链接列表突然更改C中的值

Question

I am making a simple linked list the problem is as it goes to class after class, the value of the first node changes. This is my insert code:

int adjob(int job,int time, int size,JOBS **list)
{
    JOBS *temp,*new;
    temp=*list;
    if(temp==NULL)
        *list=create(job,time,size);

    else
    {
        new=(JOBS*)malloc(sizeof(JOBS));
        new->job=job;
        new->size=size;
        new->duration=time;
        new->next=NULL;
        while(temp->next!=NULL)
            temp=temp->next;
        temp->next=new; 
    }
        return 0;
}

And this is the main:

MEM mmem[10];
int main()
{
    int i=1,jtime,jsize,status;
    char option;
    JOBS *joblist;
    joblist=NULL;
    status=initmem(mmem);
    for(;;)
    {
        clrscr();
        printer(mmem,&joblist);
        scanf("%s",&option);
        switch(option)
        {
            case 'I':       if(i<16)
                            {
                                printf("Input size of job %i: ",i);
                                scanf("%i",&jsize);
                                printf("Input duration: ");
                                scanf("%i",&jtime);
                                status=adjob(i,jtime,jsize,&joblist);
                            }
                                i++;
                                break;
            case 'S':       break;
            case 'R':       break;
            case 'E':       exit(0);
        }
    }
    free(joblist);
    joblist=NULL;
    return 0;
}

These are my structures:

struct node
{
    int job;
    int size;
    int duration;
    struct node *next;
};

struct mainmem
{
    int memblock;
    int size;
    int jobnum;
    int jobsize;
    int timer;
};
typedef struct mainmem MEM;
typedef struct node JOBS;

Then this is my printer:

int printer(MEM *mymem,JOBS **list)
{
    JOBS *temp;
    int i=0,pr=3;
    temp=*list;

    /*MEM LIST*/
    printf("\t\tMEMORY LIST\t\t\t\tJOB LIST\nMem Block    Size    Job    Job Size    Timer\n");
    while(i!=10)
    {
        printf("   %i\t     %i\n",i+1,mymem[i].size);
        i++;
    }
    /*JOB LIST*/
    gotoxy(50,2);
    printf("Job no.\tTime\tJob Size");
    if(temp==NULL)
    {
        gotoxy(50,pr);
        printf("No jobs on queue.");
    }
    else
    {
        while(temp!=NULL)
        {   
            gotoxy(50,pr);
            printf("%i\t%i\t%i",temp->job,temp->duration,temp->size);
            pr++;
            temp=temp->next;
        }
    }
    gotoxy(1,20);
    printf("[I]-INPUT   [S]-START   [R]-RESULT  [E]-EXIT\n");
    return 0;
}

The first node inserts with no problem. When I insert a second node, the first node changes. And when I try to insert beyond 2, the program functions as it should be. I tried debugging it, the value changes when the second input is inserted int he JOBS LL, but I can't figure that out from my code. What could have possibly gone wrong?

Answer 1

The problems you have is probably because you have undefined behavior in your code: The variable option is a single char , but you read into it as a string which will write two characters (the character you read and the string terminator). There's no way of telling what that will overwrite, but it will likely overwrite parts of the stack where the other local variables are stored.

You should change the input handling to read only a single character:

scanf(" %c", &option);

Note the leading space, which will tell scanf to read and discard any white-space (including newlines) before the character.