Related
I have seen many programs consisting of structures like the one below
typedef struct
{
int i;
char k;
} elem;
elem user;
Why is it needed so often? Any specific reason or applicable area?
As Greg Hewgill said, the typedef means you no longer have to write struct all over the place. That not only saves keystrokes, it also can make the code cleaner since it provides a smidgen more abstraction.
Stuff like
typedef struct {
int x, y;
} Point;
Point point_new(int x, int y)
{
Point a;
a.x = x;
a.y = y;
return a;
}
becomes cleaner when you don't need to see the "struct" keyword all over the place, it looks more as if there really is a type called "Point" in your language. Which, after the typedef, is the case I guess.
Also note that while your example (and mine) omitted naming the struct itself, actually naming it is also useful for when you want to provide an opaque type. Then you'd have code like this in the header, for instance:
typedef struct Point Point;
Point * point_new(int x, int y);
and then provide the struct definition in the implementation file:
struct Point
{
int x, y;
};
Point * point_new(int x, int y)
{
Point *p;
if((p = malloc(sizeof *p)) != NULL)
{
p->x = x;
p->y = y;
}
return p;
}
In this latter case, you cannot return the Point by value, since its definition is hidden from users of the header file. This is a technique used widely in GTK+, for instance.
UPDATE Note that there are also highly-regarded C projects where this use of typedef to hide struct is considered a bad idea, the Linux kernel is probably the most well-known such project. See Chapter 5 of The Linux Kernel CodingStyle document for Linus' angry words. :) My point is that the "should" in the question is perhaps not set in stone, after all.
It's amazing how many people get this wrong. PLEASE don't typedef structs in C, it needlessly pollutes the global namespace which is typically very polluted already in large C programs.
Also, typedef'd structs without a tag name are a major cause of needless imposition of ordering relationships among header files.
Consider:
#ifndef FOO_H
#define FOO_H 1
#define FOO_DEF (0xDEADBABE)
struct bar; /* forward declaration, defined in bar.h*/
struct foo {
struct bar *bar;
};
#endif
With such a definition, not using typedefs, it is possible for a compiland unit to include foo.h to get at the FOO_DEF definition. If it doesn't attempt to dereference the 'bar' member of the foo struct then there will be no need to include the "bar.h" file.
Also, since the namespaces are different between the tag names and the member names, it is possible to write very readable code such as:
struct foo *foo;
printf("foo->bar = %p", foo->bar);
Since the namespaces are separate, there is no conflict in naming variables coincident with their struct tag name.
If I have to maintain your code, I will remove your typedef'd structs.
From an old article by Dan Saks (http://www.ddj.com/cpp/184403396?pgno=3):
The C language rules for naming
structs are a little eccentric, but
they're pretty harmless. However, when
extended to classes in C++, those same
rules open little cracks for bugs to
crawl through.
In C, the name s appearing in
struct s
{
...
};
is a tag. A tag name is not a type
name. Given the definition above,
declarations such as
s x; /* error in C */
s *p; /* error in C */
are errors in C. You must write them
as
struct s x; /* OK */
struct s *p; /* OK */
The names of unions and enumerations
are also tags rather than types.
In C, tags are distinct from all other
names (for functions, types,
variables, and enumeration constants).
C compilers maintain tags in a symbol
table that's conceptually if not
physically separate from the table
that holds all other names. Thus, it
is possible for a C program to have
both a tag and an another name with
the same spelling in the same scope.
For example,
struct s s;
is a valid declaration which declares
variable s of type struct s. It may
not be good practice, but C compilers
must accept it. I have never seen a
rationale for why C was designed this
way. I have always thought it was a
mistake, but there it is.
Many programmers (including yours
truly) prefer to think of struct names
as type names, so they define an alias
for the tag using a typedef. For
example, defining
struct s
{
...
};
typedef struct s S;
lets you use S in place of struct s,
as in
S x;
S *p;
A program cannot use S as the name of
both a type and a variable (or
function or enumeration constant):
S S; // error
This is good.
The tag name in a struct, union, or
enum definition is optional. Many
programmers fold the struct definition
into the typedef and dispense with the
tag altogether, as in:
typedef struct
{
...
} S;
The linked article also has a discussion about how the C++ behavior of not requireing a typedef can cause subtle name hiding problems. To prevent these problems, it's a good idea to typedef your classes and structs in C++, too, even though at first glance it appears to be unnecessary. In C++, with the typedef the name hiding become an error that the compiler tells you about rather than a hidden source of potential problems.
