Cprogramming 简明教程

File Handling in C

File handling in C 是以使用C语言函数创建、打开、写入数据、读取数据、重命名和删除等文件操作处理的过程。借助这些函数，我们可以执行文件操作，以将数据存储和检索到/从程序中的文件。

File handling in C is the process of handling file operations such as creating, opening, writing data, reading data, renaming, and deleting using the C language functions. With the help of these functions, we can perform file operations to store and retrieve the data in/from the file in our program.

Need of File Handling in C

如果我们使用C程序执行输入和输出操作，数据将存在于程序运行期间，当程序终止时，我们将无法再次使用该数据。 File handling 需要处理存储在外部内存中的文件，即存储和访问计算机外部内存中的信息。你可以使用文件处理永久保存数据。

If we perform input and output operations using the C program, the data exists as long as the program is running, when the program is terminated, we cannot use that data again. File handling is required to work with files stored in the external memory i.e., to store and access the information to/from the computer’s external memory. You can keep the data permanently using file handling.

Types of Files

文件表示一系列字节。有两种类型的文件： text files 和 binary files -

A file represents a sequence of bytes. There are two types of files: text files and binary files −

FILE Pointer (File*)

在处理 file handling 时，需要一个 file pointer 来存储fopen()函数返回的 FILE 结构的引用。文件指针是所有文件处理操作所必需的。

While working with file handling, you need a file pointer to store the reference of the FILE structure returned by the fopen() function. The file pointer is required for all file-handling operations.

fopen() function 返回 FILE 类型的指针。 FILE 是 stdio.h 中预定义的结构类型，包含文件描述符、大小和位置等属性。

The fopen() function returns a pointer of the FILE type. FILE is a predefined struct type in stdio.h and contains attributes such as the file descriptor, size, and position, etc.

typedef struct {
   int fd;			/* File descriptor */
   unsigned char *buf;	/* Buffer */
   size_t size;		/* Size of the file */
   size_t pos;		/* Current position in the file */
} FILE;

Declaring a File Pointer (FILE*)

以下是声明文件指针的语法 −

Below is the syntax to declare a file pointer −

FILE* file_pointer;

Opening (Creating) a File

必须打开一个文件才能执行任何操作。 fopen() 函数用于创建新文件或打开现有文件。您需要指定您想要打开文件的模式。下面说明了各种文件打开模式，其中任何一个模式都可以在创建/打开文件期间使用。

A file must be opened to perform any operation. The fopen() function is used to create a new file or open an existing file. You need to specify the mode in which you want to open. There are various file opening modes explained below, any one of them can be used during creating/opening a file.

fopen() 函数返回 FILE 指针，该指针将用于其他操作，如读写和关闭文件。

The fopen() function returns a FILE pointer which will be used for other operations such as reading, writing, and closing the files.

Syntax

以下是打开文件的语法 −

Below is the syntax to open a file −

FILE *fopen(const char *filename, const char *mode);

此处， filename 是要打开的文件的名称， mode 定义文件的打开模式。

Here, filename is the name of the file to be opened, and mode defines the file’s opening mode.

File Opening Modes

文件访问模式默认情况下以文本或 ASCII 模式打开文件。如果您要处理二进制文件，则您将使用以下访问模式替代上述访问模式：

The file access modes by default open the file in the text or ASCII mode. If you are going to handle binary files, then you will use the following access modes instead of the above-mentioned ones:

"rb", "wb", "ab", "rb+", "r+b", "wb+", "w+b", "ab+", "a+b"

可以以各种模式打开文件。以下是不同的文件打开模式 −

There are various modes in which a file can be opened. The following are the different file opening modes −

Mode

Description

Opens an existing text file for reading purposes.

Opens a text file for writing. If it does not exist, then a new file is created. Here your program will start writing content from the beginning of the file.

Opens a text file for writing in appending mode. If it does not exist, then a new file is created. Here your program will start appending content in the existing file content.

Opens a text file for both reading and writing.

Opens a text file for both reading and writing. It first truncates the file to zero length if it exists, otherwise creates a file if it does not exist.

Opens a text file for both reading and writing. It creates the file if it does not exist. The reading will start from the beginning but writing can only be appended.

Example of Creating a File

在以下示例中，我们正在创建一个新文件。创建新文件的文件模式将是 " w "（写模式）。

In the following example, we are creating a new file. The file mode to create a new file will be "w" (write-mode).

#include <stdio.h>

int main() {
  FILE * file;

  // Creating a file
  file = fopen("file1.txt", "w");

  // Checking whether file is
  // created or not
  if (file == NULL) {
    printf("Error in creating file");
    return 1;
  }
  printf("File created.");

  return 0;
}

File created.

