C Review notes (DAY4)

1, c error handling

errno, Perror () and Sterror ()

The perror () function displays the string you passed to it, followed by a colon, a space, and a textual representation of the current errno value.

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

Error removed by 0

Program Exit status

Normally, a program that successfully executes an operation normally exits with a value of exit_success. Here, Exit_success is a macro, which is defined as 0. If there is an error condition in the program, when you exit the program, it will have a status value of Exit_failure, which is defined as-1.

2, C recursion

Factorial of the number

int factorial (unsigned int i) {   if (I <= 1)   {      return 1;   }   return I * factorial (i-1);}

Fibonacci sequence

int Fibonaci (int i) {   if (i = = 0)   {      return 0;   }   if (i = = 1)   {      return 1;   }   Return Fibonaci (i-1) + Fibonaci (i-2);}

3. variable Parameters

You need to use the stdarg.h header file, which provides functions and macros that implement the variable parameters feature. The steps are as follows:

• Define a function, the last argument is an ellipsis, and the one preceding the ellipsis is always int, indicating the number of arguments.

• Create a va_list type variable in the function definition that is defined in the Stdarg.h header file.

• Use the int parameter and the va_start macro to initialize the va_list variable as a list of arguments. The macro va_start is defined in the Stdarg.h header file.

• Use the VA_ARG macro and the va_list variable to access each item in the argument list.

• Use the macro va_end to clean up the memory assigned to the va_list variable.

#include <stdio.h> #include <stdarg.h>double average (int num,...) {    va_list valist;    Double sum = 0.0;    int i;    

/ * Initialize valist for num parameters * / Va_start

(Valist, num);    

/* Access all parameters assigned to Valist */

    for (i = 0; i < num; i++)    {       

Va_arg

(valist, int);    }    

/* Clean up the memory reserved for Valist */

   va_end (valist);    return sum/num;} int main () {   printf ("Average of 2, 3, 4, 5 =%f\n", Average (4, 2,3,4,5));   printf ("Average of 5, ten,%f\n", Average (3, 5,10,15));}

4. Memory Management

Memory management functions

Function	Describe
void *calloc (int num,int size)	The function assigns an array with function allocates an array of num elements, each of which is of size byte .
void free (void *adress)	This function releases the block of H memory pointed to by address.
void *malloc (int num)	The function allocates an array of num bytes and initializes the
void *realloc (void *adress,int newsize)	This function re-allocates memory to extend memory to newsize

Dynamically allocating memory

Without knowing the length of the text that needs to be stored, you need to define a pointer to a character that does not define the size of the memory you are learning, and then allocate memory on demand.

... char *description;

Description = malloc (* sizeof (char));

5. command Line Parameters

When executing a program, you can pass a value from the command line to the C program. These values are called command-line arguments , and they are important to the program, especially if you want to code them externally, rather than hard-coding them within the codes.

Command-line arguments are handled using the main () function parameter, whereargc refers to the number of arguments passed in, andargv[] is an array of pointers that point to each parameter passed to the program.

int main (int argc, char *argv[])  {   if (argc = = 2)   {      printf ("The argument supplied is%s\n", argv[1]); c11/>}   else if (argc > 2)   {      printf ("Too Many arguments supplied.\n");   }   else   {      printf ("One Argument expected.\n");}   }

Using a parameter to compile and execute the above code, it produces the following results:

The argument supplied is testing

Using two parameters, compile and execute the above code, which produces the following results:

Too many arguments supplied.

Without passing any parameters, compile and execute the above code, which produces the following results:

One argument expected argv[0] the name of the stored program,argv[1] is a pointer to the first command-line argument, *argv[n] is the last parameter. If no arguments are supplied, ARGC will be 1, otherwise if a parameter is passed,argc will be set to 2.

Multiple command-line arguments are separated by a space, but if the parameter itself has a space, the parameter should be passed in double quotation marks "" or single quote "Inside

C Review notes (DAY4)