Using a typedef avoids having to write struct every time you declare a variable of that type:
struct elem
{
int i;
char k;
};
elem user; // compile error!
struct elem user; // this is correct
One other good reason to always typedef enums and structs results from this problem:
enum EnumDef
{
FIRST_ITEM,
SECOND_ITEM
};
struct StructDef
{
enum EnuumDef MyEnum;
unsigned int MyVar;
} MyStruct;
Notice the typo in EnumDef in the struct (EnuumDef)? This compiles without error (or warning) and is (depending on the literal interpretation of the C Standard) correct. The problem is that I just created an new (empty) enumeration definition within my struct. I am not (as intended) using the previous definition EnumDef.
With a typdef similar kind of typos would have resulted in a compiler errors for using an unknown type:
typedef
{
FIRST_ITEM,
SECOND_ITEM
} EnumDef;
typedef struct
{
EnuumDef MyEnum; /* compiler error (unknown type) */
unsigned int MyVar;
} StructDef;
StrructDef MyStruct; /* compiler error (unknown type) */
I would advocate ALWAYS typedef'ing structs and enumerations.
Not only to save some typing (no pun intended ;)), but because it is safer.
Linux kernel coding style Chapter 5 gives great pros and cons (mostly cons) of using typedef.
Please don't use things like "vps_t".
It's a mistake to use typedef for structures and pointers. When you see a
vps_t a;
in the source, what does it mean?
In contrast, if it says
struct virtual_container *a;
you can actually tell what "a" is.
Lots of people think that typedefs "help readability". Not so. They are useful only for:
(a) totally opaque objects (where the typedef is actively used to hide what the object is).
Example: "pte_t" etc. opaque objects that you can only access using the proper accessor functions.
NOTE! Opaqueness and "accessor functions" are not good in themselves. The reason we have them for things like pte_t etc. is that there really is absolutely zero portably accessible information there.
(b) Clear integer types, where the abstraction helps avoid confusion whether it is "int" or "long".
u8/u16/u32 are perfectly fine typedefs, although they fit into category (d) better than here.
NOTE! Again - there needs to be a reason for this. If something is "unsigned long", then there's no reason to do
typedef unsigned long myflags_t;
but if there is a clear reason for why it under certain circumstances might be an "unsigned int" and under other configurations might be "unsigned long", then by all means go ahead and use a typedef.
(c) when you use sparse to literally create a new type for type-checking.
(d) New types which are identical to standard C99 types, in certain exceptional circumstances.
Although it would only take a short amount of time for the eyes and brain to become accustomed to the standard types like 'uint32_t', some people object to their use anyway.
Therefore, the Linux-specific 'u8/u16/u32/u64' types and their signed equivalents which are identical to standard types are permitted -- although they are not mandatory in new code of your own.
When editing existing code which already uses one or the other set of types, you should conform to the existing choices in that code.
(e) Types safe for use in userspace.
In certain structures which are visible to userspace, we cannot require C99 types and cannot use the 'u32' form above. Thus, we use __u32 and similar types in all structures which are shared with userspace.
Maybe there are other cases too, but the rule should basically be to NEVER EVER use a typedef unless you can clearly match one of those rules.
In general, a pointer, or a struct that has elements that can reasonably be directly accessed should never be a typedef.
It turns out that there are pros and cons. A useful source of information is the seminal book "Expert C Programming" (Chapter 3). Briefly, in C you have multiple namespaces: tags, types, member names and identifiers. typedef introduces an alias for a type and locates it in the tag namespace. Namely,
typedef struct Tag{
...members...
}Type;
defines two things. 1) Tag in the tag namespace and 2) Type in the type namespace. So you can do both Type myType and struct Tag myTagType. Declarations like struct Type myType or Tag myTagType are illegal. In addition, in a declaration like this:
typedef Type *Type_ptr;
we define a pointer to our Type. So if we declare:
Type_ptr var1, var2;
struct Tag *myTagType1, myTagType2;
then var1,var2 and myTagType1 are pointers to Type but myTagType2 not.
In the above-mentioned book, it mentions that typedefing structs are not very useful as it only saves the programmer from writing the word struct. However, I have an objection, like many other C programmers. Although it sometimes turns to obfuscate some names (that's why it is not advisable in large code bases like the kernel) when you want to implement polymorphism in C it helps a lot look here for details. Example:
typedef struct MyWriter_t{
MyPipe super;
MyQueue relative;
uint32_t flags;
...
}MyWriter;
you can do:
void my_writer_func(MyPipe *s)
{
MyWriter *self = (MyWriter *) s;
uint32_t myFlags = self->flags;
...
}
So you can access an outer member (flags) by the inner struct (MyPipe) through casting. For me it is less confusing to cast the whole type than doing (struct MyWriter_ *) s; every time you want to perform such functionality. In these cases brief referencing is a big deal especially if you heavily employ the technique in your code.
Finally, the last aspect with typedefed types is the inability to extend them, in contrast to macros. If for example, you have:
#define X char[10] or
typedef char Y[10]
you can then declare
unsigned X x; but not
unsigned Y y;
We do not really care for this for structs because it does not apply to storage specifiers (volatile and const).
