c程序中的NULL

[yu3wu4]

#include<stdio.h>
#include<malloc.h>
#define LIAN sizeof(struct lianbiao)
struct lianbiao
{
int num;
struct lianbiao *next;
struct lianbiao *before;
};
void inputlian(struct lianbiao*);//输入链表
void outputlianfromhead(struct lianbiao*);//输出链表
void outputlianfromend(struct lianbiao*);//输出链表
//void insertnum();//插入数字
//void deletenum();//删除数字
int total=0;
int main()
{
struct lianbiao *head,*end;
//l1=(struct lianbiao*)malloc(100*sizeof(struct lianbiao));
end=((struct lianbiao*)malloc(LIAN));
head=end;
head->before=NULL;
end->next=NULL;
inputlian(end);
outputlianfromhead(head);
outputlianfromend(end);
return 0;
}
//----------------------------------------------------------------------------------------------------------------------
void inputlian(struct lianbiao* l1)
{
int a;
struct lianbiao *l2;
scanf("%d",&a);
while(a!=0)
{
++total;
l1->num=a;
printf("---%d\n",l1->num);
l2=((struct lianbiao*)malloc(LIAN));
l1->next=l2;
l2->before=l1;
l1=l2;
scanf("%d",&a);
};
l1=NULL;
}
//----------------------------------------------------------------------------------------------------------------------
void outputlianfromend(struct lianbiao* e)
{
struct lianbiao* p;
p=e;
p=p->before;
while(p!=NULL)
{
printf("%d\n",p->num);
p=p->before;
}
}
//----------------------------------------------------------------------------------------------------------------------
void outputlianfromhead(struct lianbiao* h)
{
struct lianbiao* p;
p=h;
while(p!=NULL)
{
printf("%d\n",p->num);
p=p->next;
}
}//该程序输出链表时while（p！=NULL）好像没有用一样，当链表输出完时，指针接着往下指，然后结果就显示该内存不可读了，用的是vc++6.0编译器。。

[赵4老师]

仅供参考

//将c:\\tmp文件夹下的所有文件的内容全部放到用malloc分配的内存中
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <io.h>
struct FB {
    char fn[256];
    size_t fl;
    char *b;
    struct FB *next;
    struct FB *prev;
} *fh,*fb,*ft;
char ln[256];
char fpn[256];
FILE *af;
FILE *f;
int L,n;
int main() {
    system("dir /b /a-d c:\\tmp\\*.* >c:\\allfn.txt");
    af=fopen("c:\\allfn.txt","r");
    if (NULL==af) {
        printf("Can not open file c:\\allfn.txt!\n");
        return 1;
    }
    fh=NULL;
    fb=NULL;
    n=0;
    while (1) {
        if (NULL==fgets(ln,256,af)) break;
        L=strlen(ln);
        if ('\n'==ln[L-1]) ln[L-1]=0;
        printf("read %s\n",ln);
        strcpy(fpn,"c:\\tmp\\");
        strcat(fpn,ln);
        ft=(struct FB *)malloc(sizeof(struct FB));
        if (NULL==ft) {
            printf("Can not malloc ft!\n");
            fclose(af);
            return 2;//之前的malloc在main退出后由操作系统自动free
        }
        printf("ft[%d]==%p\n",n,ft);
        strcpy(ft->fn,fpn);
        f=fopen(fpn,"rb");
        if (NULL==f) {
            printf("Can not open file %s!\n",fpn);
            fclose(af);
            return 3;//之前的malloc在main退出后由操作系统自动free
        }
        ft->fl=_filelength(fileno(f));
        ft->b=malloc(ft->fl);
        if (NULL==ft->b) {
            printf("Can not malloc ft->b!\n");
            fclose(f);
            fclose(af);
            return 4;//之前的malloc在main退出后由操作系统自动free
        }
        printf("ft[%d]->b==%p\n",n,ft->b);
        if (ft->fl!=fread(ft->b,1,ft->fl,f)) {
            printf("fread error!\n");
            fclose(f);
            fclose(af);
            return 5;//之前的malloc在main退出后由操作系统自动free
        }
        fclose(f);
        ft->next=NULL;

