I'm dealing with some pre-ANSI C syntax. See I have the following function call in one conditional
BPNN *net;
// Some more code
double val;
// Some more code, and then,
if (evaluate_performance(net, &val, 0)) {
But then the function evaluate_performance was defined as follows (below the function which has the above-mentioned conditional):
evaluate_performance(net, err)
BPNN *net;
double *err;
{
How come evaluate_performance was defined with two parameters but called with three arguments? What does the '0' mean?
And, by the way, I'm pretty sure that it isn't calling some other evaluate_performance defined elsewhere; I've greped through all the files involved and I'm pretty sure the we are supposed to be talking about the same evaluate_performance here.
Thanks!
If you call a function that doesn't have a declared prototype (as is the case here), then the compiler assumes that it takes an arbitrary number and types of arguments and returns an int. Furthermore, char and short arguments are promoted to ints, and floats are promoted to doubles (these are called the default argument promotions).
This is considered bad practice in new C code, for obvious reasons -- if the function doesn't return int, badness could ensure, you prevent the compiler from checking that you're passing the correct number and types of parameters, and arguments might get promoted incorrectly.
C99, the latest edition of the C standard, removes this feature from the language, but in practice many compilers still allow them even when operating in C99 mode, for legacy compatibility.
As for the extra parameters, they are technically undefined behavior according to the C89 standard. But in practice, they will typically just be ignored by the runtime.
The code is incorrect, but in a way that a compiler is not required to diagnose. (A C99 compiler would complain about it.)
Old-style function definitions don't specify the number of arguments a function expects. A call to a function without a visible prototype is assumed to return int and to have the number and type(s) of arguments implied by the calls (with narrow integer types being promoted to int or unsigned int, and float being promoted to double). (C99 removed this; your code is invalid under the C99 standard.)
This applies even if the definition precedes the call (an old-style definition doesn't provide a prototype).
If such a function is called incorrectly, the behavior is undefined. In other words, it's entirely the programmer's responsibility to get the arguments right; the compiler won't diagnose errors.
This obviously isn't an ideal situation; it can lead to lots of undetected errors.
Which is exactly why ANSI added prototypes to the language.
Why are you still dealing with old-style function definitions? Can you update the code to use prototypes?
Even standard C compilers are somewhat permissive when it comes to this. Try running the following:
int foo()
{
printf("here");
}
int main()
{
foo(3,4);
return 0;
}
It will, to some's surprise, output "here". The extra arguments are just ignored. Of course, it depends on the compiler.
Overloading doesn't exist in C so having 2 declarations would not work in the same text.
That must be a quite old compiler to not err on this one or it did not find the declaration of the function yet!
Some compilers would not warn/err when calling an undefined function. That's probably what you're running into. I would suggest you look at the command line flags of the compiler to see whether there is a flag you can use to get these warnings because you may actually find quite a few similar mistakes (too many parameters is likely to work just fine, but too few will make use of "undefined" values...)
Note that it is possible to do such (add extra parameters) when using the ellipsis as in printf():
printf(const char *format, ...);
I would imagine that the function had 3 parameters at some point and the last was removed because it was unused and some parts of the code was not corrected as it ought to be. I would remove that 3rd parameter, just in case the stack goes in the wrong order and thus fails to send the correct parameters to the function.
Related
In C when a function is declared like void main(); trying to input an argument to it(as the first and the only argument) doesn't cause a compilation error and in order to prevent it, function can be declared like void main(void);. By the way, I think this also applies to Objective C and not to C++. With Objective C I am referring to the functions outside classes. Why is this? Thanks for reaching out. I imagine it's something like that in Fortran variables whose names start with i, j, k, l, m or n are implicitly of integer type(unless you add an implicit none).
Edit: Does Objective C allow this because of greater compatibility with C, or is it a reason similar to the reason for C having this for having this?
Note: I've kept the mistake in the question so that answers and comments wouldn't need to be changed.