I don't think forward declarations are even possible with typedef. Use of struct, enum, and union allow for forwarding declarations when dependencies (knows about) is bidirectional.
Style:
Use of typedef in C++ makes quite a bit of sense. It can almost be necessary when dealing with templates that require multiple and/or variable parameters. The typedef helps keep the naming straight.
Not so in the C programming language. The use of typedef most often serves no purpose but to obfuscate the data structure usage. Since only { struct (6), enum (4), union (5) } number of keystrokes are used to declare a data type there is almost no use for the aliasing of the struct. Is that data type a union or a struct? Using the straightforward non-typdefed declaration lets you know right away what type it is.
Notice how Linux is written with strict avoidance of this aliasing nonsense typedef brings. The result is a minimalist and clean style.
Let's start with the basics and work our way up.
Here is an example of Structure definition:
struct point
{
int x, y;
};
Here the name point is optional.
A Structure can be declared during its definition or after.
Declaring during definition
struct point
{
int x, y;
} first_point, second_point;
Declaring after definition
struct point
{
int x, y;
};
struct point first_point, second_point;
Now, carefully note the last case above; you need to write struct point to declare Structures of that type if you decide to create that type at a later point in your code.
Enter typedef. If you intend to create new Structure ( Structure is a custom data-type) at a later time in your program using the same blueprint, using typedef during its definition might be a good idea since you can save some typing moving forward.
typedef struct point
{
int x, y;
} Points;
Points first_point, second_point;
A word of caution while naming your custom type
Nothing prevents you from using _t suffix at the end of your custom type name but POSIX standard reserves the use of suffix _t to denote standard library type names.
The name you (optionally) give the struct is called the tag name and, as has been noted, is not a type in itself. To get to the type requires the struct prefix.
GTK+ aside, I'm not sure the tagname is used anything like as commonly as a typedef to the struct type, so in C++ that is recognised and you can omit the struct keyword and use the tagname as the type name too:
struct MyStruct
{
int i;
};
// The following is legal in C++:
MyStruct obj;
obj.i = 7;
typedef will not provide a co-dependent set of data structures. This you cannot do with typdef:
struct bar;
struct foo;
struct foo {
struct bar *b;
};
struct bar {
struct foo *f;
};
Of course you can always add:
typedef struct foo foo_t;
typedef struct bar bar_t;
What exactly is the point of that?
A>
a typdef aids in the meaning and documentation of a program by allowing creation of more meaningful synonyms for data types. In addition, they help parameterize a program against portability problems (K&R, pg147, C prog lang).
B>
a structure defines a type. Structs allows convenient grouping of a collection of vars for convenience of handling (K&R, pg127, C prog lang.) as a single unit
C>
typedef'ing a struct is explained in A above.
D> To me, structs are custom types or containers or collections or namespaces or complex types, whereas a typdef is just a means to create more nicknames.
In 'C' programming language the keyword 'typedef' is used to declare a new name for some object(struct, array, function..enum type). For example, I will use a 'struct-s'.
In 'C' we often declare a 'struct' outside of the 'main' function. For example:
struct complex{ int real_part, img_part }COMPLEX;
main(){
struct KOMPLEKS number; // number type is now a struct type
number.real_part = 3;
number.img_part = -1;
printf("Number: %d.%d i \n",number.real_part, number.img_part);
}
Each time I decide to use a struct type I will need this keyword 'struct 'something' 'name'.'typedef' will simply rename that type and I can use that new name in my program every time I want. So our code will be:
typedef struct complex{int real_part, img_part; }COMPLEX;
//now COMPLEX is the new name for this structure and if I want to use it without
// a keyword like in the first example 'struct complex number'.
main(){
COMPLEX number; // number is now the same type as in the first example
number.real_part = 1;
number.img)part = 5;
printf("%d %d \n", number.real_part, number.img_part);
}
If you have some local object(struct, array, valuable) that will be used in your entire program you can simply give it a name using a 'typedef'.
Turns out in C99 typedef is required. It is outdated, but a lot of tools (ala HackRank) use c99 as its pure C implementation. And typedef is required there.
I'm not saying they should change (maybe have two C options) if the requirement changed, those of us studing for interviews on the site would be SOL.
