Saturday, July 02, 2005

My recent experience of programming in C tell me following things:

1. ALWAYS! ALWAYS!! ALWAYS!!! initialize local variables in C.
pointer, integers, chars, user defined structures whatever it is.
Initialize. Uninitialized variables are especially dangerous in
highly recursive programs because somewhere, at some invocation
the variable assumes the 'bad' value and catastrophic results happen
somewhere down in the call stack. You can initialize local
structured data-types such as array and structures using following syntax.

Message m = { 0 } ;
This makes all the elements of the structure equal to zero.
(Message is a type definition for a struct Message_tag)

int i[5] = { 10 } ;
This will make only first element of array i equal to 10, all other will be
zero. Also note that, globals, statics are always by default initialized to
zero. This is not the case with locals. But little more typing can save you
lot of trouble.

2. Containers should hold your data, followed by local variables and lastly
local pointers. I prefer to allocated as much data on stack as possible. The language
takes care of deallocating memory for you. For example, I have a list of
messages in C. This is not a std::list<> in C++. Following way avoids
many pitfalls.

1. Declare a local variable: "Message m;"
2. Ask the list to make a copy of message to be added and store
the copy in the list. List takes care of creating a copy,
copying data. You have to implement list in that way.
3. When extracting out from the list, extract message in a local
variable again. (Use pass by reference technique) List copies the
data for you in the (reference passed) local variable. List also
takes care of deallocating the copy of message it has.

This means you are not holding your data except one local variable at
a time. This will save you especially when a recursive function is
manipulating lists.


Anonymous said...

Are these struct and array initializers part of the C standard or are they GCC extensions?

Sumant said...

Yes, it is a C standard. You can find more detailed initialization styles supported by C99 standard in section 6.7.8:Initialization Watch out for a post on incompatibilities between C and C++ initializers.

One strange extension supported by gcc is zero sized arrays. ANSI standard forbids zero sized arrays. In gcc you can still
get address of zero sized array!!

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