C-傳遞TMP值通過函數到另一個函數

Question

在這個方案：

#include <stdio.h> 
#include <stdlib.h> /* for malloc */ 
#include <ctype.h> 

#define STD_WORK_WEEK 40 
#define OVERTIME_RATE 1.5 


struct employee 
{ 
char first_name[10]; 
char last_name[10]; 
long id_number; 
float wage; 
float hours; 
float overtime; 
float gross; 

struct employee *next; 
}; 

/************************************************************************/ 
/*      Function: Gross_pay_calc      */ 
/*                  */ 
/* Purpose: Calculates gross pay of an employee using hours and  */ 
/*    wage data that is already stored      */ 
/*                  */ 
/*                  */ 
/* Parameters: wage - Array of employee wages       */ 
/*    hours- Array of number of hours worked by an employee */ 
/*    grossPay- Gross Pay of an employee      */ 
/*    size-number of employees to process      */ 
/*                  */ 
/* Returns: Stores grossPay to be referenced in another function output */ 
/*                  */ 
/************************************************************************/ 

/* Function call to calculate gross pay. */ 
void Gross_pay_calc (struct employee *emp1) 
{ 
    /*Local Variable Declaration */ 
    struct employee *tmp; /* tmp pointer value to current node */ 

      if (tmp->hours <= STD_WORK_WEEK) /* Calculates grossPay based on <40 or  >40 for OT */ 

      { 
      tmp->gross = tmp->wage*tmp->hours; 
      } 

      else 

      { 
      tmp->gross = (tmp->wage*STD_WORK_WEEK)+((tmp->hours-STD_WORK_WEEK)* (tmp->wage*OVERTIME_RATE)); 
      } 

    } 

/*-----------------------------------------------------------------------------*/ 
/*                    */ 
/* FUNCTION: print_list              */ 
/*                    */ 
/* DESCRIPTION: This function will print the contents of a linked    */ 
/*    list. It will traverse the list from beginning to the  */ 
/*    end, printing the contents at each node.      */ 
/*                    */ 
/* PARAMETERS: emp1 - pointer to a linked list        */ 
/*                    */ 
/* OUTPUTS:  None               */ 
/*                    */ 
/* CALLS:  None               */ 
/*                    */ 
/*-----------------------------------------------------------------------------*/ 
void print_list(struct employee *emp1) 
{ 
    struct employee *tmp; /* tmp pointer value to current node */ 
    int i = 0;    /* counts the nodes printed   */ 

     printf ("--------------------------------------------------------------\n");   /*Print Header To Screen */ 
     printf ("  Name \t  Clock# \t Wage \t Hours \t OT \t Gross\n"); 
     printf ("---------------------------------------------------------------\n"); 

    /* Start a beginning of list and print out each value    */ 
    /* loop until tmp points to null (remember null is 0 or false)  */ 

    for(tmp = emp1; tmp ; tmp = tmp->next) 
    { 
     i++; 

     /* TODO - print other members as well */ 
     printf("\n %s %s %6d %8.2f %5.2f %7.2f\n",tmp->first_name,tmp->last_name,tmp->id_number, 
                tmp->wage,tmp->hours,tmp->gross); 

    } 

    printf("\n\nTotal Number of Employees = %d\n", i); 

    } 

/*----------------------------------------------------------------------------*/ 
/*                   */ 
/* FUNCTION: main               */ 
/*                   */ 
/* DESCRIPTION: This function will prompt the user for an employee   */ 
/*    id and wage until the user indicates they are finished.  */ 
/*    At that point, a list of id and wages will be    */ 
/*    generated.             */ 
    /*                   */ 
/* PARAMETERS: None               */ 
/*                   */ 
/* OUTPUTS:  None               */ 
/*                   */ 
/* CALLS:  print_list             */ 
/*                   */ 
/*----------------------------------------------------------------------------*/ 
int main() 
{ 

char answer[80];  /* to see if the user wants to add more employees */ 
int more_data = 1; /* flag to check if another employee is to be processed */ 
char value;    /* gets the first character of answer */ 

struct employee *current_ptr, /* pointer to current node */ 
       *head_ptr;  /* always points to first node */ 

    /* Set up storage for first node */ 
head_ptr = (struct employee *) malloc (sizeof(struct employee)); 
current_ptr = head_ptr; 

while (more_data) 
{ 
    /* Read in Employee ID and Hourly Wage */ 

    printf("\nEnter employee ID: "); 
    scanf("%li", & current_ptr -> id_number); 

    printf("\nEnter employee weekly wage: "); 
    scanf("%f", & current_ptr -> wage); 

    printf("\nEnter employee weekly hours: "); 
    scanf("%f", & current_ptr -> hours); 

    printf("\nEnter First Name: "); 
    scanf("%s", & current_ptr -> first_name); 

    printf("\nEnter Last Name: "); 
    scanf("%s", & current_ptr -> last_name); 

    printf("Would you like to add another employee? (y/n): "); 
    scanf("%s", answer); 