Example of Opening a File

在以下示例中，我们正在打开一个现有文件。打开现有文件的文件模式将是 " r "（只读）。您还可以使用上面说明的其他文件打开模式选项。

In the following example, we are opening an existing file. The file mode to open an existing file will be "r" (read-only). You may also use other file opening mode options explained above.

Note: 必须有一个文件要打开。

Note: There must be a file to be opened.

#include <stdio.h>

int main() {
  FILE * file;

  // Opening a file
  file = fopen("file1.txt", "r");

  // Checking whether file is
  // opened or not
  if (file == NULL) {
    printf("Error in opening file");
    return 1;
  }
  printf("File opened.");

  return 0;
}

File opened.

Closing a File

在对文件执行操作后必须关闭每个文件。 fclose() function 关闭打开的文件。

Each file must be closed after performing operations on it. The fclose() function closes an opened file.

Syntax

以下是 fclose() 函数的语法 −

Below is the syntax of fclose() function −

int fclose(FILE *fp);

fclose() 函数在成功时返回零，如果关闭文件时出错则返回 EOF。

The fclose() function returns zero on success, or EOF if there is an error in closing the file.

fclose() 函数实际上将缓冲区中仍然挂起的任何数据刷新到文件，关闭文件，并释放用于该文件的任何内存。EOF 是在头文件 stdio.h. 中定义的常量

The fclose() function actually flushes any data still pending in the buffer to the file, closes the file, and releases any memory used for the file. The EOF is a constant defined in the header file stdio.h.

Example of Closing a File

在下面的示例中，我们将关闭一个已打开的文件 -

In the following example, we are closing an opened file −

#include <stdio.h>

int main() {
  FILE * file;

  // Opening a file
  file = fopen("file1.txt", "w");

  // Checking whether file is
  // opened or not
  if (file == NULL) {
    printf("Error in opening file");
    return 1;
  }
  printf("File opened.");

  // Closing the file
  fclose(file);
  printf("\nFile closed.");

  return 0;
}

File opened.
File closed.

Writing to a Text File

提供了以下库函数来写入以可写模式打开的文件中的数据 -

The following library functions are provided to write data in a file opened in writeable mode −

fputc() : Writes a single character to a file.
fputs(): Writes a string to a file.
fprintf(): Writes a formatted string (data) to a file.

Writing Single Character to a File

fputc() function 是一个未格式化的函数，它将参数 "c" 的单个字符值写入由 " fp " 引用的输出流中。

The fputc() function is an unformatted function that writes a single character value of the argument "c" to the output stream referenced by "fp".

int fputc(int c, FILE *fp);

在以下代码中，将给定字符数组中的一个字符写入以 " w " 模式打开的文件中：

In the following code, one character from a given char array is written into a file opened in the "w" mode:

#include <stdio.h>

int main() {

   FILE *fp;
   char * string = "C Programming tutorial from TutorialsPoint";
   int i;
   char ch;

   fp = fopen("file1.txt", "w");

   for (i = 0; i < strlen(string); i++) {
      ch = string[i];
      if (ch == EOF)
         break;
      fputc(ch, fp);
   }
   printf ("\n");
   fclose(fp);

   return 0;
}

Output

After executing the program, "file1.txt" will be created in the current folder and the string is written to it.

Writing String to a File

fputs() function 将字符串 "s" 写入由 "fp" 引用的输出流中。成功时它返回一个非负值，否则在发生任何错误的情况下返回 EOF。

The fputs() function writes the string "s" to the output stream referenced by "fp". It returns a non-negative value on success, else EOF is returned in case of any error.

int fputs(const char *s, FILE *fp);

以下程序将给定二维字符数组中的字符串写入文件中 -

The following program writes strings from the given two-dimensional char array to a file −

#include <stdio.h>

int main() {

   FILE *fp;
   char *sub[] = {"C Programming Tutorial\n", "C++ Tutorial\n", "Python Tutorial\n", "Java Tutorial\n"};
   fp = fopen("file2.txt", "w");

   for (int i = 0; i < 4; i++) {
      fputs(sub[i], fp);
   }

   fclose(fp);

   return 0;
}

Output

当程序运行时，将在当前文件夹中创建一个名为 "file2.txt" 的文件，并保存以下行 -

When the program is run, a file named "file2.txt" is created in the current folder and save the following lines −

C Programming Tutorial
C++ Tutorial
Python Tutorial
Java Tutorial

Writing Formatted String to a File

fprintf() function 将格式化的数据流发送到由 FILE 指针表示的磁盘文件。

The fprintf() function sends a formatted stream of data to the disk file represented by the FILE pointer.

int fprintf(FILE *stream, const char *format [, argument, ...])

