. You can make fp point to the file information area of a certain file (a structure variable), through this file **#
2. Open and close files
1. Open and close files
**对文件读写之前应该“打开”该文件，在使用结束之后应“关闭”该文件。 所谓“打开”是指为文件建立相应的信息区(用来存放有关文件的信息)和文件缓冲区(用来暂时存放输入输出的数据)。 在编写程序时，在打开文件的同时，一般都指定一个指针变量指向该文件，也就是建立起指针变量与文件之间的联系，这样，就可以通过该指针变量对文件进行读写了。 所谓“关闭”是指撤销文件信息区和文件缓冲区，使文件指针变量不再指向该文件，显然就无法进行对文件的读写了。**
2. Open the data file with the fopen function
FILE *fp; //定义一个指向文件的指针变量fp
fp=fopen(″a1″,″r″); //将fopen函数的返回值赋给指针变量fp
When opening a file, notify the compilation system of the following three information:
① The name of the file to be opened, that is, the name of the file to be accessed
② The way to use the file ("read" or "write", etc.)
③ Which pointer variable to use Points to the file to be opened
using the file mode
file usage mode
meaning
if the specified file does not exist
"r" (read-only)
for input data, open an existing text file
error
"w" (write-only)
for output data, open an TEXT FILE
Create a new file
"a" (append)
append data to the end of the text file
error
"rb" (read-only)
open a binary file for input data
error
"wb" (write-only)
open a binary file for output data
create new file
"ab" (append)
appends data to the end of the binary file
error
"r+" (read-write)
opens a text file for reading and writing
error
"w+" (read-write)
creates a new text file for reading and writing
create new file
"a+" (read-write)
for reading and writing, open a text file
error
"rb+" (read-write)
Open a binary file for reading and writing
Error
"wb+" (read-write)
Create a new binary file for reading and writing
Create new file
"ab+" (read-write)
Open a binary file for reading and writing
Error
(1) with The file opened in the "r" mode can only be used for input to the computer but not for output data to the file, and the file should already exist and contain data, so that the program can read data from the file. Cannot open a non-existent file in "r" mode, otherwise an error occurs.
(2) The file opened with "w" can only be used to write data to the file (that is, the output file), but not to input to the computer. If the file does not exist, create a new file with the specified name before opening the file. If there is already a file named with this file name, delete the file before opening the file, and then create a new file.
(3) If you want to add new data to the end of the file (do not want to delete the original data), you should open it with "a". But at this time, you should ensure that the file already exists; otherwise, you will get an error message. An implicit "file reading and writing position mark" is automatically set in each data file, and the position it points to is the current reading and writing position. If the "file read and write position mark" is at the beginning of the file, the next read and write is the data at the beginning of the file. Then the "file reading and writing position mark" automatically moves to the next reading and writing position, so as to read and write the next data. When a file is opened for append, the file read/write mark is moved to the end of the file. (4) Files opened with "r+", "w+" and "a+" can be used for both input and output data.
(5) If the task of "opening" cannot be realized, the fopen function will return a null pointer value NULL.
(6) The C standard recommends using the file usage methods listed in Table 10.1 to open text files or binary files, but some C compilation systems currently in use may not fully provide all these functions, and you need to pay attention to the regulations of the system used.
(7) There are 12 file usage methods, 6 of which add the letter b after the first letter (such as rb, wb, ab, rb+, wb+, ab+), b means binary mode. In fact, there is only one difference between with b and without b, that is, the handling of newlines. Since the newline can be realized with a '\n' in the C language, in order to realize the newline in the Windows system, two characters of "carriage return" and "newline" must be used, namely '\r' and '\n'. Therefore, if you use a text file and open it with "w", when outputting to the file, when a newline character '\n' is encountered, the system will convert it into two characters '\r' and '\n' , otherwise, when viewing the file in the Windows system, each line will be connected into one piece and cannot be read. Similarly, if there is a text file and it is opened with "r", when reading from the file, if two consecutive characters '\r' and '\n' are encountered, convert them into one character '\n'. If you are using binary files, no such conversion is required when reading and writing to the file. Adding b means that the binary file is used, and the system will not convert it.
(8) If the "wb" file usage method is used, it does not mean that the data stored in the memory in ASCII format will be automatically converted into binary format when the file is output. The output data format is determined by the read and write statements used in the program. For example, the fscanf and fprintf functions are used for input and output in ASCII mode, while the fread and fwrite functions are used for binary input and output.
