Why doesn’t putchar require a header? - c

Reading this answer which explains the polyglot program on page not found on Stack Overflow I was surprised to read putchar was used because you don't need any #include to use it. This seems to be the case, although en.cppreference.com reference and www.cplusplus.com reference show putchar as defined in the stdio.h header.
How can a function be used (correctly) without having a declaration in C? Or is putchar something inbuilt in compiler (like sizeof operator)?

In c, you can use any function without a declaration.
The compiler then assumes, that the function has a return type of int. The parameters are passed to the function as given. Since there is no function declaration, the compiler cannot verify, if the parameters are correct.
putchar is not builtin into the compiler.
However, since
The function call putchar(c) shall be equivalent to putc(c,stdout).
it might be defined as a macro, e.g.
#define putchar(c) putc(c, stdout)
In this case, you must include stdio.h to have the correct definition for putchar.

Some compilers do weird, non-standard things such as automatically including various common headers. It is possible that the code was compiled on one such compiler.
Otherwise, in the old obsolete C90 standard, you didn't need to have a function prototype visible: if you had not, the compiler would start to assume that the return type was int. Which doesn't make any sense. This nonsense was removed from the C language with the C99 standard.
So the reason the code compiled, was because you used a crappy compiler. There are no guarantees that the code will compile/link or work as predicted.
For example:
int main ()
{
putchar('a');
}
This compiles with gcc as well as gcc -std=c90. But if you compile it as standard C,
gcc -std=c99 -pedantic-errors
you'll get error: implicit declaration of function 'putchar'.

Related

Is it correct to drop #include <stdio.h> in C?

If I use printf, scanf, puts or some other function in C (not C++) and don't write an include line, can it be treated as unspecified or undefined behaviour?
As I remember, C didn't require porotope declaration at all, but it was recommended to have them to allow compiler to make type casts on calling. And prototypes for printf and other such functions are not required still, not sure about custom functions.
PS: This question relates to discussion in comments of https://codegolf.stackexchange.com/a/55989/32091.
Is it correct to drop #include in C?
No, it's not correct. Always include it if you use a stdio.h function like printf.
C has removed implicit declarations (with C99) and the includes are required. The only other alternative is to have a visible prototyped declaration for printf.
Moreover even when C had implicit declarations, implicit declarations are not OK for variable argument functions; so in C89 not adding a stdio.h include and not having a visible prototype (for printf example) is undefined behavior.
For professional development, no.
For codegolfing, it is ok.
If you don't declare a function, the compiler automatically generates one, which may or may not match its real declaration. If it doesn't, it may or may not produce segfault or a software bug. gcc also gives in this case a warning.
No and Yes
stdio.h has an explicit declaration of the functions you want to use, such things are prohibited if it was a C++ compiler (g++ for example).
Since c++ requires explicit declarations of all functions, but any proper C compiler will create an implicit declaration of those functions, compile the code into object file, and when linked with the standard library, it will find a definition of those functions that accidentally will match the implicit declaration, probably gcc will give you a warning.
So if you are writing software that you want to be maintainable and readable, that's not an option to drop it, however for fast prototyping or code challenges, this might not be that important.
Technically you can skip #include in many cases. But for some functions the compiler cannot generate correct function call without prototype. E.g. if a parameter is double and you put 0 - with prototype it will be converted and stored as double value in stack and otherwise there will be int which will produce wrong calculations.

Do I really need to include string.h? [duplicate]