At all, in C language, struct/union/enum are macro instruction processed by the C language preprocessor (do not mistake with the preprocessor that treat "#include" and other)
so :
struct a
{
int i;
};
struct b
{
struct a;
int i;
int j;
};
struct b is expended as something like this :
struct b
{
struct a
{
int i;
};
int i;
int j;
}
and so, at compile time it evolve on stack as something like:
b:
int ai
int i
int j
that also why it's dificult to have selfreferent structs, C preprocessor round in a déclaration loop that can't terminate.
typedef are type specifier, that means only C compiler process it and it can do like he want for optimise assembler code implementation. It also dont expend member of type par stupidly like préprocessor do with structs but use more complex reference construction algorithm, so construction like :
typedef struct a A; //anticipated declaration for member declaration
typedef struct a //Implemented declaration
{
A* b; // member declaration
}A;
is permited and fully functional. This implementation give also access to compilator type conversion and remove some bugging effects when execution thread leave the application field of initialisation functions.
This mean that in C typedefs are more near as C++ class than lonely structs.
Why do these two struct definitions compile fine:
struct foo {
int a;
} __attribute__((packed));
typedef struct __attribute__((packed)) {
int a;
} bar;
While this one gives a warning:
typedef struct {
int a;
} baz __attribute__((packed));
warning: ‘packed’ attribute ignored [-Wattributes]
And this one gives an error and a warning:
typedef struct qux __attribute__((packed)) {
int a;
} qux;
error: expected identifier or ‘(’ before ‘{’ token
warning: data definition has no type or storage class [enabled by default]
As a novice C programmer, the fact that the last two definitions don't work seems like a fairly arbitrary choice on the parts of the language designers/compiler writers. Is there a reason for this? I'm using gcc 4.7.3.
typedef struct __attribute__((packed)) {
int a;
} bar;
vs
typedef struct {
int a;
} baz __attribute__((packed));
In the first you said "consider this anonymous structure, with the attribute packed, that has a member a. then create an alias for this structure, and name that 'bar'". The normal syntax for describing a struct without doing it anonymously and having to typedef it is
struct bar { int a; };
in the second declaration you said "consider this anonymous structure, that has a member a. then create an alias for this structure, and name that 'baz who is packed'".
It doesn't work because you're trying to apply the attribute to the alias in a typedef, not to the structure definition.
typedef <entity> <alias>;
I would advise putting on a hit on whomever suggested you use the "typedef" syntax for describing structs :)
The reason that it works in the first two instances is because __attribute__((packed)) can only be applied to a struct, union or enum. In the first example, you're declaring a struct called foo, in the second you are declaring a struct called bar. In this second example, typedef converts a declaration of a variable into a declaration of a type.
Your third example is declaring a variable called baz and trying to declare it as packed. Since packing info is attached to the type, not an instance, this makes no sense, and the compiler ignores it.
Here are details on how attributes work in gcc.
You really shouldn't be even using packed unless you know exactly what you are doing. For one thing, __attribute__ is not standard C. If is a gcc extension and so won't work with any other compiler.
For another, there are very few situations where it is actually needed. In your code above, for instance, it is a no-op even in the first two instances as packing removes space between members, and you've only got one member in each struct. The reason that packing structures isn't the default is because common datatypes just work better when aligned on a certain boundary. For instance, access to an int will perform better if aligned on a memory location that is a multiple of four. See the wiki entry on data structure alignment for more information.
The keyword __attribute__((packed)) applies to struct.
In
typedef struct {
int a;
} baz __attribute__((packed));
typedef associates baz with the struct - the attributes comes after, and applies to nothing - gcc ignores it. To fix this, allow typedef to associate the whole struct declaration, including the attribute, by placing baz after the attribute:
typedef struct {
int a;
} __attribute__((packed)) baz;
In the 2nd example, the struct declaration is wrong
struct qux __attribute__((packed)) {
int a;
}
as quz should appear after the attribute:
struct __attribute__((packed)) qux {
int a;
}
It is usually better to let the compiler optimizes the structure and align the inner elements in memory in a way the CPU processes them more efficiently.
Packing a structure, however, may be important at times when building a data structure that has to be packed, in order to cope, for instance, with a driver demands.
This is a rather odd question because it's all very well explained in the manual. However to there isn't anything arbitrary about it, you're just not exploring the complete use of this built-in keyword:
__attribute__(()) is a general purpose gcc extension. There are function attributes, type attributes and variable attributes.
Where you place the word __attribute__ determines what it applies to. You could potentially have several __attribute__s in a given declaration so it's not arbitrary to make some of it syntactically unacceptable for easier internal parsing.
I tried to compile something like:
struct A
{ int a;
struct B
{ int c;
};
};
Now when I compile this code the compiler gives me a warning message that:
declaration does not declare anything [enabled by default]
I know that I have not defined any instance of struct B. That will mean that I shall not be able to access variable c. Still compiler compiles this code with a warning. What's the whole point ? Why does not the compiler give a compilation error instead ?
ADDED Info:
The size of the struct A is equal to the size of int on my machine!!