        if (NULL==fh) {
            ft->prev=NULL;
            fh=ft;
        } else {
            fb->next=ft;
            ft->prev=fb;
        }
        fb=ft;
        n++;
    }
    fclose(af);
    printf("-----list-----\n");
    for (ft=fh;NULL!=ft;ft=ft->next) {
        printf("%8d %s\n",ft->fl,ft->fn);
        if (NULL!=ft) fb=ft;
    }
    printf("-----free-----\n");
    n--;
    if (NULL!=fh) {
        for (ft=fb->prev;NULL!=ft;ft=ft->prev) {
            if (NULL!=ft->next->b) {
                printf("ft[%d]->b==%p\n",n,ft->next->b);
                free(ft->next->b);
            }
            if (NULL!=ft->next) {
                printf("ft[%d]==%p\n",n,ft->next);
                free(ft->next);
            }
            n--;
        }
        if (NULL!=fh->b) {
            printf("ft[0]->b==%p\n",fh->b);
            free(fh->b);
        }
        printf("ft[0]==%p\n",fh);
        free(fh);
    }
    return 0;
}
//C:\tmp\tmp\Debug>dir /a-d c:\tmp
// 驱动器 C 中的卷是 C_HD5_1
// 卷的序列号是 1817-D526
//
// c:\tmp 的目录
//
//找不到文件
//
//C:\tmp\tmp\Debug>tmp
//找不到文件
//-----list-----
//-----free-----
//
//C:\tmp\tmp\Debug>dir /a-d c:\tmp
// 驱动器 C 中的卷是 C_HD5_1
// 卷的序列号是 1817-D526
//
// c:\tmp 的目录
//
//2011-06-30  18:04            44,840 my_c.rar
//2011-06-30  17:18             1,036 err.frm
//2011-06-30  14:32            14,243 出租.txt
//2011-06-28  12:08            23,681 MSDN98书签.txt
//             4 个文件         83,800 字节
//             0 个目录 17,041,870,848 可用字节
//
//C:\tmp\tmp\Debug>tmp
//read my_c.rar
//ft[0]==00421800
//ft[0]->b==00520068
//read err.frm
//ft[1]==00421670
//ft[1]->b==0052AFC0
//read 出租.txt
//ft[2]==00421530
//ft[2]->b==00378F28
//read MSDN98书签.txt
//ft[3]==004213F0
//ft[3]->b==0052B3F8
//-----list-----
// 44840 c:\tmp\my_c.rar
//  1036 c:\tmp\err.frm
// 14243 c:\tmp\出租.txt
// 23681 c:\tmp\MSDN98书签.txt
//-----free-----
//ft[3]->b==0052B3F8
//ft[3]==004213F0
//ft[2]->b==00378F28
//ft[2]==00421530
//ft[1]->b==0052AFC0
//ft[1]==00421670
//ft[0]->b==00520068
//ft[0]==00421800
//
//C:\tmp\tmp\Debug>

[Carl_CCC]

指针作为函数参数的时候如果要返回值的话，单级指针是不行的。因为那只是指针的一份拷贝 #include<stdio.h> #include<malloc.h> #define LIAN sizeof(struct lianbiao) struct lianbiao { int num; struct lianbiao *next; struct lianbiao *before; }; void inputlian(struct lianbiao**);//输入链表 void outputlianfromhead(struct lianbiao*);//输出链表 void outputlianfromend(struct lianbiao*);//输出链表 //void insertnum();//插入数字 //void deletenum();//删除数字 int total=0; int main() { struct lianbiao *head,*end; //l1=(struct lianbiao*)malloc(100*sizeof(struct lianbiao)); end=((struct lianbiao*)malloc(LIAN)); head=end; head->before=NULL; end->next=NULL; inputlian( &end); outputlianfromhead(head); outputlianfromend(end); return 0; } //---------------------------------------------------------------------------------------------------------------------- void inputlian(struct lianbiao ** ppl1) { int a; struct lianbiao *l2; struct lianbiao *l1=*ppl1; scanf("%d",&a); while(a!=0) { ++total; l1->num=a; printf("---%d\n",l1->num); l2=((struct lianbiao*)malloc(LIAN)); l1->next=l2; l2->before=l1; l1=l2; scanf("%d",&a); }; l1->next=NULL; } //---------------------------------------------------------------------------------------------------------------------- void outputlianfromend(struct lianbiao* e) { struct lianbiao* p; p=e; p=p->before; while(p!=NULL) { printf("%d\n",p->num); p=p->before; } } //---------------------------------------------------------------------------------------------------------------------- void outputlianfromhead(struct lianbiao* h) { struct lianbiao* p; p=h; while(p!=NULL) { printf("%d\n",p->num); p=p->next; } }