What will happen if I don't include the header files when running a c program? I know that I get warnings, but the programs runs perfectly.
I know that the header files contain function declarations. Therefore when I don't include them, how does the compiler figure it out? Does it check all the header files?
I know that I get warnings, but the programs runs perfectly.
That is an unfortunate legacy of pre-ANSI C: the language did not require function prototypes, so the standard C allows it to this day (usually, a warning can be produced to find functions called without a prototype).
When you call a function with no prototype, C compiler makes assumptions about the function being called:
Function's return type is assumed to be int
All parameters are assumed to be declared (i.e. no ... vararg stuff)
All parameters are assumed to be whatever you pass after default promotions, and so on.
If the function being called with no prototype fits these assumptions, your program will run correctly; otherwise, it's undefined behavior.
Before the 1989 ANSI C standard, there was no way to declare a function and indicate the types of its parameters. You just had to be very careful to make each call consistent with the called function, with no warning from the compiler if you got it wrong (like passing an int to sqrt()). In the absence of a visible declaration, any function you call was assumed to return int; this was the "implicit int" rule. A lot of standard functions do return int, so you could often get away with omitting a #include.
The 1989 ANSI C standard (which is also, essentially, the 1990 ISO C standard) introduced prototypes, but didn't make them mandatory (and they still aren't). So if you call
int c = getchar();
it would actually work, because getchar() returns an int.
The 1999 ISO C standard dropped the implicit int rule, and made it illegal (actually a constraint violation) to call a function with no visible declaration. So if you call a standard function without the required #include, a C99-conforming compiler must issue a diagnostic (which can be just a warning). Non-prototype function declarations (ones that don't specify the types of the arguments) are still legal, but they're considered obsolescent.
(The 2011 ISO C standard didn't change much in this particular area.)
But there's still plenty of code out there that was written for C90 compilers, and most modern compilers still support the older standard.
So if you call a standard function without the required #include, what will probably happen is that (a) the compiler will warn you about the missing declaration, and (b) it will assume that the function returns int and takes whatever number and type(s) of arguments you actually passed it (also accounting for type promotion, such as short to int and float to double). If the call is correct, and if you compiler is lenient, then your code will probably work -- but you'll have one more thing to worry about if it fails, perhaps for some unrelated reason.
Variadic functions like printf are another matter. Even in C89/C90, calling printf with no visible prototype had undefined behavior. A compiler can use an entirely different calling convention for variadic functions, so printf("hello") and puts("hello") might generate quite different code. But again, for compatibility with old code, most compilers use a compatible calling convention, so for example the first "hello world" program in K&R1 will probably still compile and run.
You can also write your own declarations for standard functions; the compiler doesn't care whether it sees a declaration in a standard header or in your own source file. But there's no point in doing so. Declarations have changed subtly from one version of the standard to the next, and the headers that came with your implementation should be the correct ones.
So what will actually happen if you call a standard function without the corresponding #include?
In a typical working environment, it doesn't matter, because with any luck your program won't survive code review.
In principle, any compiler that conforms to C99 or later may reject your program with a fatal error message. (gcc will behave this way with -std=c99 -pedantic-errors) In practice, most compilers will merely print a warning. The call will probably work if the function returns int (or if you ignore the result) and if you get all the argument types correct. If the call is incorrect, the compiler may not be able to print good diagnostics. If the function doesn't return int, the compiler will probably assume that it does, and you'll get garbage results, or even crash your program.
So you can study this answer of mine, follow up by reading the various versions of the C standard, find out exactly which edition of the standard your compiler conforms to, and determine the circumstances in which you can safely omit a #include header -- with the risk that you'll mess something up next time you modify your program.
Or you can pay attention to your compiler's warnings (Which you've enabled with whatever command-line options are available), read the documentation for each function you call, add the required #includes at the top of each source file, and not have to worry about any of this stuff.
First of all: just include them.
If you don't the compiler will use the default prototype for undeclared functions, which is:
int functionName(int argument);
So it will compile, and link if the functions are available. But you will have problems at runtime.
There are a lot of things you can't do if you leave out headers:
(I'm hoping to get some more from the comments since my memory is failing on this ...)
You can't use any of the macros defined in the headers. This can be significant.
The compiler can't check that you are calling functions properly since the headers define their parameters for it.
For compatibility with old program C compilers can compile code calling functions which have not been declared, assuming the parameters and return value is of type int. What can happen? See for example this question: Troubling converting string to long long in C I think it's a great illustration of the problems you can run into if you don't include necessary headers and so don't declare functions you use. What happened to the guy was he tried to use atoll without including stdlib.h where atoll is declared:
char s[30] = { "115" };
long long t = atoll(s);
printf("Value is: %lld\n", t);
Surprisingly, this printed 0, not 115, as expected! Why? Because the compiler didn't see the declaration of atoll and assumed it's return value is an int, and so picked only part of the value left on stack by the function, in other words the return value got truncated.
That's why of the reasons it is recommended to compile your code with -Wall (all warnings on).