Because you can do this:
struct A
{ int a;
struct B
{ int c;
};
};
int main()
{
struct A a = {1};
struct B b = {2};
return a.a + b.c;
}
Note:
you need a semicolon after declaring B, which your code is missing
this isn't particularly useful, but I suppose it might serve some documentary purpose (ie,to suggest a relationship or grouping between types)
in C++, the second variable would have type A::B, but C doesn't have the same scoping rules (all structs just belong to the global struct namespace, in effect)
As to the motivation for allowing it ...
struct Outer {
struct {
int b;
} anon;
/* this ^ anonymous struct can only be declared inside Outer,
because there's no type name to declare anon with */
struct Inner {
int c;
} named;
/* this ^ struct could be declared outside, but why force it to be
different than the anonymous one? */
struct Related {
double d;
};
/* oh no we have no member declared immediately ... should we force this
declaration to be outside Outer now? */
struct Inner * (*function_pointer)(struct Related *);
/* no member but we are using it, now can it come back inside? */
struct Related excuse;
/* how about now? */
};
Once you've allowed nested type declarations like this, I doubt there's any particular motivation to require there be a member of that type right away.
It's legal (but extremely bad style) to do:
struct A {
int a;
struct B {
int c;
};
};
struct B B_instance;
struct A A_instance;
And the compiler doesn't know about the later variables that use the struct types, so it really should not error out.
Generally, a warning means the code likely does not do what you intended but is legal in the language. The compiler is saying, “This is likely not what you really wanted to do, but I must allow you to do it because the language says it is allowed.” The compiler cannot give you an error for this code because the C standard permits it, so it must be allowed (unless you specifically ask for errors for such things, as by using GCC’s -Werror option to turn warnings into errors).
The C standard does not attempt to define everything that makes sense in a program. For example, these things are legal in C:
3;
if (x) then foo(); else foo();
x = 4*0;
The first statement has no side effects, and its return value is not used. But it is legal in C, since a statement may be just an expression. The second statement just calls foo(), so the if is pointless. In the third statement, multiplying by four is pointless.
It would be extremely difficult to write a C standard that prohibited all things that did not make sense. And it is certainly not worth the effort. So this is part of your answer: When the committee writing the C standard builds the language, do they want to spend a lot of time rewriting the technical specification to exclude things that do not make sense? Sometimes yes, if it seems valuable to avoid something that could cause serious bugs. But much of the time, it is just not worth their time and would complicate the specification unnecessarily.
However, compilers can recognize some of these things and warn you. This helps catch many typographical errors or other mistakes.
On the other hand, sometimes these constructions arise from unusual circumstances. For example, a program may have preprocessor statements that define struct A in different ways when building for different targets or different features. In some of those targets, it may be that the struct B member is not needed in struct A, so it is not declared, but the declaration of struct B (the type, not the object) remains present just because it was easier to write the preprocessor statements that way.
So the compiler needs to permit these things, to avoid interfering with programmers writing a wide variety of programs.
You are, in fact, declaring struct B here, but you are not declaring a variable of that type.
This is a warning, but one you should fix. Perhaps you meant:
struct A
{ int a;
struct B
{
int c;
} c;
};
If I declare and never use variable then gcc will give me a warning.
But if I have a struct with several members and some of those are not used, gcc will not warn about it...
Is there an option (or another method) to check these?
(Of course I can manually delete some entries and try to compile again, but I am looking for this kind of approach).
Thanks
No GCC won't warn about this. Mostly because in the majority case whether or not a member is used can't be determined. A good portion of struct are defined in a header file. This can be used by not just your application but by any other application referencing your .lib or using the same header file. Hence just because the current piece of code being compiled doesn't use the member it doesn't mean that the member is not used by some other piece of code.
Local variables are different. Whether or not they are used easily determined by compiling only the function in question. Hence GCC, and many other compilers, give a warning.
You can legally and portably access the first member of a struct without using its name, simply by casting the struct to the type of the first member.
typedef struct {
int x;
} mystruct;
mystruct s;
*(int*)&s = 3;
You can also non-portably, but with virtually 100% reliability, access any field in the struct without using its name by casting the struct to another struct type with a compatible structure.
typedef struct {
int x;
char y;
} mystruct;
typedef struct {
int a;
char b;
} otherstruct;
mystruct s;
((otherstruct*)&s)->b = 'C';
I'm afraid that this means that neither searching the source for the field's name, nor removing it, are completely reliable.
I have seen many programs consisting of structures like the one below
typedef struct
{
int i;
char k;
} elem;
elem user;
Why is it needed so often? Any specific reason or applicable area?
As Greg Hewgill said, the typedef means you no longer have to write struct all over the place. That not only saves keystrokes, it also can make the code cleaner since it provides a smidgen more abstraction.
Stuff like
typedef struct {
int x, y;
} Point;
Point point_new(int x, int y)
{
Point a;
a.x = x;
a.y = y;
return a;
}
becomes cleaner when you don't need to see the "struct" keyword all over the place, it looks more as if there really is a type called "Point" in your language. Which, after the typedef, is the case I guess.
