Why do I get these warnings?

Question

The compiler showed the following warnings for the following code segment. Please help me correct it.

if((tmp_n = (struct dot *)shmat(shm_net, NULL, 0)) == (int *) -1) { }
warning: comparison of distinct pointer types lacks a cast [enabled by default]

Its a C program, this code segment is for attaching a shared memory segment to a pointer **tmp_n which is of the type struct dot.

struct dot {int weight; int tmv;};

Answer 1

Try this one

if((tmp_n = (struct dot *)shmat(shm_net, NULL, 0)) == (void *) -1) { }

and look at the man-page , it states:

Return Value
On success shmat() returns the address of the attached shared memory segment; 
on error (void *) -1 is returned, and errno is set to indicate the cause of the error. 

Answer 2

您需要将-1转换为与要比较的变量相同的指针类型：

if((tmp_n = (struct dot *)shmat(shm_net, NULL, 0)) == (struct dot *) -1) { }