(9) Three standard stream files can be used in the program - standard input stream, standard output stream and standard error output stream. The system has specified the corresponding relationship with the terminal for these 3 files. The standard input stream is the input from the terminal, the standard output stream is the output to the terminal, and the standard error output stream is the error message sent to the terminal when the program fails. The system automatically opens these 3 standard stream files when the program starts running.
3. Close the data file with the fclose function
fclose(文件指针)；
fclose(fp);
A file should be closed after use to prevent it from being misused. "Close" is to revoke the file information area and file buffer, so that the file pointer variable no longer points to the file, that is, the file pointer variable is "decoupled" from the file, and the original associated file cannot be read or written through the pointer. Operation, unless opened again, make the pointer variable point to the file again. If you end the program without closing the file, you will lose data. Because, when writing data to a file, the data is first output to the buffer, and it is not officially output to the file until the buffer is full. If the program ends when the buffer is not filled with data, the data in the buffer may be lost.
When using the fclose function to close a file, first output the data in the buffer to the disk file, and then cancel the file information area. Some compilation systems will automatically write the data in the buffer to the file before the program ends, thus avoiding this problem, but you should still develop the habit of closing all files before the program terminates. The fclose function also returns a value. When the close operation is successfully performed, the return value is 0; otherwise, EOF (-1) is returned.
3. Sequential reading and writing of data files
1. How to read and write characters to a file
The function of reading and writing a character. Function
name call form function return value
fgetc fgetc(fp) Read a character from the file pointed to by fp. If the read is successful, the read character will be returned. If it fails, the end-of-file flag EOF (ie -1) will be returned.
fputc fputc(ch,fp) writes the character ch to the file pointed to by the file pointer variable fp. The output is successful, and the return value is the output character; if the output fails, it returns EOF (ie -1). The first letter f of fgetc represents the
file (file), the middle get means "get", the last letter c means character (character), the meaning of fgetc is very clear: read a character from the file. fputc is also similar. Function name
for reading and writing a character string Function
name
Calling form Function Return value fgets fgets(str,n,fp) reads a character string of length (n-1) from the file pointed to by fp, stores it in the character array str, and reads successfully , return the address str, and return
NULL if it fails
s represents a character string (string). The meaning of fgets is: read a string from a file
2. How to read and write a string to a file
The function prototype of the **fgets function is char *fgets(char str, int n, FILE fp);
its function is to read a string from the file. Calling can be written in the following form:
fgets(str,n,fp);
Among them, n is the number of characters required, but actually only n-1 characters are read from the file pointed to by fp, and then a '\0' character is added at the end, so that the resulting string has a total of n characters , put them into the character array str. If a newline character "\n" or the end-of-file character EOF is encountered before reading n-1 characters, the reading ends, but the encountered newline character "\n" is also read as a character. If the execution of the fgets function is successful, the return value is the address of the first element of the str array, and if the end of the file or an error in reading data is encountered at the beginning, NULL is returned
3. Read and write text files in a formatted way
You can format the input and output of the file. At this time, you need to use the fprintf function and the fscanf function. As you can see from the function name, they just add a letter f in front of printf and scanf. Their functions are similar to the printf function and scanf function, and they are both formatted read and write functions. There is only one difference: the read and write objects of fprintf and fscanf functions are not terminals but files. They are generally called as:
fprintf (fp,″%d,%6.2f″,i,f); //将int型变量i和float型变量f的值按%d和%6.2f的格式输出到fp指向的文件中
fscanf (fp,″%d,%f″,&i,&f);
//磁盘文件上如果有字符“3,4.5”，则从中读取整数3送给整型变量i，读取实数4.5送给float型变量f
4. Read and write a set of data to the file in binary mode
The C language allows reading a block of data from a file with the fread function, and writing a block of data to the file with the fwrite function. When reading and writing, it is performed in binary form. When writing data to the disk, a group of data in the memory is directly copied to the disk file without conversion, and when reading, the contents of several bytes in the disk file are also read into the memory in batches.
fread(buffer, size, count, fp);
fwrite(buffer, size, count, fp);
buffer: is an address. For fread, it is the address of the memory area used to store the data read from the file. For fwrite, it is to output the data in the storage area starting from this address to the file (the above refers to the starting address).
size: The number of bytes to read and write.
count: How many data items to read and write (the length of each data item is size).
fp: FILE type pointer.
