020 basics of C language: C language forced type conversion and error handling

One ： summary

Cast is to convert a variable from one type to another data type . for example , If you want to store one long Type value into a simple integer , You need to put long Type cast to int type . You can use the cast operator to explicitly cast a value from one type to another .(type_name) expression

example ：

 Use cast operators to divide one integer variable by another , Get a floating point number ：
#include <stdio.h>
int main(){
    
	int sum = 17, count = 5;
	double mean;
	mean = (double) sum / count;
	printf("mean: %f \n", mean);
}
 result ：mean: 3.400000
 It should be noted here that cast operators take precedence over division , therefore  sum  The value of is first converted to  double  type , Then divide by  count, Get a type of  double  Value .
 Type conversion can be implicit , Executed automatically by the compiler , It can also be explicit , Specify by using a cast operator . In programming , Cast operators are used when type conversion is needed , Is a good programming habit .

 

Two ： Integer promotion

 Integer promotion is to put less than int or unsigned int The integer type of is converted to int or unsigned int The process of 
	#include <stdio.h>
	int main(){
    
		int i = 17;
		char c = 'c';   // ascii The value is 99
		int sum;
		sum = i + c;
		printf("value of sum: %d \n", sum);
	}
	 result ：value of sum: 116
	 ad locum ,sum The value of is 116, Because the compiler does integer Promotion , When performing an actual addition operation , hold 'c' The value of is converted to the corresponding ascii value .

 

3、 ... and ： The usual arithmetic conversion

Implicitly cast values to the same type . The compiler first performs integer Promotion , If the operand types are different , Then they will be converted to the type of the highest level in the following levels ：int --> unsigned int --> long --> unsigned long --> long long --> unsigned long long --> float --> double --> long double

Common arithmetic transformations are not applicable to assignment operators , Logical operators && and ||.

Let's take a look at the following examples to understand the concept ：

#include <stdio.h>
int main()
{
    
   int  i = 17;
   char c = 'c'; /* ascii  The value is  99 */
   float sum;
   sum = i + c;
   printf("Value of sum : %f\n", sum );
}
 When the above code is compiled and executed , It will produce the following results ：
Value of sum : 116.000000

ad locum ,c First it's converted to an integer , But because the final value is double Type , So I can use the common arithmetic conversion , The compiler will put i and c Convert to floating point , And add them up to get a floating point number .

 

Four ： Error handling

4.1： summary

C Language does not provide direct support for error handling , But as a system programming language , It allows you to access the underlying data in the form of a return value . When something goes wrong , Most of the C or UNIX Function call returns 1 or NULL, At the same time, an error code will be set errno, The error code is a global variable , Indicates that an error occurred during a function call . You can go to <error.h> Various error codes are found in the header file .

therefore ,C The programmer can check the return value , Then decide which appropriate action to take according to the return value . Developers should be at program initialization time , hold errno Set to 0, It's a good programming habit .0 A value indicates that there are no errors in the program .

 

4.2：errno/perror()/strerror()

C Language provides perror() and strerror() Function to display and errno Related text messages .

perror() The function displays the string passed to it , Followed by a colon , A space and current errno The text representation of the value .

strerror() function , Return a pointer , The pointer points to the current errno The text representation of the value .

#include <stdio.h>
#include <errno.h>
#include <string.h>
extern int errno;
int main(){
    
	FILE *pf;
	int errnum;
	pf = fopen("unexist.txt", "rb");
	if(pf == NULL){
    
		errnum = errno;
		fprintf(stderr, "Value of errno: %d \n", errno);
		perror("Error printed by perror");
		fprintf(stderr, "Error opening file: %s \n", strerror(errnum));
	}else{
    
		fclose(pf);
	}
	return 0;
}
 result ：
┌──(rootkali)-[~/Desktop/c_test]
└─# ./cuowuchuli     
Value of errno: 2 
Error printed by perror: No such file or directory
Error opening file: No such file or directory