What will happen if I don't include header files

What will happen if I don't include the header files when running a c program? I know that I get warnings, but the programs runs perfectly.
I know that the header files contain function declarations. Therefore when I don't include them, how does the compiler figure it out? Does it check all the header files?
I know that I get warnings, but the programs runs perfectly.
That is an unfortunate legacy of pre-ANSI C: the language did not require function prototypes, so the standard C allows it to this day (usually, a warning can be produced to find functions called without a prototype).
When you call a function with no prototype, C compiler makes assumptions about the function being called:
Function's return type is assumed to be int
All parameters are assumed to be declared (i.e. no ... vararg stuff)
All parameters are assumed to be whatever you pass after default promotions, and so on.
If the function being called with no prototype fits these assumptions, your program will run correctly; otherwise, it's undefined behavior.
Before the 1989 ANSI C standard, there was no way to declare a function and indicate the types of its parameters. You just had to be very careful to make each call consistent with the called function, with no warning from the compiler if you got it wrong (like passing an int to sqrt()). In the absence of a visible declaration, any function you call was assumed to return int; this was the "implicit int" rule. A lot of standard functions do return int, so you could often get away with omitting a #include.
The 1989 ANSI C standard (which is also, essentially, the 1990 ISO C standard) introduced prototypes, but didn't make them mandatory (and they still aren't). So if you call
int c = getchar();
it would actually work, because getchar() returns an int.
The 1999 ISO C standard dropped the implicit int rule, and made it illegal (actually a constraint violation) to call a function with no visible declaration. So if you call a standard function without the required #include, a C99-conforming compiler must issue a diagnostic (which can be just a warning). Non-prototype function declarations (ones that don't specify the types of the arguments) are still legal, but they're considered obsolescent.
(The 2011 ISO C standard didn't change much in this particular area.)
But there's still plenty of code out there that was written for C90 compilers, and most modern compilers still support the older standard.
So if you call a standard function without the required #include, what will probably happen is that (a) the compiler will warn you about the missing declaration, and (b) it will assume that the function returns int and takes whatever number and type(s) of arguments you actually passed it (also accounting for type promotion, such as short to int and float to double). If the call is correct, and if you compiler is lenient, then your code will probably work -- but you'll have one more thing to worry about if it fails, perhaps for some unrelated reason.
Variadic functions like printf are another matter. Even in C89/C90, calling printf with no visible prototype had undefined behavior. A compiler can use an entirely different calling convention for variadic functions, so printf("hello") and puts("hello") might generate quite different code. But again, for compatibility with old code, most compilers use a compatible calling convention, so for example the first "hello world" program in K&R1 will probably still compile and run.
You can also write your own declarations for standard functions; the compiler doesn't care whether it sees a declaration in a standard header or in your own source file. But there's no point in doing so. Declarations have changed subtly from one version of the standard to the next, and the headers that came with your implementation should be the correct ones.
So what will actually happen if you call a standard function without the corresponding #include?
In a typical working environment, it doesn't matter, because with any luck your program won't survive code review.
In principle, any compiler that conforms to C99 or later may reject your program with a fatal error message. (gcc will behave this way with -std=c99 -pedantic-errors) In practice, most compilers will merely print a warning. The call will probably work if the function returns int (or if you ignore the result) and if you get all the argument types correct. If the call is incorrect, the compiler may not be able to print good diagnostics. If the function doesn't return int, the compiler will probably assume that it does, and you'll get garbage results, or even crash your program.
So you can study this answer of mine, follow up by reading the various versions of the C standard, find out exactly which edition of the standard your compiler conforms to, and determine the circumstances in which you can safely omit a #include header -- with the risk that you'll mess something up next time you modify your program.
Or you can pay attention to your compiler's warnings (Which you've enabled with whatever command-line options are available), read the documentation for each function you call, add the required #includes at the top of each source file, and not have to worry about any of this stuff.
First of all: just include them.
If you don't the compiler will use the default prototype for undeclared functions, which is:
int functionName(int argument);
So it will compile, and link if the functions are available. But you will have problems at runtime.
There are a lot of things you can't do if you leave out headers:
(I'm hoping to get some more from the comments since my memory is failing on this ...)
You can't use any of the macros defined in the headers. This can be significant.
The compiler can't check that you are calling functions properly since the headers define their parameters for it.
For compatibility with old program C compilers can compile code calling functions which have not been declared, assuming the parameters and return value is of type int. What can happen? See for example this question: Troubling converting string to long long in C I think it's a great illustration of the problems you can run into if you don't include necessary headers and so don't declare functions you use. What happened to the guy was he tried to use atoll without including stdlib.h where atoll is declared:
char s[30] = { "115" };
long long t = atoll(s);
printf("Value is: %lld\n", t);
Surprisingly, this printed 0, not 115, as expected! Why? Because the compiler didn't see the declaration of atoll and assumed it's return value is an int, and so picked only part of the value left on stack by the function, in other words the return value got truncated.
That's why of the reasons it is recommended to compile your code with -Wall (all warnings on).

