如何在使用malloc時正確使用free（）來釋放內存？

Question

我一直在使用指針和malloc，我不知道如何正確使用free（）。我有一個程序，允許用戶在選擇特定選項時添加數據記錄。我的程序允許這個功能只發生一次，然後我得到一個段錯誤。我不認爲我正在使用免費（），並希望有人能指出我的問題。我有全局變量：

int records = 5; 
int access = 0;     //initialize access counter to 0 
int count = 0; 

struct childrensBook *first; //initialize first pointer 
struct childrensBook *last;  //initialize last pointer 
struct childrensBook *temp;  //initialize temp pointer 

然後我有，我有預定記錄我的主要功能的開始：

int main(void) 
{ 
    last = (struct childrensBook *) malloc(records * sizeof(struct childrensBook));  //allocate memory to "last" pointer 
    first = last;  

    memcpy(last->title, "We're Going on a Bear Hunt", 27);        //Beginning of all predefined Records 
    memcpy(last->author, "Michael Rosen", 14); 
    last->price = 7.99; 
    last++;                    //incrememnts pointer 
    count = count + 1;                 //begins counting of records 

    memcpy(last->title, "Goodnight Moon", 15); 
    memcpy(last->author, "Margaret Wise Brown", 20); 
    last->price = 8.99; 
    last++;                    //incrememnts pointer 
    count = count + 1;                 //adds 1 to record count 

    memcpy(last->title, "One Fish\n Two Fish\nRed Fish\n Blue Fish", 38); 
    memcpy(last->author, "Dr. Seuss", 10); 
    last->price = 8.99; 
    last++;                    //incrememnts pointer 
    count = count + 1;                 //adds 1 to record count 

    memcpy(last->title, "Snowmen At Night", 17); 
    memcpy(last->author, "Caralyn Buehner", 16); 
    last->price = 16.99; 
    last++;                    //incrememnts pointer 
    count = count + 1;                 //adds 1 to record count 

    memcpy(last->title, "Harold and the Purple Crayon", 29); 
    memcpy(last->author, "Crockett Johnson", 17); 
    last->price = 6.99 

下面這只是我的菜單循環，我還沒有包括在內。我的問題在於我的附加功能，其中我使用免費的（），它是在這裏：

addRecord()                   //Add Function 
{              
    last=first;                   //get pointer in correct position 
    memcpy(&temp, &last, records *sizeof(struct childrensBook));      //use memcpy to copy info from last to temp 

    fprintf(stderr, "\nYou have added the record:\n==========================\nBook: %s\nWritten by: %s\nPrice: $%.2f\n", temp->title, temp->author, 
     temp->price); 

    temp++; 
    free(last);  //problem?? I have tried using free(first), free(last) and free(temp) and none work.... 
    count = count + 1; 

}//end addRecord 

我也這樣做了，它仍然不工作：

  addRecord()                      //Add Function 
    {              
    last=first;                   //get pointer in correct position 
    temp = (struct childrensBook *) malloc(records * sizeof(struct childrensBook)); 
    memcpy(&temp, &last, records *sizeof(struct childrensBook));      //use memcpy to copy info from last to temp 

    fprintf(stderr, "\nYou have added the record:\n==========================\nBook: %s\nWritten by: %s\nPrice: $%.2f\n", temp->title, temp->author, 
     temp->price); 

    temp++; 
    free(last); 
    count = count + 1; 

}//end addRecord 

Answer 1

last點first在第一次致電addRecord()結束時獲得釋放。在第二次調用中，您將會崩潰，因爲last不再指向分配的內存。但是，更有問題的是，您沒有爲新記錄分配任何新內存。請參閱realloc()。