Also note that while your example (and mine) omitted naming the struct itself, actually naming it is also useful for when you want to provide an opaque type. Then you'd have code like this in the header, for instance:
typedef struct Point Point;
Point * point_new(int x, int y);
and then provide the struct definition in the implementation file:
struct Point
{
int x, y;
};
Point * point_new(int x, int y)
{
Point *p;
if((p = malloc(sizeof *p)) != NULL)
{
p->x = x;
p->y = y;
}
return p;
}
In this latter case, you cannot return the Point by value, since its definition is hidden from users of the header file. This is a technique used widely in GTK+, for instance.
UPDATE Note that there are also highly-regarded C projects where this use of typedef to hide struct is considered a bad idea, the Linux kernel is probably the most well-known such project. See Chapter 5 of The Linux Kernel CodingStyle document for Linus' angry words. :) My point is that the "should" in the question is perhaps not set in stone, after all.
It's amazing how many people get this wrong. PLEASE don't typedef structs in C, it needlessly pollutes the global namespace which is typically very polluted already in large C programs.
Also, typedef'd structs without a tag name are a major cause of needless imposition of ordering relationships among header files.
Consider:
#ifndef FOO_H
#define FOO_H 1
#define FOO_DEF (0xDEADBABE)
struct bar; /* forward declaration, defined in bar.h*/
struct foo {
struct bar *bar;
};
#endif
With such a definition, not using typedefs, it is possible for a compiland unit to include foo.h to get at the FOO_DEF definition. If it doesn't attempt to dereference the 'bar' member of the foo struct then there will be no need to include the "bar.h" file.
Also, since the namespaces are different between the tag names and the member names, it is possible to write very readable code such as:
struct foo *foo;
printf("foo->bar = %p", foo->bar);
Since the namespaces are separate, there is no conflict in naming variables coincident with their struct tag name.
If I have to maintain your code, I will remove your typedef'd structs.
From an old article by Dan Saks (http://www.ddj.com/cpp/184403396?pgno=3):
The C language rules for naming
structs are a little eccentric, but
they're pretty harmless. However, when
extended to classes in C++, those same
rules open little cracks for bugs to
crawl through.
In C, the name s appearing in
struct s
{
...
};
is a tag. A tag name is not a type
name. Given the definition above,
declarations such as
s x; /* error in C */
s *p; /* error in C */
are errors in C. You must write them
as
struct s x; /* OK */
struct s *p; /* OK */
The names of unions and enumerations
are also tags rather than types.
In C, tags are distinct from all other
names (for functions, types,
variables, and enumeration constants).
C compilers maintain tags in a symbol
table that's conceptually if not
physically separate from the table
that holds all other names. Thus, it
is possible for a C program to have
both a tag and an another name with
the same spelling in the same scope.
For example,
struct s s;
is a valid declaration which declares
variable s of type struct s. It may
not be good practice, but C compilers
must accept it. I have never seen a
rationale for why C was designed this
way. I have always thought it was a
mistake, but there it is.
Many programmers (including yours
truly) prefer to think of struct names
as type names, so they define an alias
for the tag using a typedef. For
example, defining
struct s
{
...
};
typedef struct s S;
lets you use S in place of struct s,
as in
S x;
S *p;
A program cannot use S as the name of
both a type and a variable (or
function or enumeration constant):
S S; // error
This is good.
The tag name in a struct, union, or
enum definition is optional. Many
programmers fold the struct definition
into the typedef and dispense with the
tag altogether, as in:
typedef struct
{
...
} S;
The linked article also has a discussion about how the C++ behavior of not requireing a typedef can cause subtle name hiding problems. To prevent these problems, it's a good idea to typedef your classes and structs in C++, too, even though at first glance it appears to be unnecessary. In C++, with the typedef the name hiding become an error that the compiler tells you about rather than a hidden source of potential problems.
Using a typedef avoids having to write struct every time you declare a variable of that type:
struct elem
{
int i;
char k;
};
elem user; // compile error!
struct elem user; // this is correct
One other good reason to always typedef enums and structs results from this problem:
enum EnumDef
{
FIRST_ITEM,
SECOND_ITEM
};
struct StructDef
{
enum EnuumDef MyEnum;
unsigned int MyVar;
} MyStruct;
Notice the typo in EnumDef in the struct (EnuumDef)? This compiles without error (or warning) and is (depending on the literal interpretation of the C Standard) correct. The problem is that I just created an new (empty) enumeration definition within my struct. I am not (as intended) using the previous definition EnumDef.
