尝试将结构指针memset（）指向0时出现分段错误

Question

I just want to set 0 to a pointer to a structure like this:

#include <stdio.h>
#include <string.h>
struct apple
{
     int f;
     int d;
     size_t tree;
};
struct apple *app;
int main(void)
{
 memset(&app,0,sizeof(app));
 app->tree = 1;
 app->d = 4;
 app->f = 2;
 printf("%d %d %lu\n",app->d, app->f, app->tree);
}

It compiles fine with gcc -o foo foo.c but when I run I get Segmentaion fault. i tried memset(app, 0 , sizeof(app)); but it complains, I tried memset(*app, 0,sizeof(app)); also complains. This one compiles fine but I still get seg fault memset((void *) &app, 0, sizeof(app));

What should i do to set this pointer to struct 0 ? If I have just a simple struct like this : struct apple app; (without pointer) it should go like memset(&app, 0, sizeof(app));

Because it's a pointer and memset has the first parameter a void* , why isn't working like passing just a simple variable without the "&" thing?

Answer 1

This:

memset(&app,0,sizeof(app));

Overwrites app (a pointer) with zeroes. We're off to a bad start.

Then we try and reference app (which is now NULL ):

app->tree = 1;

And things have gotten worse.

You want to allocate something for app to point to:

app = malloc(sizeof(struct apple));
memset(app, 0, sizeof(struct apple));

or

app = calloc(1, sizeof(struct apple));   // calloc zeroes things out for you

Answer 2

You are passing &app as a parameter to memset here:

memset(&app,0,sizeof(app));

This means you're setting the value of that pointer to 0 . You then proceed to de-reference it.

app->tree = 1;

It is extremely unlikely that the address 0 points to a place you can write to (in fact, it is possible, but not given, that this address represent the null pointer.) This is enough to cause a segmentation violation.

You need to decide what you mean with "to set 0 to a pointer to a structure". If you want the contents of the pointee set to zero, then you can use memset thus, after making it point to some allocated memory:

app = malloc(sizeof(*app));  // allocate space for an apple object
memset(app, 0, sizeof(*app)); // app, not &app!!!

Note that you can allocate and zero initialize the allocated memory using calloc .

app = calloc(1, sizeof(*app));

If you really want to set the value of the pointer to 0, then perhaps you mean setting it to the null pointer, which you can achieve like this:

app = NULL;

But now you can't de-reference the pointer.