Cprogramming 简明教程

Error Handling in C

因此，C 编程并不直接支持错误处理，因为在 C 中没有可以防止错误或异常突然终止程序的关键字。但是，程序员可以使用其他函数进行错误处理。

As such, C programming does not provide direct support for error handling, as there are no keywords in C that can prevent errors or exceptions from abruptly terminating the program. However, a programmer can use other functions for error handling.

您可以使用 errno 在 C 中高效地进行错误处理。此外，其他可用于错误处理的函数包括 perror, strerror, ferror, 和 clearererr 。

You can use errno for efficient error handling in C. Additionally other functions that can be used for error handling include perror, strerror, ferror, and clearererr.

The errno Variable

C 是一种系统编程语言。它以返回值的形式提供较底层的访问。如果发生任何错误，大多数 C 或甚至 Unix 函数调用将返回 -1 或 NULL，并设置错误代码 errno。它被设置为一个全局变量，并表示在任何函数调用期间发生的错误。您可以在 <error.h> 头文件中找到各种定义的错误代码。

C is a system programming language. It provides you access at a lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and set an error code errno. It is set as a global variable and indicates an error occurred during any function call. You can find various error codes defined in <error.h> header file.

因此，C 程序员可以检查返回的值，并可以根据返回的值采取适当的措施。这是一个好习惯，在初始化程序时将 errno 设置为 0。0 的值表示程序中没有错误。

So, a C programmer can check the returned values and can take appropriate action depending on the return value. It is a good practice, to set errno to 0 at the time of initializing a program. A value of 0 indicates that there is no error in the program.

下表显示了 errno 的值和与之关联的错误消息−

The following table shows the errno values and error messages associated with them −

errno value

Error

1

Operation not permitted

2

No such file or directory

3

No such process

4

Interrupted system call

5

I/O error

6

No such device or address

7

The argument list is too long

8

Exec format error

9

Bad file number

10

No child processes

11

Try again

12

Out of memory

13

Permission denied

Example

请看以下示例：

Take a look at the following example −

#include <stdio.h>
#include <errno.h>

int main() {

   FILE* fp;

   // opening a file which does not exist
   fp = fopen("nosuchfile.txt", "r");
   printf("Value of errno: %d\n", errno);

   return 0;
}

它将生成如下输出：

It will produce the following output −

Value of errno: 2

C 编程语言提供了 perror() 和 strerror() 函数，可用于显示与 errno 关联的文本消息。

The C programming language provides perror() and strerror() functions which can be used to display the text message associated with errno.

The perror() Function

显示传递给它的字符串，后跟冒号、空格，然后是当前 errno 值的文本表示形式。

displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value.

void perror(const char *str);

Example

在上面的示例中，“errno = 2”与消息 No such file or directory, 关联，可以使用 perror() 函数打印该消息

In the above example, the "errno = 2" is associated with the message No such file or directory, which can be printed with perror() function

#include <stdio.h>
#include <errno.h>

int main(){

   FILE* fp;

   // opening a file which does not exist
   fp = fopen("nosuchfile.txt", "r");

   printf("Value of errno: %d\n", errno);
   perror("Error message:");

   return 0;
}

当你运行这段代码时，它将产生以下输出：

When you run this code, it will produce the following output −

Value of errno: 2
Error message: No such file or directory

The strerror() Function

这将返回一个指向当前 errno 值的文本表示形式的指针。

This returns a pointer to the textual representation of the current errno value.

char *strerror(int errnum);

让我们使用此函数显示 errno=2 的文本表示形式

Let us use this function to display the textual representation of errno=2

Example

请看以下示例：

Take a look at the following example −

#include <stdio.h>
#include <errno.h>

int main() {

   FILE* fp;

   // opening a file which does not exist
   fp = fopen("nosuchfile.txt", "r");

   printf("Value of errno: %d\n", errno);
   printf("The error message is : %s\n", strerror(errno));

   return 0;
}
Value of errno: 2
he error message is : No such file or directory

The ferror() Function

此函数用于检查文件操作期间是否发生了错误。

This function is used to check whether an error occurred during a file operation.

int ferror(FILE *stream);

Example

在这里，我们尝试从以“w”模式打开的文件中读取。ferror() 函数用于打印错误消息

Here, we try to read from a file opened in ‘w’ mode. The ferror() function is used to print the error message

#include <stdio.h>

int main(){

   FILE *fp;
   fp = fopen("test.txt","w");
   char ch = fgetc(fp);  // Trying to read data, from writable file
   if(ferror(fp)){
      printf("File is opened in writing mode! You cannot read data from it!");
   }
   fclose(fp);

   return(0);
}

运行代码并检查其输出：

Run the code and check its output −

File is opened in writing mode! You cannot read data from it!

The clearerr() Function

clearerr() 函数用于清除文件流的末尾和错误指示符。

The clearerr() function is used to clear both end-of-file and error indicators for a file stream.

void clearerr(FILE *stream);

Example

请看以下示例：

Take a look at the following example −

#include <stdio.h>

int main(){

   FILE *fp;
   fp = fopen("test.txt","w");

   char ch = fgetc(fp);  // Trying to read data, from writable file

   if(ferror(fp)){
      printf("File is opened in writing mode! You cannot read data from it!\n");
   }

   // Clears error-indicators from the file stream
   // Subsequent ferror() doesn't show error
   clearerr(fp);

   if(ferror(fp)){
      printf("Error again in reading from file!");
   }
   fclose(fp);

   return(0);
}

Divide by Zero Errors

当对任何数字进行除法时，一个常见的问题是，程序员不会检查除数是否为 0，最终导致运行时错误。

It is a common problem that at the time of dividing any number, programmers do not check if a divisor is zero and finally it creates a runtime error.

Example 1

下面的代码通过在除法之前检查除数是否为 0 来修复此错误 -

The following code fixes this error by checking if the divisor is zero before dividing −

#include <stdio.h>
#include <stdlib.h>

int main() {
   int dividend = 20;
   int divisor = 0;
   int quotient;

   if( divisor == 0){
      fprintf(stderr, "Division by zero! Exiting...\n");
      exit(-1);
   }
   quotient = dividend / divisor;
   fprintf(stderr, "Value of quotient : %d\n", quotient );
   exit(0);
}

编译并执行上述代码后，将产生以下结果 −

When the above code is compiled and executed, it produces the following result −

Division by zero! Exiting...
Program Exit Status

通常，在程序在成功操作后退出时，将使用 EXIT_SUCCESS 的值退出。此处，EXIT_SUCCESS 是一个宏，定义为 0。

It is a common practice to exit with a value of EXIT_SUCCESS in case of program coming out after a successful operation. Here, EXIT_SUCCESS is a macro and it is defined as 0.

Example 2

如果您的程序中存在错误条件并且您要退出，那么您应该以 EXIT_FAILURE 的状态退出，该状态定义为“ -1”。因此，我们按如下方式编写上述程序 -

If you have an error condition in your program and you are coming out then you should exit with a status EXIT_FAILURE which is defined as "-1". So let’s write the above program as follows −

#include <stdio.h>
#include <stdlib.h>

int main() {

   int dividend = 20;
   int divisor = 5;
   int quotient;

   if(divisor == 0) {
      fprintf(stderr, "Division by zero! Exiting...\n");
      exit(EXIT_FAILURE);
   }

   quotient = dividend / divisor;
   fprintf(stderr, "Value of quotient: %d\n", quotient );

   exit(EXIT_SUCCESS);
}

编译并执行上述代码后，将产生以下结果 −

When the above code is compiled and executed, it produces the following result −

Value of quotient: 4