5. Read attention
(1) Data storage method Text method: Data is stored in the file in character mode (ASCII code). For example, the integer 12 occupies 2 bytes when sent to the file instead of 4 bytes. Data saved in text format is easy to read. Binary mode: The data is copied to the file as it is stored in memory. For example, the integer 12 occupies 4 bytes when sent to the file and in memory.
(2) Classification of files Text files (ASCII files): All of the files are ASCII characters. Binary file: The data in the memory is copied to the file in a binary manner, which is called a binary file, that is, an image file.
(3) Opening method of the file Text method: the method without b, the newline character is converted when reading and writing the file. Binary mode: In the mode with b, newline characters are not converted when reading and writing files.
(4) File reading and writing functions Text reading and writing functions: functions used to read and write character data to text files (such as fgetc, fgets, fputc, fputs, fscanf, fprintf, etc.). Binary read and write functions: functions used to read and write binary data to binary files (such as getw, putw, fread, fwrite, etc.).
4. Randomly read and write data files
Sequential reading and writing of files is relatively easy to understand and easy to operate, but sometimes it is not efficient. Random access does not read and write according to the order of physical positions of data in the file, but can access data at any position. Obviously, this method is much more efficient than sequential access.
1. File location mark and its positioning
1. File location marker
In order to control reading and writing, the system sets a file reading and writing position mark (referred to as file position mark or file mark) for each file, which is used to indicate "the position of the next character to be read and written next". Under normal circumstances, when reading and writing character files sequentially, the file position mark points to the beginning of the file. At this time, if the file is read/written, after the first character is read/written, the order of the file position mark moves backward. A position, when the next read/write operation is performed, the second character pointed to by the position mark is read or written. And so on, until the end of the file is encountered, at this time the file position mark is after the last data. Streaming files can be read and written sequentially or randomly. The key is to control the position mark of the file. If the file position mark is moved in byte position order, it is sequential read and write. If the file position mark can be moved to any position as needed, random read and write can be realized. The so-called random reading and writing means that after reading and writing the previous character (byte), it is not necessary to read and write the subsequent character (byte), but can read and write the required character (byte) at any position in the file. ). That is, the order of reading and writing data to a file is generally inconsistent with the physical order of the data in the file. Data can be written anywhere, and data can be read anywhere.
2. Positioning of the file position mark
Use the rewind function to make the file position mark point to the beginning of the file. The function of the rewind function is to make the file position mark return to the beginning of the file. This function has no return value.
Use the fseek function to change the file position mark, fseek(file type pointer, displacement, starting point); the fseek function is generally used for binary files.
fseek (fp,100L,0); //将文件位置标记向前移到离文件开头100个字节处
fseek (fp,50L,1); //将文件位置标记向前移到离当前位置50个字节处
fseek (fp,-10L,2); //将文件位置标记从文件末尾处向后退10个字节
Use the ftell function to measure the current position of the file position mark: the function of the ftell function is to obtain the current position of the file position mark in the streaming file, which is represented by the displacement relative to the beginning of the file. If an error occurs when calling the function (for example, the file pointed to by fp does not exist), the return value of the ftell function is -1L.
5. Error detection of file reading and writing
1. ferror function
ferror(fp);
When calling various input and output functions (such as putc, getc, fread, fwrite, etc.), if an error occurs, in addition to the reflection of the function return value, you can also use the ferror function to check. If ferror returns a value of 0 (false), it means that no error occurred; if it returns a non-zero value, it means that an error occurred.
2. clearerr function
The role of clearerr is to set the file error flag and the end of file flag to 0. Assuming an error occurs while calling an input-output function, the ferror function evaluates to a non-zero value. The clearerr(fp) should be called immediately to make the value of ferror(fp) become 0 for the next detection. As long as the file read and write error flag appears, it remains until the clearerr function or rewind function is called on the same file, or any other input and output function.
Each call to the input/output function for the same file will generate a new value of the ferror function, so the value of the ferror function should be checked immediately after calling an input/output function, otherwise the information will be lost. When the fopen function is executed, the initial value of the ferror function is automatically set to 0.