Why do I get these warnings?

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;};


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. 

You need to cast -1 to the same pointer type as the variable you're comparing with:

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

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