在以下程序中，我们有一个名为 " employee " 的结构类型数组。该结构有一个字符串、一个整数和一个浮点元素。使用 fprintf() 函数，将数据写入文件。

In the following program, we have an array of struct type called "employee". The structure has a string, an integer, and a float element. Using the fprintf() function, the data is written to a file.

#include <stdio.h>

struct employee {
   int age;
   float percent;
   char *name;
};

int main() {

   FILE *fp;
   struct employee emp[] = {
      {25, 65.5, "Ravi"},
      {21, 75.5, "Roshan"},
      {24, 60.5, "Reena"}
   };

   char *string;
   fp = fopen("file3.txt", "w");

   for (int i = 0; i < 3; i++) {
      fprintf(fp, "%d %f %s\n", emp[i].age, emp[i].percent, emp[i].name);
   }
   fclose(fp);

   return 0;
}

Output

When the above program is executed, a text file is created with the name "file3.txt" that stores the employee data from the struct array.

Reading from a Text File

提供了以下库函数来从以读取模式打开的文件中读取数据 -

The following library functions are provided to read data from a file that is opened in read mode −

fgetc(): Reads a single character from a file.
fgets(): Reads a string from a file.
fscanf(): Reads a formatted string from a file.

Reading Single Character from a File

fgetc() function 从由 " fp " 引用的输入文件中读取一个字符。返回值是读取的字符，或者在发生任何错误的情况下，它返回 EOF。

The fgetc() function reads a character from the input file referenced by "fp". The return value is the character read, or in case of any error, it returns EOF.

int fgetc(FILE * fp);

以下示例以逐个字符的方式读取给定文件，直到文件末尾。

The following example reads the given file in a character by character manner till it reaches the end of file.

#include <stdio.h>

int main(){

   FILE *fp ;
   char ch ;
   fp = fopen ("file1.txt", "r");

   while(1) {
      ch = fgetc (fp);
      if (ch == EOF)
         break;
      printf ("%c", ch);
   }
   printf ("\n");
   fclose (fp);
}

Output

运行代码并检查其输出：

Run the code and check its output −

C Programming tutorial from TutorialsPoint

Reading String from a File

fgets() function 从由 “fp” 引用的输入流中最多读取 “n - 1” 个字符。它将读取的字符串复制到缓冲区 “ buf ”，附加一个空字符以终止字符串。

The fgets() function reads up to "n − 1" characters from the input stream referenced by "fp". It copies the read string into the buffer "buf", appending a null character to terminate the string.

下面这个程序读取给定文件中的每一行，直到检测到文件末尾−

This following program reads each line in the given file till the end of the file is detected −

# include <stdio.h>

int main() {

   FILE *fp;
   char *string;
   fp = fopen ("file2.txt", "r");

   while (!feof(fp)) {
      fgets(string, 256, fp);
      printf ("%s", string) ;
   }
   fclose (fp);
}

Output

运行代码并检查其输出：

Run the code and check its output −

C Programming Tutorial
C++ Tutorial
Python Tutorial
Java Tutorial

Reading Formatted String from a File

C 编程语言中的 fscanf() function 用于从文件中读取格式化输入。

The fscanf() function in C programming language is used to read formatted input from a file.

int fscanf(FILE *stream, const char *format, ...)

在下面的程序中，我们利用 fscanf() 函数以不同类型的变量读取格式化数据。通常使用格式说明符 (%d, %f, %s 等) 来指示字段类型。

In the following program, we use the fscanf() function to read the formatted data in different types of variables. Usual format specifiers are used to indicate the field types (%d, %f, %s, etc.)

#include <stdio.h>

int main() {

   FILE *fp;
   char *s;
   int i, a;
   float p;

   fp = fopen ("file3.txt", "r");

   if (fp == NULL) {
      puts ("Cannot open file"); return 0;
   }

   while (fscanf(fp, "%d %f %s", &a, &p, s) != EOF)
   printf ("Name: %s Age: %d Percent: %f\n", s, a, p);
   fclose(fp);

   return 0;
}

Output

执行以上程序时，它将打开文本文件 “file3.txt”，并在屏幕上打印其内容。运行代码后，你将得到类似以下的输出−

When the above program is executed, it opens the text file "file3.txt" and prints its contents on the screen. After running the code, you will get an output like this −

Name: Ravi Age: 25 Percent: 65.500000
Name: Roshan Age: 21 Percent: 75.500000
Name: Reena Age: 24 Percent: 60.500000

File Handing Binary Read and Write Functions

在二进制文件的情况下，读/写操作以二进制形式完成。你需要在访问模式中包含字符 “ b ” (用于写入二进制文件的 “ wb ”，用于读取二进制文件的 “ rb ”)。

The read/write operations are done in a binary form in the case of a binary file. You need to include the character "b" in the access mode ("wb" for writing a binary file, "rb" for reading a binary file).

