在C中读取和写入

Question

I'm trying to make a program that creates and reads a binary file, which contains "struct elements"; can you please tell me what I did wrong? I got errors telling me that "s" is not a pointer in function fread()... so I declared ELEM *s; instead of ELEM s;

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

typedef struct element{
    char name[80];
    int p;
}ELEM;

void create()
{
    FILE *f;
    int d=0;
    char c;
    ELEM *s;
    f=fopen("file.bin","wb");
    do{
    printf("Add elements to file?: (y/n)");
    fflush(stdin);
    scanf("%c",&c);
    if (c=='y')
    {
        printf("Name=");
        gets((*s).name);
        printf("P=");
        scanf("%d",(*s).p);
        fwrite(s,sizeof(ELEM),1,f);
    }
    } while(d==0);
    fclose(f);
}

void show()
{
    FILE *f;
    ELEM *s;
    f=fopen("file.bin","rb");
    while(feof(f)!=NULL)
    {
        fread(s,sizeof(ELEM),1,f);
        puts((*s).name);
        printf("\t%d\n",(*s).p);
    }
    fclose(f);
}

void add()
{
    FILE *f;
    int d=0;
    char c;
    ELEM *s;
    f=fopen("file.bin","ab");
    do{
    printf("Add elements to file?: (y/n)");
    fflush(stdin);
    scanf("%c",&c);
    if (c=='y')
    {
        printf("Name=");
        gets((*s).name);
        printf("P=");
        scanf("%d",(*s).p);
        fwrite(s,sizeof(ELEM),1,f);
    }
    } while(d==0);
    fclose(f);
}


/*void function()
{

}*/

int main()
{
    int k=0,r;
    do{
        printf("1 - create file\n2 - add elements to fil\n3 - show elements\n4 - put unique elements in another file\n5 - exit program\n");
        scanf("%d",&r);
        switch(r)
        {
            case 1 : create(); break;
            case 2 : add(); break;
            case 3 : show(); break;
            case 4 : printf("Function not defined!\n"); break;
            case 5 : k=1; break;
            default : printf("Command unrecognized!\n");
        }
    } while(k==0);
    return 0;
}

Answer 1

You declared a pointer but assigned no memory to it. You should revert to a normal variable:

ELEM s;

/* ... */

fwrite(&s,sizeof(ELEM),1,f);
       ^

Alternatively, in your current code you should do this:

ELEM *s = calloc(1, sizeof *s);

Answer 2

The first parameter passed to fwrite should be an address, and the variable whose address you pass must have enough memory to hold the number of objects you plan to read.

So there are two ways:

Creating Variable on Stack:

You allocate the variable on stack and pass its address to fwrite

ELEM s;
fwrite(&s,sizeof(ELEM),1,f);

or

Dynamic Memory allocation:

ELEM *s;   

Should be allocated an memory equivalent to hold no of objects of type ELEM that you want to read.

ELEM *s = malloc(sizeof *s);

In this case remember to free the memory once done with your use:

free(s);

Answer 3

You are not allocating the pointer s in your create function. You probably want something like

 s = malloc(sizeof(*s));
 memset (s, 0, sizeof(*s));

etc.

And you really should learn to compiler with gcc -Wall -g and to use the gdb debugger.

Also, take time to read a good book about programming in C.

Answer 4

Well, your problems is in the scanf. scanf should get a pointer, so change (*s).p to &(*s).p (or even &s->p )

Answer 5

You should have allocated the pointer to struct ELEM Assuming this is C:

 ELEM *s = malloc(sizeof(ELEM));

If it is C++ just add a cast in front

 ELEM *s = (ELEM*) malloc(sizeof(ELEM));