MACRO question

[Jonathan100]

Hi guys.
I have question regarding macro:

#define          tcp_sndbuf(pcb)          (TCPWND16((pcb)->snd_buf))

 #define TCPWND16(x)             (x) 

What do they mean? is it function inside macro?

[salem c]

gcc -E is your friend.
It just runs the pre-processor step, so all #includes and #defines are fully processed.

$ cat bar.c
#define tcp_sndbuf(pcb)          (TCPWND16((pcb)->snd_buf))
#define TCPWND16(x)             (x) 

int main()
{
    int foo;
    tcp_sndbuf(foo);
    return 0;
}

$ gcc -E bar.c
# 1 "bar.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "bar.c"



int main()
{
    int foo;
    (((foo)->snd_buf));
    return 0;
}

All TCPWND16 does is put parentheses around the macro parameter.
All tcp_sndbuf does is append ->snd_buf to the now parenthesised parameter, then put parentheses around the whole lot.

[jonnin]

macros can be functions.
here, it says that tcp_sndbuf is the 'function', with a parameter called pcb.
the other one does not do anything and is probably a hack to make something that no longer exists work in old code(?).

Macros are best avoided unless there is no other way; you have lost type safety (what is the type of pcb?) and get weird errors (try tcp_sndbuf("fubar") and look at the confusing error message) and they are prone to creating bugs and causing problems. There really is no reason not to do this some other way.

[George P]

That is the advantage and disadvantage of using macros. You can do virtually anything programmatic using a macro. Since macros are mere text replacement by the preprocessor they aren't type safe and can end up making maintenance a PITA.

https://stackoverflow.com/questions/1351051/problem-with-macros

The C++ stdlib has lambdas that help correct many of the issues with macros.

https://en.cppreference.com/w/cpp/language/lambda

[Jonathan100]

so in tcp_sndbuf(pcb), pcb argument needs to be a pointer to struct
(unless what is the point of pointing to attribute: (pcb)->snd_buf)?

[salem c]

> so in tcp_sndbuf(pcb), pcb argument needs to be a pointer to struct
Yes it does.

After all the substitution, the result still needs to make sense in the context that it's used in.

[Jonathan100]

OK i get it. I hope you can help me with yet another unclear DEFINE phrase:

/**
* members common to struct tcp_pcb and struct tcp_listen_pcb
*/

#define TCP_PCB_COMMON(type) \
  type *next; /* for the linked list */ \
  void *callback_arg; \
  TCP_PCB_EXTARGS \
  enum tcp_state state; /* TCP state */ \
  u8_t prio; \
  /* ports are in host byte order */ \
  u16_t local_port

specifically, i didn't understand backslash use
How is this DEFINE is structured? why it is uses ; ?

[seeplus]

\ just means line continuation when used as the last char on a line.

A #define needs to be on one line so to split it across multiple lines for ease of reading \ is used as a line continuation so that effectively as far as the preprocessor is concerned this is all just one line.

[Jonathan100]

I see, ok, so why DEFINE line is so long and contain ;?

we have a macro: MACRO TCP_PCB_COMMON(type)

What is the definition of that?

[AbstractionAnon]

so why DEFINE line is so long and contain ;

The DEFINE contains 5 statements and comments.
That makes it lengthy;
The author of the macro decided that the 5 lines occurred together often enough that they should be substituted by a single line or that it was important that the 5 lines always occur together.

Note that line 5 appears to be another macro which is expanded in place.
We can't know the full expansion without knowing the expansion of TCP_PCB_EXTARGS.

What is the definition of that?

The definition of the macro is exactly what is shown.
Note that the argument type is used on line 3.
Note that line 9 does not have a ; therefore one is needed after invoking the macro;

As Salem C suggested above, your compiler option to display macro expansions is your friend. The option will show you exactly what the preprocessor sees.

[Jonathan100]

Thanks :)