[starytx]

那只能说明你的链表没有建好，具体就是头尾节点指针没有处理好，头结点的before必须是NULL，尾节点的next必须是NULL

[yu3wu4]

#include<stdio.h> #include<malloc.h> #define LIAN sizeof(struct lianbiao) struct lianbiao { int num; struct lianbiao *next; struct lianbiao *before; }; void inputlian(struct lianbiao**);//输入链表 void outputlianfromhead(struct lianbiao*);//输出链表 void outputlianfromend(struct lianbiao*);//输出链表 //void insertnum();//插入数字 //void deletenum();//删除数字 int total=0; int main() { struct lianbiao *head,*end; //l1=(struct lianbiao*)malloc(100*sizeof(struct lianbiao)); end=((struct lianbiao*)malloc(LIAN)); head=end; head->before=NULL; end->next=NULL; printf("%d",end); inputlian( &end); printf("%d",end); outputlianfromhead(head); outputlianfromend(end); return 0; } //---------------------------------------------------------------------------------------------------------------------- void inputlian(struct lianbiao ** ppl1) { int a; struct lianbiao *l2; struct lianbiao *l1=*ppl1; scanf("%d",&a); while(a!=0) { ++total; l1->num=a; printf("---%d\n",l1->num); l2=((struct lianbiao*)malloc(LIAN)); l1->next=l2; l2->before=l1; l1=l2; scanf("%d",&a); }; l1->next=NULL; } //---------------------------------------------------------------------------------------------------------------------- void outputlianfromend(struct lianbiao* e) { struct lianbiao* p; p=e; p=p->before; while(p->before!=NULL) { printf("%d\n",p->num); p=p->before; } } //---------------------------------------------------------------------------------------------------------------------- void outputlianfromhead(struct lianbiao* h) { struct lianbiao* p; p=h; while(p->next!=NULL) { printf("%d\n",p->num); p=p->next; } } 这里按照二楼改了，为什么两次输出的end地址一样呢，还有就是void outputlianfromend(struct lianbiao* e) 这里不能正常输出。。

[starytx]

p=p->before; // 如果你需要输出e中的num的话，这句是多余的 while(p->before!=NULL) // 这里改成 p != NULL 同样 while(p->next!=NULL) 也改成 p != NULL

[yu3wu4]

指针那里是懂了，能说说为什么两次end的地址输出一样吗？不是用了双重指针了吗？ #include<stdio.h> #include<malloc.h> #define LIAN sizeof(struct lianbiao) struct lianbiao { int num; struct lianbiao *next; struct lianbiao *before; }; void inputlian(struct lianbiao**);//输入链表 void outputlianfromhead(struct lianbiao*);//输出链表 void outputlianfromend(struct lianbiao*);//输出链表 //void insertnum();//插入数字 //void deletenum();//删除数字 int total=0; int main() { struct lianbiao *head,*end; //l1=(struct lianbiao*)malloc(100*sizeof(struct lianbiao)); end=((struct lianbiao*)malloc(LIAN)); head=end; head->before=NULL; end->next=NULL; printf("%d",end); inputlian( &end); printf("%d",end); outputlianfromhead(head); outputlianfromend(end); return 0; } //---------------------------------------------------------------------------------------------------------------------- void inputlian(struct lianbiao ** ppl1) { int a; struct lianbiao *l2; struct lianbiao *l1=*ppl1; scanf("%d",&a); while(a!=0) { ++total; l1->num=a; printf("---%d\n",l1->num); l2=((struct lianbiao*)malloc(LIAN)); l1->next=l2; l2->before=l1; l1=l2; scanf("%d",&a); }; l1->next=NULL; } //---------------------------------------------------------------------------------------------------------------------- void outputlianfromend(struct lianbiao* e) { struct lianbiao* p; p=e; p=p->before; while(p->before!=NULL) { printf("%d\n",p->num); p=p->before; } } //---------------------------------------------------------------------------------------------------------------------- void outputlianfromhead(struct lianbiao* h) { struct lianbiao* p; p=h; while(p->next!=NULL) { printf("%d\n",p->num); p=p->next; } }