如何在C中将结构作为参数传递给函数以进行错误检查？

Question

General description of code:-

It is a simple program in C using structures to get the input from    
     the user and print the output.Both input and output are defined as    
     separate functions in separate files. Additional user defined error 
     header file is created for error handling which checks error of C 
     function calls, 'malloc' in this case.

Environment: Linux, OS: fedora

error

input.c: In function ‘input’:
    input.c:17:8: error: incompatible type for argument 1 of ‘error’
      error(*val, -1, "malloc");    //error because of this line //usage error(int return_variable, value, "func_name")
            ^
    In file included from input.c:3:0:
    error.h:7:6: note: expected ‘int’ but argument is of type ‘struct Eq’
     void error(int val, int ret, char* func_name)
          ^~~~~
    make: *** [Makefile:10: input.o] Error 1

main.c

//********* headers **********//
#include<stdio.h>
#include<stdlib.h>

//********** structure *********//
struct Eq
{
    int *x;
    int *y;
};

//********** Function prototypes ***********//

struct Eq* input();             
int output(struct Eq*);


//*********** Main function *************//
int main()
{
    struct Eq* num;

    num = input();          //takes user input, returns to num

    output(num);            //prints the output, num passed as arg.

    return 0;
}

input.c input function

#include<stdio.h>
#include<stdlib.h>
#include"error.h"                   //user defined header file

//********** structure *********//
struct Eq
{
    int *x;
    int *y;
};

struct Eq* input()
{
    struct Eq *val;

    val = (struct Eq*)malloc(sizeof(struct Eq));
    error(*val, -1, "malloc");          //error because of this line    //usage error(int return_variable, value, "func_name")

    val->x = (int*)malloc(sizeof(int));
// *val->x = -1;                    //!! try changing this and func below to invoke error function !!
    error(*val->x, -1, "malloc");           //no errors because of this

    val->y = (int*)malloc(sizeof(int));
    error(*val->y, -1, "malloc");           //no errors because of this

    printf("Enter the value of x:");
    scanf("%d", val->x);

    printf("Enter the value of y:");
    scanf("%d", val->y);

    return val;
}

output.c

#include<stdio.h>
#include<stdlib.h>

//********** structure *********//
struct Eq
{
        int *x;
        int *y;
};

int output(struct Eq *num)
{
        printf("value of x=%d\n",*num->x);
        printf("value of y=%d\n",*num->y);

        return 0;
}

error.h Error Handler header file

#include<stdio.h>
#include<stdlib.h>

void error(int, int, char*);

void error(int val, int ret, char* func_name)
{
        if(val == ret)
        {
                switch(ret)
                {
                        case 0:
                                printf("fatal error at %s, return 0\n", func_name);
                                exit(1);

                        case -1:
                                printf("fatal error at %s, return -1\n", func_name);
                                exit(1);
                }
        }
  return 0;
 }

Answer 1

malloc returns a null pointer on failure, see here . Your error function has to take a pointer, as you can't derefernce a null pointer, and then check whether it is NULL . Something like this:

void error(void *ptr, char *func_name)
{
    if(ptr == NULL)
    {     
        printf("fatal error at %s, return NULL\n", func_name);
        exit(1);
    }
}

// call like this:
error(val, "malloc"); // after val = malloc...
error(val->x, "malloc"); // after val->x = malloc...