有两个函数可用于二进制输入和输出： fread() 函数和 fwrite() 函数。这两个函数都应用于读取或写入内存块，通常是数组或结构。

There are two functions that can be used for binary input and output: the fread() function and the fwrite() function. Both of these functions should be used to read or write blocks of memories, usually arrays or structures.

Writing to Binary File

fwrite() function 将一个指定字节块从一个缓冲区写入到以二进制写入模式打开的文件中。下面是使用此函数的原型：

The fwrite() function writes a specified chunk of bytes from a buffer to a file opened in binary write mode. Here is the prototype to use this function:

fwrite(*buffer, size, no, FILE);

在下面的程序中，声明了一个名为 “ employee ” 的结构类型数组。我们使用 fwrite() 函数以一个 employee 数据大小的一个字节块写入以 “ wb ” 模式打开的文件中。

In the following program, an array of a struct type called "employee" has been declared. We use the fwrite() function to write one block of byte, equivalent to the size of one employee data, in a file that is opened in "wb" mode.

#include <stdio.h>

struct employee {
   int age;
   float percent;
   char name[10];
};

int main() {

   FILE *fp;
   struct employee e[] = {
      {25, 65.5, "Ravi"},
      {21, 75.5, "Roshan"},
      {24, 60.5, "Reena"}
   };

   char *string;

   fp = fopen("file4.dat", "wb");
   for (int i = 0; i < 3; i++) {
      fwrite(&e[i], sizeof (struct employee), 1, fp);
   }

   fclose(fp);

   return 0;
}

Output

When the above program is run, the given file will be created in the current folder. It will not show the actual data, because the file is in binary mode.

Reading from Binary File

fread() function 将一个指定字节块从以二进制读取模式打开的文件中读取到一个指定大小的缓冲区中。下面是使用此函数的原型：

The fread() function reads a specified chunk of bytes from a file opened in binary read mode to a buffer of the specified size. Here is the prototype to use this function:

fread(*buffer, size, no, FILE);

在下面的程序中，声明了一个名为 “ employee ” 的结构类型数组。我们使用 fread() 函数以一个 employee 数据大小的一个字节块读取以 “ rb ” 模式打开的文件中。

In the following program, an array of a struct type called "employee" has been declared. We use the fread() function to read one block of byte, equivalent to the size of one employee data, in a file that is opened in "rb" mode.

#include <stdio.h>

struct employee {
   int age;
   float percent;
   char name[10];
};

int main() {

   FILE *fp;
   struct employee e;

   fp = fopen ("file4.dat", "rb");

   if (fp == NULL) {
      puts ("Cannot open file");
      return 0;
   }

   while (fread (&e, sizeof (struct employee), 1, fp) == 1)
   printf ("Name: %s Age: %d Percent: %f\n", e.name, e.age, e.percent);

   fclose(fp);

   return 0;
}

Output

执行以上程序时，它将打开文件 “file4.dat”，并在屏幕上打印其内容。运行代码后，你将得到类似以下的输出−

When the above program is executed, it opens the file "file4.dat" and prints its contents on the screen. After running the code, you will get an output like this −

Name: Ravi Age: 25 Percent: 65.500000
Name: Roshan Age: 21 Percent: 75.500000
Name: Reena Age: 24 Percent: 60.500000

Renaming a File

rename() function 用于将一个现有文件从一个旧文件名重命名为一个新文件名。

The rename() function is used to rename an existing file from an old file name to a new file name.

Syntax

以下是重命名文件的语法−

Below is the syntax to rename a file −

int rename(const char *old_filename, const char *new_filename)

Example

#include <stdio.h>

int main() {
  // old and new file names
  char * file_name1 = "file1.txt";
  char * file_name2 = "file2.txt";

  // Renaming old file name to new one
  if (rename(file_name1, file_name2) == 0) {
    printf("File renamed successfully.\n");
  } else {
    perror("There is an error.");
  }

  return 0;
}

如果存在一个文件 (file1.txt)，则输出如下−

If there is a file (file1.txt) available, the following will be the output −

Name: Ravi Age: 25 Percent: 65.500000
Name: Roshan Age: 21 Percent: 75.500000
Name: Reena Age: 24 Percent: 60.500000