In my code, I call functions from the string.h library (specifically, strcmp()), but I forgot to add the string library. Even without including the string library, my code compiles and runs properly as if I had included the string library.
What is happening?
A header file (e.g., string.h) only includes function declarations: that is, the return type and types and number of parameters.
The functions are defined in the C library (which is linked against your code by default), so they are are available to you whether or not you #include <string.h>. However, if you fail to #include the header file, the compiler might warn you about missing function declarations, and it can also cause problems if the return type of a function is other than int (the default for functions not otherwised declared in C).
Starting with the 1999 ISO C standard, calling a function with no visible declaration is a constraint violation, requiring a diagnostic. The diagnostic may be a non-fatal warning.
Some C compilers do not enforce the newer C99 rules by default. For example, until recently gcc's default behavior was -std=gnu89, which supports the 1989/1990 C standard with GNU-specific extensions.
Under the older rules, if you call a function with no visible declaration, an implicit declaration is created assuming that the function returns int and expects the number and types of arguments you've passed it. If that assumption is incorrect, the call's behavior is undefined. It happens that strcmp() returns an int, so if the compiler accepts the call you'll probably get away with it.
You should find out how to get your compiler to at least warn you about calls to undeclared functions. Once you've confirmed that it will do so, you should add the requires #include <string.h> to your code.
Note that the #include directive only includes the header file that declares strcmp and other standard functions, not the library. The definitions of those functions (the code that implements them) is commonly included as part of the C standard library. Linking your program to the standard library is handled by the linker, not by the compiler, and it's usually done implicitly (because you asked to compile and link a C program). (The math library, depending on the implementation, might not be linked automatically, but the implementation of the string functions almost always is.)
The string library is in fact included if you include the stdio library. But the stdio does not include the string library(not directly)
By default compiler include all necessary header file and program will successfully run.
Which Compiler you are using ?
you should use C99 compiler with -strict flags and -error flags then compiler will give you error if you call a function without including header file..
Error will look like this
implicit declaration of strcmp() found
Related
As a beginner in C I was experimenting something, so I excluded the standard library <stdio.h> and still there is no error. Can someone explain it?
Here is the sample code:
main()
{
printf("hello, world!\n");
}
This program is working same with or without library. Why?
TL;DR -- You're excluding the header file, not the standard library.
If you exclude the header file where the function is having a forward declaration, you'll receive a warning for sure mentioning the "implicit declaration" of the function.
In that case, (invalid as per the latest standards), the function will be assumed to return int and no check on the number of parameters passed will be there.
However, by default, the generated object file from your source is linked with the default C library libc which has the function definition present. In this case, the function return type matches the implicit case, so linker happily links the object files together.
So, it successfully finishes the linking and works same.
That said, main() should be int main(void), at least to conform to the standards.
While it is recommended to include the standard header files when appropriate, it is not necessarily required. The default return value for a function is int (which you ignore in your code and is perfectly allowable). The notion of function prototypes was added to the language well after there was a substantial body of existing programs and so in order to not break them, function prototypes are optional and the default is to have no prototype which means there is no compiler validation that the types of arguments are correct.
Your program is able to link properly against the standard library and execute correctly.
"Hello World!" is a very simple program, so it would be a mistake to rely on this behavior for anything substantial.
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.
When I am going through a code snippet I have seen some functions like
#include <stdio.h>
int main() {
printf( "Upper case of a is %c\n", toupper('a'));
printf( "Upper case of 9 is %c\n", toupper('9'));
printf( "Upper case of g is %c\n", toupper('g'));
return 0;
}
being used in the source file without any header file being included.
So is there any default header file that gets added to source when compiling. I am using GNU C.
Please don't mind if the syntax of the function is wrong as that is not the important point.
No, there are no implicit #include directives.
What you're probably running into is that, prior to the 1999 ISO C standard, C permitted functions to be called with no visible declaration. The compiler would assume that the called function returns int and takes arguments compatible with the (promoted) arguments passed in the call.
gcc by default supports the 1990 ISO C standard plus GNU-specific extensions.
If you compile with something like gcc -std=c99 -pedantic, you'll get warnings about calls to functions with no visible declarations. (Use -std=gnu99 if you need GNU-specific extensions as well.)
Calling undeclared functions was a bad idea even before the 1999 standard. You should correct your code so there's a visible declaration (probably via a #include for the appropriate header) for each function you call or otherwise refer to.
Your original question asked about toUppercase, which is not a standard function; it may or may not be defined somewhere else.
Your revised question uses toupper, which is a standard C function declared in <ctype.h> and defined in the standard C library.
It's not surprising that you can get away with calling toupper with no visible declaration -- but you should still add
#include <ctype.h>
to the top of your source file.
Before you do that, try compiling with gcc -std=c99; you should get a warning.
One more thing: It's important to keep in mind that headers and libraries are two different things.
Headers, like <stdio.h> and <stdlib.h> are generally text files containing just declarations of functions and other entities specified by the C standard library.
Libraries, which have system-specific names like, for example, libc.so, contain the actual executable code that implements those functions.
Headers are handled by the compiler; libraries are handled by the linker.
There are generally no default headers; every header you use has to be explicitly #included, either directly or indirectly. libc.so (or whatever it's called) is typically linked by default; you don't have to specify it. (Though for the functions declared in <math.h>, you often have to specify -lm to link the corresponding library.)
As forum people are asking not to discuss in comments so i have no other option than replying to " Keith Thompson" last post. I am not sure what you meant by c standard library. look here
C standard library
It clearly says 27 header files are part of standard library and stdlib.h is one of them. See my point is not to argue with you. I am trying to have clarity in mind. You are saying something like libc.so as standard library but the wikipedia clearly states something else and now i am totally confused.
When I create a simple C program in Visual Studio 2010,
http://debugmode.net/2012/02/06/how-to-write-and-run-a-c-program-in-visual-studio-2010/
I remove the "#include < stdio.h > ",
My program still runs successfully, I could not understand how is it possible?
Any help is appreciated.
Regards,
The stdio.h header isn't strictly required unless you use functions declared in it, such as the following:
http://www.acm.uiuc.edu/webmonkeys/book/c_guide/2.12.html
Further looking at the code I believe the default behaviour when you don't have a prototype is to assume an int return type and to derive the types of parameters from the types of arguments which will work in this particular case. But it's generally a bad practice and should be avoided.
If that passed through the compiler / linker without any warnings you may want to check your environment settings. It's easy to forget to include an header and it can cause a lot of unintended and hard to track down side effects if you don't notice it.
The primary purpose of including standard header files is to include the declarations of standard functions into your source file.
However, the original standard C language (C89/90) did not require functions to be declared before they are called (aside from variadic functions, which have to be pre-declared with prototype to avoid undefined behavior). For this reason, as long as we are talking about non-variadic function calls, it is perfectly possible to write a correct program without pre-declaring standard functions, i.e. without including standard header files.
For example, calling strcmp function with two char * arguments is perfectly legal in C89/90 without pre-declaring strcmp. Meanwhile, printf has to be pre-declared with prototype, if you want your program to remain a valid C program with defined behavior.
This header file provides prototypes for a number of common functions and macros.
If you don't call any of those functions or macros, then you don't need it. If you do call them, it can still work as long as you are linking with the right libraries. But you could get some compiler errors or warnings if the compiler doesn't have those definitions.
#include < stdio.h >
It is a header file known as standard input output file. The input,output funcation are written in this file. funcations like printf,scanf etc.
Refere this http://computer.howstuffworks.com/c2.htm
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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.