    Gross_pay_calc(head_ptr); 

    /* Ask user if they want to add another employee */ 
    if ((value = toupper(answer[0])) != 'Y') 
     { 
     current_ptr->next = (struct employee *) NULL; 
     more_data = 0; 
     } 
    else 
     { 
     /* set the next pointer of the current node to point to the new node */ 
     current_ptr->next = (struct employee *) malloc (sizeof(struct employee)); 
     /* move the current node pointer to the new node */ 
     current_ptr = current_ptr->next; 
     } 
} /* while */ 

/* print out listing of all employee id's and wages that were entered */ 


print_list(head_ptr); 

    printf("\n\nEnd of program\n"); 

return 0; 
} 

我試圖用gross_pay_calc來計算，我在請求進入每個員工的工資總額。但是，當我嘗試在print_list函數中打印tmp-> gross值時，它不能正確計算。我不確定爲什麼會發生這種情況，並且很樂意提供關於如何使其準確計算的建議！

Answer 1

兩件事情，首先，你的直接問題。正如評論中提到的，在Gross_pay_calc中，您通過一個不確定的指針tmp訪問內存。它在解除引用前從未設置爲有效內存，因此它是未定義的行爲。嘗試使用傳入的參數emp1代替：

/* Function call to calculate gross pay. */ 
void Gross_pay_calc (struct employee *emp1) 
{ 
    if (emp1->hours <= STD_WORK_WEEK) /* Calculates grossPay based on <40 or  >40 for OT */ 
    { 
     emp1->gross = emp1->wage*emp1->hours; 
    } 
    else 
    { 
     emp1->gross = (emp1->wage*STD_WORK_WEEK) + 
         ((emp1->hours-STD_WORK_WEEK) * (emp1->wage*OVERTIME_RATE)); 
    } 
} 

其次，算法用於填充你的鏈表是過度勞累。一個簡單的指針到指針可以使循環多更簡潔：

int main() 
{ 
    char answer[80];  /* to see if the user wants to add more employees */ 

    struct employee *head_ptr = NULL; /* the list head pointer */ 
    struct employee **next_ptr = &head_ptr; /*always holds the address of the next pointer to fill */ 

    do 
    { /* allocate new node wherever *next_ptr says to. initially it will be 
      the head pointer. after that, the next-ptr of the last node added. */ 
     *next_ptr = calloc(1, sizeof(**next_ptr)); 

     /* Read in Employee ID and Hourly Wage */ 
     printf("\nEnter employee ID: "); 
     scanf("%li", &(*next_ptr)->id_number); 

     printf("\nEnter employee weekly wage: "); 
     scanf("%f", &(*next_ptr)->wage); 

     printf("\nEnter employee weekly hours: "); 
     scanf("%f", &(*next_ptr)->hours); 

     printf("\nEnter First Name: "); 
     scanf("%s", (*next_ptr)->first_name); 

     printf("\nEnter Last Name: "); 
     scanf("%s", (*next_ptr)->last_name); 

     Gross_pay_calc(*next_ptr); 

     // move to the next pointer we'll be filling 
     next_ptr = &(*next_ptr)->next; 

     /* Ask user if they want to add another employee */ 
     printf("Would you like to add another employee? (y/n): "); 
     scanf("%s", answer); 
     if (toupper(answer[0]) != 'Y') 
      break; 

    } while (1); 

    // always set the last pointer to nothing. 
    *next_ptr = NULL; 

    /* print out listing of all employee id's and wages that were entered */ 
    print_list(head_ptr); 

    printf("\n\nEnd of program\n"); 

    return 0; 
} 

同樣，打印設備可以清理有點以及：

void print_list(struct employee *emp1) 
{ 
    int i = 0;    /* counts the nodes printed   */ 

    printf ("--------------------------------------------------------------\n"); 
    printf ("  Name \t  Clock# \t Wage \t Hours \t OT \t Gross\n"); 
    printf ("---------------------------------------------------------------\n"); 

    /* Start a beginning of list and print out each value    */ 
    /* loop until tmp points to null (remember null is 0 or false)  */ 
    for (;emp1; ++i, emp1 = emp1->next) 
    { 
     printf("\n %s %s %6ld %8.2f %5.2f %7.2f\n", 
       emp1->first_name,emp1->last_name,emp1->id_number, 
       emp1->wage,emp1->hours,emp1->gross); 
    } 
    printf("\n\nTotal Number of Employees = %d\n", i); 
}