With a typdef similar kind of typos would have resulted in a compiler errors for using an unknown type:
typedef
{
FIRST_ITEM,
SECOND_ITEM
} EnumDef;
typedef struct
{
EnuumDef MyEnum; /* compiler error (unknown type) */
unsigned int MyVar;
} StructDef;
StrructDef MyStruct; /* compiler error (unknown type) */
I would advocate ALWAYS typedef'ing structs and enumerations.
Not only to save some typing (no pun intended ;)), but because it is safer.
Linux kernel coding style Chapter 5 gives great pros and cons (mostly cons) of using typedef.
Please don't use things like "vps_t".
It's a mistake to use typedef for structures and pointers. When you see a
vps_t a;
in the source, what does it mean?
In contrast, if it says
struct virtual_container *a;
you can actually tell what "a" is.
Lots of people think that typedefs "help readability". Not so. They are useful only for:
(a) totally opaque objects (where the typedef is actively used to hide what the object is).
Example: "pte_t" etc. opaque objects that you can only access using the proper accessor functions.
NOTE! Opaqueness and "accessor functions" are not good in themselves. The reason we have them for things like pte_t etc. is that there really is absolutely zero portably accessible information there.
(b) Clear integer types, where the abstraction helps avoid confusion whether it is "int" or "long".
u8/u16/u32 are perfectly fine typedefs, although they fit into category (d) better than here.
NOTE! Again - there needs to be a reason for this. If something is "unsigned long", then there's no reason to do
typedef unsigned long myflags_t;
but if there is a clear reason for why it under certain circumstances might be an "unsigned int" and under other configurations might be "unsigned long", then by all means go ahead and use a typedef.
(c) when you use sparse to literally create a new type for type-checking.
(d) New types which are identical to standard C99 types, in certain exceptional circumstances.
Although it would only take a short amount of time for the eyes and brain to become accustomed to the standard types like 'uint32_t', some people object to their use anyway.
Therefore, the Linux-specific 'u8/u16/u32/u64' types and their signed equivalents which are identical to standard types are permitted -- although they are not mandatory in new code of your own.
When editing existing code which already uses one or the other set of types, you should conform to the existing choices in that code.
(e) Types safe for use in userspace.
In certain structures which are visible to userspace, we cannot require C99 types and cannot use the 'u32' form above. Thus, we use __u32 and similar types in all structures which are shared with userspace.
Maybe there are other cases too, but the rule should basically be to NEVER EVER use a typedef unless you can clearly match one of those rules.
In general, a pointer, or a struct that has elements that can reasonably be directly accessed should never be a typedef.
It turns out that there are pros and cons. A useful source of information is the seminal book "Expert C Programming" (Chapter 3). Briefly, in C you have multiple namespaces: tags, types, member names and identifiers. typedef introduces an alias for a type and locates it in the tag namespace. Namely,
typedef struct Tag{
...members...
}Type;
defines two things. 1) Tag in the tag namespace and 2) Type in the type namespace. So you can do both Type myType and struct Tag myTagType. Declarations like struct Type myType or Tag myTagType are illegal. In addition, in a declaration like this:
typedef Type *Type_ptr;
we define a pointer to our Type. So if we declare:
Type_ptr var1, var2;
struct Tag *myTagType1, myTagType2;
then var1,var2 and myTagType1 are pointers to Type but myTagType2 not.
In the above-mentioned book, it mentions that typedefing structs are not very useful as it only saves the programmer from writing the word struct. However, I have an objection, like many other C programmers. Although it sometimes turns to obfuscate some names (that's why it is not advisable in large code bases like the kernel) when you want to implement polymorphism in C it helps a lot look here for details. Example:
typedef struct MyWriter_t{
MyPipe super;
MyQueue relative;
uint32_t flags;
...
}MyWriter;
you can do:
void my_writer_func(MyPipe *s)
{
MyWriter *self = (MyWriter *) s;
uint32_t myFlags = self->flags;
...
}
So you can access an outer member (flags) by the inner struct (MyPipe) through casting. For me it is less confusing to cast the whole type than doing (struct MyWriter_ *) s; every time you want to perform such functionality. In these cases brief referencing is a big deal especially if you heavily employ the technique in your code.
Finally, the last aspect with typedefed types is the inability to extend them, in contrast to macros. If for example, you have:
#define X char[10] or
typedef char Y[10]
you can then declare
unsigned X x; but not
unsigned Y y;
We do not really care for this for structs because it does not apply to storage specifiers (volatile and const).
I don't think forward declarations are even possible with typedef. Use of struct, enum, and union allow for forwarding declarations when dependencies (knows about) is bidirectional.
Style:
Use of typedef in C++ makes quite a bit of sense. It can almost be necessary when dealing with templates that require multiple and/or variable parameters. The typedef helps keep the naming straight.