C program is running without including Header files? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
C program without header
I have been studying C for a long time . but one thing that bothering me is that , today I made a C program and forget to include the stdio.h and conio.h header files I saved the file as kc.c ? when I compiled and run this .c file the output was as I was expecting to be.
but how can a C program could run without using the standard header file?
or I am not aware with the concepts that I am missing hear?
EDIT: the program
int main()
{
int i=12,j=34;
int *pa=&i,*pb=&j;
printf("the value of i and j is %d %d respectively ",*pa,*pb);
getch();
return 0;
}
because I have used the printf() function of STDIO.H header here ,but without including it how can It got compiled and run successfully?
The compiler is allowed to make things work, but is under no obligation to do so.
You are supposed to declare all variable argument list functions before using them; not declaring printf() properly leads to undefined behaviour (and one valid undefined behaviour is to work as expected).
You should be getting warnings about the undeclared functions if you compile in C99 mode (but Turbo C probably doesn't have a C99 mode).
Nit-picking:
[H]ow can a C program could run without using the standard header file?
All programs run without using any headers whatsoever. However, most programs are compiled using the standard headers to declare the standard functions, and the better programs ensure that all functions are declared before they are used (or are defined as static functions before they are used).
C99 requires this, though many compilers will allow errant programs to compile. However, the compilation should produce a diagnostic; the diagnostic may or may not lead to the compilation failing. In practice, it usually doesn't cause the compilation to fail, but it could and with some compilers (GCC, for example) you can force the compiler's hand (e.g. with GCC's -Werror=missing-prototypes -Werror=old-style-definition options).
When the language standard being applied pre-dates ISO C99, C does not require a function to be declared or defined before it is referenced.
However when the compiler encounters such a function call, it simply assumes that the function returns an int and that it takes an indeterminate number and type of parameters. This is called am implicit declaration. If you later declare the function, or call it with a different number of parameters or incompatible parameters, you may get a warning in some compilers that the second call does not match declaration implied by the first, but the ISO C89 standard treats the function as variadic [like printf()] where any number and type of parameters are allowed.
Moreover if the actual return value is not an int, any return value accepted and processed may not make much sense one way or another.
It is bad form to rely on an implicit declaration, and most compilers would issue a warning. If your compiler did not, you need to increase the warning level; those diagnostics are there to help improve your code quality. If you simply ignored the warning (any warning for that matter), then you shouldn't!
In C++ the rules are tighter and failure to declare or define a function before referencing it is an error, since it is necessary to allow function overloading.
A header file is nothing more than a listing of constants, preprocessor macros and function prototypes. The function prototypes tell C what arguments each function takes.
If the compiler sees a function being used without a corresponding prototype or function definition, it generates an implicit declaration of the form int func(). Since C functions are linked solely by name and not by function signature (as is the case with C++), the linker later locates the function definitions in the standard library.

Code still runs without any includes (Bloodshed's Dev-C++)

I am currently learning and experimenting with C and am using Bloodshed's DEV-C++ as an IDE.
Now, I just realized that the following piece of code (as it is...no includes or nothing) compiles and runs :
main ()
{
printf("%d", strlen("hello"));
}
Now, if I'm not mistaken, shouldn't two header files be included in this source for it to work ? stdio.h and string.h...but as you can see, I did not add them and the code still compiled and ran successfully.
My complaint is that I want the compiler to be "strict" because since I'm still learning C, I don't want the code to run if normally it shouldn't.
So, is there any way to prevent Dev-C++ from 'correcting my mistakes' when it comes to includes, ie making it more kinda "strict" ?
C90 had a feature (absent of C99 and C++) called implicit function declaration: when you used a name not declared yet in a function call, the compiler behaved as if
extern int identifier();
had been seen. That feature has been dropped from C99 and most compilers had option to warn about this even before C99 was promulgated.
Even when staying in C90, it is not recommended style to use this. If you have to maintain code making use of this and can't add prototypes, check that:
the functions returns an int (it is the case for printf but the validity is implementation dependent for strlen which returns a size_t which can be int or something else)
the function isn't variadic (it is the case for strlen but not printf)
the type of the arguments is not modified by default argument promotions (char, short, float are) and you must pay attention to cast pointers to void* when needed when the expected type is void*, you have to pay attention to cast NULL to the correct pointer type. (These are the same things you have to pay attention for variadic arguments BTW).
If those conditions aren't met -- and they aren't for any calls in your code -- you enter in the realm of undefined behavior.
I don't know if this actually is a DevC++ issue, but in any case you should consider ditching it. It is no longer being developed and is very buggy. I recommend changing to Code::Blocks, which is better in every way and alows you to use the very latest GCC compiler.
One of the possibilities for 'undefined behaviour' - which you get if you call a variadic function without a visible prototype - is that your code compiles and runs successfully.
If you're using gcc as the underlying compiler then you should be able to pass flags such as -std=c89 -pedantic -Wall -Wextra and get warnings about code such as the snippet that you've posted.

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