Allocate memory ：malloc() and free()

​ Memory storage categories have one thing in common : After determining which storage category to use , According to your own memory management rules , Scope and storage period will be automatically selected .

malloc() function

​ This function takes a parameter : Number of memory bytes required . use malloc() Create an array with the following code :

double * ptd;
ptd = (double *)malloc(30 * sizeof(double));

The above code is 30 individual double The value of type requests memory space , And set up ptd Point to this location . You can also use expressions ptd[0] Represents an array element .

Three ways to create arrays :

1. When you declare an array , The dimension of an array is represented by a constant expression , Use the array name to access the elements of the array . You can create this array with static memory or automatic memory .

2. When declaring a variable length array , The dimension of an array is represented by a variable expression , Use the array name to access the elements of the array . Arrays with this feature can only be created in automatic memory .

3. Declare a pointer , call malloc(), Assign its return value to the pointer , Using pointers to access elements of an array . The pointer can be static or automatic .

free() function

​ Usually malloc() To work with free() Matching use of . among free() The argument to the function is before malloc() Return address , Mainly to release malloc() Allocated memory . Call... At the end of the function free() Functions can avoid Memory leak The problem of

exit() function

​ If memory allocation fails , You can call exit() The function ends the program , Its prototype is also stdlib.h in , The standard provides two return values to ensure normal operation in all operating systems :

​ EXIT_SUCCESS( perhaps , amount to 0) Indicates the end of a normal program .

​ EXIT_FAILURE Indicates that the program aborts abnormally .

Other operating systems also accept integer values that indicate other operational errors .

Run a piece of code and use the above functions to realize the program ：

// Assign arrays dynamically 
#include <stdio.h>
#include <stdlib.h>// by malloc()、free() Provide prototypes 

int main(void)
{
    
    double * ptd;
    int max;
    int number;
    int i = 0;

    puts("What is the maxium nuber of type double enteries?");
    if(scanf("%d",&max) != 1)
    {
    
        puts("Number not correctly entered -- bye.");
        exit(EXIT_FAILURE);
    }
    ptd = (double *)malloc(max * sizeof(double));
    if(ptd == NULL)
    {
    
        puts("Memory allocation failed. Goodbye.");
        exit(EXIT_FAILURE);
    }
    //ptd  Now point to max Array of elements 
    puts("Enter the Values (q to quit):");
    while(i < max && scanf("%lf",&ptd[i]) == 1)
        ++i;
    printf("Here are your %d enteries:\n", number = i);
    for (i = 0; i < number;i++)
    {
    
        printf("%7.2f", ptd[i]);
        if(i % 7 == 6)
            putchar('\n');
    }
    if(i % 7 != 0)
        putchar('\n');
    puts("Done.");
    free(ptd);

    return 0;
}

Running results ：

What is the maxium nuber of type double enteries?
5
Enter the Values (q to quit):
20 30 35 25 40 80
Here are your 5 enteries:
  20.00  30.00  35.00  25.00  40.00
Done.