Another note: As pointed out by #Steve Summit and #matt (here), Objective-C is a strict superset of C, which means that all C code is also valid Objective-C code and thus has to show this behavior regarding functions.
Because function prototypes were not a part of pre-standard C, functions could be declared only with empty parentheses:
extern double sin();
All existing code used that sort of notation. The standard would have failed had such code been made invalid, or made to mean “zero arguments”.
So, in standard C, a function declaration like that means “takes an undefined list of zero or more arguments”. The standard does specify that all functions with a variable argument list must have a prototype in scope, and the prototype will end with , ...). So, a function declared with an empty argument list is not a variadic function (whereas printf() is variadic).
Because the compiler is not told about the number and types of the arguments, it cannot complain when the function is called, regardless of the arguments in the call.
In early (pre-ANSI) C, a correct match of function arguments between a function's definition and its calls was not checked by the compiler.
I believe this was done for two reasons:
It made the compiler considerably simpler
C was always designed for separate compilation, and checking consistency across translation units (that is, across multiple source files) is a much harder problem.
So, in those early days, making sure that a function's call(s) matched its definition was the responsibility of the programmer, or of a separate program, lint.
The lax checking of function arguments also made varargs functions like printf possible.
At any rate, in the original C, when you wrote
extern int f();
, you were not saying "f is a function accepting no arguments and returning int". You were simply saying "f is a function returning int". You weren't saying anything about the arguments.
Basically, early C's type system didn't even have a way of recording the parameters expected by a function. And that was especially true when separate compilation came into play, because the linker resolved external symbols based pretty much on their names only.
C++ changed this, of course, by introducing function prototypes. In C++, when you say extern int f();, you are declaring a function that explicitly takes 0 arguments. (Also a scheme of "name mangling" was devised, which among other things let the linker do some consistency checking at link time.)
Now, this was all somewhat of a deficiency in old C, and the biggest change that ANSI C introduced was to adopt C++'s function prototype notation into C. It was slightly different, though: to maintain compatibility, in C saying extern int f(); had to be interpreted as meaning "function returning int and taking unspecified arguments". If you wanted to explicitly say that a function took no arguments, you had to (and still have to) say extern int f(void);.
There was also a new ... notation to explicitly mark a function as taking variable arguments, like printf, and the process of getting rid of "implicit int" in declarations was begun.
All in all it was a significant improvement, although there are still a few holes. In particular, there's still some responsibility placed on the programmer, namely to ensure that accurate function prototypes are always in scope, so that the compiler can check them. See also this question.
Two additional notes: You asked about Objective C, but I don't know anything about that language, so I can't address that point. And you said that for a function without a prototype, "trying to input an argument to it (as the first and the only argument) doesn't cause a compilation error", but in fact, you can pass any number or arguments to such a function, without error.
If "a function" were compiled separately, the mismatch would not be detected, "the function" would return a double that main would treat as an int... In the light of what we have said about how declarations must match definitions this might seems surprising. The reason a mismatch can happen is that if there is no function prototype, a function is implicitly declared by its first appearance in an expression, such as
sum += "the function"(line);
If a name that has not been previously declared occurs in an expression and is followed by a left parenthesis, it is declared by context to be a function name, the function is assumed to return an int, and nothing is assumed about its arguments.
I apologize beforehand for the ambiguous question, but what does this mean?
By the way this is page 73 chapter 4.3 from Brian W. Kernighan and Dennis M. Ritchie's C Programming Language book, 2nd edition.
K&R2 covers the 1989/1990 version of the language. The current ISO C standard, published in 1999 2011, drops the "implicit int" rule, and requires a visible declaration for any function you call. Compilers don't necessarily enforce this by default, but you should be able to request more stringent warnings -- and you definitely should. In well-written new code, the rule is irrelevant (but it is necessary to understand it).
An example: the standard sqrt() function is declared in <math.h>:
double sqrt(double);
If you write a call without the required #include <math.h>:
double x = 64.0;
double y = sqrt(x);
a C90 compiler will assume that sqrt returns int -- and it will generate code to convert the result from int to double. The result will be garbage, or perhaps a crash.