Not so in the C programming language. The use of typedef most often serves no purpose but to obfuscate the data structure usage. Since only { struct (6), enum (4), union (5) } number of keystrokes are used to declare a data type there is almost no use for the aliasing of the struct. Is that data type a union or a struct? Using the straightforward non-typdefed declaration lets you know right away what type it is.
Notice how Linux is written with strict avoidance of this aliasing nonsense typedef brings. The result is a minimalist and clean style.
Let's start with the basics and work our way up.
Here is an example of Structure definition:
struct point
{
int x, y;
};
Here the name point is optional.
A Structure can be declared during its definition or after.
Declaring during definition
struct point
{
int x, y;
} first_point, second_point;
Declaring after definition
struct point
{
int x, y;
};
struct point first_point, second_point;
Now, carefully note the last case above; you need to write struct point to declare Structures of that type if you decide to create that type at a later point in your code.
Enter typedef. If you intend to create new Structure ( Structure is a custom data-type) at a later time in your program using the same blueprint, using typedef during its definition might be a good idea since you can save some typing moving forward.
typedef struct point
{
int x, y;
} Points;
Points first_point, second_point;
A word of caution while naming your custom type
Nothing prevents you from using _t suffix at the end of your custom type name but POSIX standard reserves the use of suffix _t to denote standard library type names.
The name you (optionally) give the struct is called the tag name and, as has been noted, is not a type in itself. To get to the type requires the struct prefix.
GTK+ aside, I'm not sure the tagname is used anything like as commonly as a typedef to the struct type, so in C++ that is recognised and you can omit the struct keyword and use the tagname as the type name too:
struct MyStruct
{
int i;
};
// The following is legal in C++:
MyStruct obj;
obj.i = 7;
typedef will not provide a co-dependent set of data structures. This you cannot do with typdef:
struct bar;
struct foo;
struct foo {
struct bar *b;
};
struct bar {
struct foo *f;
};
Of course you can always add:
typedef struct foo foo_t;
typedef struct bar bar_t;
What exactly is the point of that?
A>
a typdef aids in the meaning and documentation of a program by allowing creation of more meaningful synonyms for data types. In addition, they help parameterize a program against portability problems (K&R, pg147, C prog lang).
B>
a structure defines a type. Structs allows convenient grouping of a collection of vars for convenience of handling (K&R, pg127, C prog lang.) as a single unit
C>
typedef'ing a struct is explained in A above.
D> To me, structs are custom types or containers or collections or namespaces or complex types, whereas a typdef is just a means to create more nicknames.
In 'C' programming language the keyword 'typedef' is used to declare a new name for some object(struct, array, function..enum type). For example, I will use a 'struct-s'.
In 'C' we often declare a 'struct' outside of the 'main' function. For example:
struct complex{ int real_part, img_part }COMPLEX;
main(){
struct KOMPLEKS number; // number type is now a struct type
number.real_part = 3;
number.img_part = -1;
printf("Number: %d.%d i \n",number.real_part, number.img_part);
}
Each time I decide to use a struct type I will need this keyword 'struct 'something' 'name'.'typedef' will simply rename that type and I can use that new name in my program every time I want. So our code will be:
typedef struct complex{int real_part, img_part; }COMPLEX;
//now COMPLEX is the new name for this structure and if I want to use it without
// a keyword like in the first example 'struct complex number'.
main(){
COMPLEX number; // number is now the same type as in the first example
number.real_part = 1;
number.img)part = 5;
printf("%d %d \n", number.real_part, number.img_part);
}
If you have some local object(struct, array, valuable) that will be used in your entire program you can simply give it a name using a 'typedef'.
Turns out in C99 typedef is required. It is outdated, but a lot of tools (ala HackRank) use c99 as its pure C implementation. And typedef is required there.
I'm not saying they should change (maybe have two C options) if the requirement changed, those of us studing for interviews on the site would be SOL.
At all, in C language, struct/union/enum are macro instruction processed by the C language preprocessor (do not mistake with the preprocessor that treat "#include" and other)
so :
struct a
{
int i;
};
struct b
{
struct a;
int i;
int j;
};
struct b is expended as something like this :
struct b
{
struct a
{
int i;
};
int i;
int j;
}
and so, at compile time it evolve on stack as something like:
b:
int ai
int i
int j
that also why it's dificult to have selfreferent structs, C preprocessor round in a déclaration loop that can't terminate.
typedef are type specifier, that means only C compiler process it and it can do like he want for optimise assembler code implementation. It also dont expend member of type par stupidly like préprocessor do with structs but use more complex reference construction algorithm, so construction like :
typedef struct a A; //anticipated declaration for member declaration
typedef struct a //Implemented declaration
{
A* b; // member declaration
}A;
is permited and fully functional. This implementation give also access to compilator type conversion and remove some bugging effects when execution thread leave the application field of initialisation functions.
This mean that in C typedefs are more near as C++ class than lonely structs.