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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: My C program does not compile or gives segmentation fault
Date: Thu, 23 Mar 2017 13:24:25 -0700
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acidlategamer@gmail.com writes:
[...]
> I just came to senses.
> The malloc I did in create_queue() is a local memory allocation. q
> will be again uninitialized after the function returns, hence I was
> getting segmentation faults. I did correct way.

Almost.  The memory allocated by malloc() isn't local, it's on the
"heap", and its lifetime continues until it's deallocated by free() or
equivalent, or until the program completes.

What's local is your only pointer to it.  When that pointer object
vanishes, you've lost your only reference to the allocated memory.
It's a memory leak.

And q isn't "again uninitialized".  You actually have two distinct
objects named "q".  One is a global (file-scope) variable.  The other
is the parameter to the create_queue() function, which is initialized
with a *copy* of the value of the argument, the global "q".  (Parameters
are like local variables, except that they're initialized to the value
of the corresponding argument.)

> http://codepad.org/sqUT3bx4

Here's your updated create_queue() function:

struct queue *create_queue(struct queue *q)
{
        q = malloc(sizeof(struct queue));
        q -> rear = NULL;//how this happened without any error??
        q -> front = NULL;
        return q;
}

It takes an argument of type "struct queue*", but it doesn't do anything
with that value; it immediately clobbers it with the value returned by
malloc().  This isn't exactly wrong, but it's useless.  You can drop the
parameter and change it to a local variable:

struct queue *create_queue(void)
{
    struct queue q = malloc(sizeof(struct queue));
    q -> rear = NULL;//how this happened without any error??
    q -> front = NULL;
    return q;
}

Then the call in your main function becomes:

    q = create_queue();

Your insert() function always returns the value of its first parameter,
which it never changes.  It might as well be a void function, not
returning a value.  (Unless it *should* modify the value of q; I haven't
studied the logic closely enough to be sure.)

Consistent indentation would make your code much easier to read.


-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"