(You could manually declare sqrt yourself, but that's the wrong solution.)
So don't do that. Always include whatever header is required for any function you call. You might get away with calling an undeclared function if it does return int (and if your compiler doesn't enforce strict C99 or C11 semantics, and if a few other conditions are satisfied), but there's no good reason to do so.
Understanding the "implicit int" rule is still useful for understanding the behavior of old or poorly written code, but you should never depend on it in new code.
Function prototypes were introduced into the language late.
Before prototypes, the compiler would assume that every argument passed to every unknown function should be passed as an integer and would assume that the return value was also an integer.
This worked fine for the few cases where it was correct, but meant people had to write programs in an awkward order so that functions would never rely on unknown functions that did not match this expectation.
When prototypes were introduced into C89 (aka ANSI C or ISO C), the prototypes allow the compiler to know exactly what types of arguments are expected and what types of results will be returned.
It is strongly recommended that you use function prototypes for all new code; when working on an entirely old code base, the prototypes might be harmful. (Or, if the code must be compilable on a pre-ANSI C compiler, then you might wish to leave off the prototypes so it can be built on the ancient software. gcc is the only place I've seen this in a long time.)
It's just stating that if the compiler comes across code that calls an unknown function, then it implicitly treats it as if it had already seen a declared prototype of the form int unknown();
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 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).
If "a function" were compiled separately, the mismatch would not be detected, "the function" would return a double that main would treat as an int... In the light of what we have said about how declarations must match definitions this might seems surprising. The reason a mismatch can happen is that if there is no function prototype, a function is implicitly declared by its first appearance in an expression, such as
sum += "the function"(line);
If a name that has not been previously declared occurs in an expression and is followed by a left parenthesis, it is declared by context to be a function name, the function is assumed to return an int, and nothing is assumed about its arguments.
I apologize beforehand for the ambiguous question, but what does this mean?
By the way this is page 73 chapter 4.3 from Brian W. Kernighan and Dennis M. Ritchie's C Programming Language book, 2nd edition.
K&R2 covers the 1989/1990 version of the language. The current ISO C standard, published in 1999 2011, drops the "implicit int" rule, and requires a visible declaration for any function you call. Compilers don't necessarily enforce this by default, but you should be able to request more stringent warnings -- and you definitely should. In well-written new code, the rule is irrelevant (but it is necessary to understand it).
An example: the standard sqrt() function is declared in <math.h>:
double sqrt(double);
If you write a call without the required #include <math.h>:
double x = 64.0;
double y = sqrt(x);
a C90 compiler will assume that sqrt returns int -- and it will generate code to convert the result from int to double. The result will be garbage, or perhaps a crash.
(You could manually declare sqrt yourself, but that's the wrong solution.)
So don't do that. Always include whatever header is required for any function you call. You might get away with calling an undeclared function if it does return int (and if your compiler doesn't enforce strict C99 or C11 semantics, and if a few other conditions are satisfied), but there's no good reason to do so.
Understanding the "implicit int" rule is still useful for understanding the behavior of old or poorly written code, but you should never depend on it in new code.
Function prototypes were introduced into the language late.
Before prototypes, the compiler would assume that every argument passed to every unknown function should be passed as an integer and would assume that the return value was also an integer.
This worked fine for the few cases where it was correct, but meant people had to write programs in an awkward order so that functions would never rely on unknown functions that did not match this expectation.
When prototypes were introduced into C89 (aka ANSI C or ISO C), the prototypes allow the compiler to know exactly what types of arguments are expected and what types of results will be returned.
It is strongly recommended that you use function prototypes for all new code; when working on an entirely old code base, the prototypes might be harmful. (Or, if the code must be compilable on a pre-ANSI C compiler, then you might wish to leave off the prototypes so it can be built on the ancient software. gcc is the only place I've seen this in a long time.)
It's just stating that if the compiler comes across code that calls an unknown function, then it implicitly treats it as if it had already seen a declared prototype of the form int unknown();