Can someone help explain these basic consepts to me please?

Hi,

Im trying to learn C and ive been reading through the tutorials on this site but im getting stuck with a few things so I wonder if anyone would try to help me understand them.

First thing, int main(). From what i understand this line is used to define if there are any command line arguments for the program. Is that its only job? Why is it always int, what about if there are letters in the arguments, would it be char main()? If there are no arguments, would it really matter what is put at the start (int, float etc)?

Secondly, when using scanf, im having a hard time understanding why a & sign is used before the variable. It seems to work without it, is it bad practice?

Finally with the functions, why is the function defined as a prototype above with pretty much the same information inlcuded in the actual function, for example:

int mult ( int x, int y );

Is this really nessesary or is it justgood coding practice?

Thank you for any help you can give.

Quote:

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Originally Posted by spadez

First thing, int main(). From what i understand this line is used to define if there are any command line arguments for the program. Is that its only job? Why is it always int, what about if there are letters in the arguments, would it be char main()? If there are no arguments, would it really matter what is put at the start (int, float etc)?

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int is the return type, and the main function must be defined with that return type (with an exception that you need not be concerned with at the moment). You might want to read the tutorial on command line arguments.

Quote:

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Originally Posted by spadez

Secondly, when using scanf, im having a hard time understanding why a & sign is used before the variable. It seems to work without it, is it bad practice?

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You need to understand a little on pointers to understand why this is so. Whether to use the address of operator depends on context. I suggest that you read the tutorial on pointers.

Quote:

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Originally Posted by spadez

Finally with the functions, why is the function defined as a prototype above with pretty much the same information inlcuded in the actual function, for example:

int mult ( int x, int y );

Is this really nessesary or is it justgood coding practice?

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It is not necessary since you can declare the function as having an unknown number of parameters, but that is generally bad practice.

Quote:

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Originally Posted by spadez

Hi,

Im trying to learn C and ive been reading through the tutorials on this site but im getting stuck with a few things so I wonder if anyone would try to help me understand them.

First thing, int main(). From what i understand this line is used to define if there are any command line arguments for the program. Is that its only job? Why is it always int, what about if there are letters in the arguments, would it be char main()? If there are no arguments, would it really matter what is put at the start (int, float etc)?

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No. The int is the return type of the program (which is required to always be int). The parameters that the program accepts from the command line go inside the parentheses. So in this case int main() doesn't take any command line parameters.
int main( int argc, char* argv[] ) does take command line parameters; argc tells you how many parameters, and argv is an array of strings for each parameter.

Quote:

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Originally Posted by spadez

Secondly, when using scanf, im having a hard time understanding why a & sign is used before the variable. It seems to work without it, is it bad practice?

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Putting & before a variable name returns the address of that variable, which is what scanf() needs in order to change that variable. If you just pass the variable by itself, you're passing a copy of that object and scanf() would be modifying the copy instead of the original.

Quote:

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Originally Posted by spadez

Finally with the functions, why is the function defined as a prototype above with pretty much the same information inlcuded in the actual function, for example:

int mult ( int x, int y );

Is this really nessesary or is it justgood coding practice?

Thank you for any help you can give.

---

Code is compiled from top to bottom, so if a function tries to call mult() before it's declared, the compiler will complain that it doesn't know what mult() is.

1) I think this has sufficiently been covered above. Some implementations allow you to do void main(), and you'll see this in a lot of code, but it's a bad practice, and it's actually against the standard. Your OS expects a return value when it runs the process.

2) Putting the name of a char array "decomposes" into a pointer to the first object, so my_array and &my_array[0] are effectively the same thing - that's probably why it works for you. Pointers / references like this are used to pass strings, so that's why functions require a pointer, and why it works when you specify the name of the array

3) Can't add much...

Hi, thank you for the help.

I know now that main has to be an int, but i dont really understand why. Is it because the program always returns a number? Is this going back to return 0 means the program is completed sucessfully and none zero is an error. What exactly is the porpose of the int main, just to signify the start of the body of the code?.

For part two, am i right in saying that the & sign points to the location of the variable instead of the actual variable itself?

Finally for part three the code is as follows:

Code:

---

#include <stdio.h>

int mult ( int x, int y );

int main()
{
  int x;
  int y;
 
  printf( "Please input two numbers to be multiplied: " );
  scanf( "%d", &x );
  scanf( "%d", &y );
  printf( "The product of your two numbers is %d\n", mult( x, y ) );
  getchar();
}

int mult (int x, int y)
{
  return x * y;
}

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However since it is being defined at the start (because the program executes from the top), would it not be possible to code it like this:

Code:

---

#include <stdio.h>

int mult ( int x, int y );

int main()
{
  int x;
  int y;
 
  printf( "Please input two numbers to be multiplied: " );
  scanf( "%d", &x );
  scanf( "%d", &y );
  printf( "The product of your two numbers is %d\n", mult( x, y ) );
  getchar();
}

int mult
{
  return x * y;
}

---

I have removed a portion of the code because it is already being defined at the start of the code. Is this not possible? It seems like its defining it twice.

Again thank you for your help, im sorry my questions are probably dumb but ive been reading the tutorials on this site and some things are still not clear.

It's not defining it twice, it's declaring it once and then defining it once.
You could get rid of the declaration by moving the definition above main() like this:

Code:

---

#include <stdio.h>

int mult (int x, int y)
{
  return x * y;
}

int main()
{
  int x;
  int y;
 
  printf( "Please input two numbers to be multiplied: " );
  scanf( "%d", &x );
  scanf( "%d", &y );
  printf( "The product of your two numbers is %d\n", mult( x, y ) );
  getchar();
}

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Usually you would put the declaration into a header (.h) file and the definition into a source (.c) file.

Ok, that makes more sence. Is anyone able to help me with the first two questions?

Quote:

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Originally Posted by spadez

Hi, thank you for the help.

I know now that main has to be an int, but i dont really understand why. Is it because the program always returns a number? Is this going back to return 0 means the program is completed sucessfully and none zero is an error. What exactly is the porpose of the int main, just to signify the start of the body of the code?.

.

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every function is called by another function.but main is a special case, the OS calls it and in turn expects it to return some value and that value is 0 if the program has successfully terminated and a non-zero value signal abnormal termination.since 0 is taken as int that's why int main(void) is used.

Quote:

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Originally Posted by spadez

For part two, am i right in saying that the & sign points to the location of the variable instead of the actual variable itself?

.

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yes you're right.'&' points to the variable's address in the memory where it is stored.for eg
int i=10;
then '&i' is not 10 but its address where 'i' is stored.and scanf requires this address to store i's value.

Quote:

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Originally Posted by BEN10

every function is called by another function.but main is a special case, the OS calls it and in turn expects it to return some value and that value is 0 if the program has successfully terminated and a non-zero value signal abnormal termination.since 0 is taken as int that's why int main(void) is used.

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That is simply not true. There are no defined values for success or error and the OS usually doesn't even care what it returns. It's simply a user-defined value that you return.
Now, other programs can pick up this value that you return and interpret it in some way. That's what its purpose usually is. What the numbers means is up to the implementor, the coder - ie YOU.

Quote:

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Originally Posted by BEN10

every function is called by another function.but main is a special case, the OS calls it and in turn expects it to return some value and that value is 0 if the program has successfully terminated and a non-zero value signal abnormal termination.since 0 is taken as int that's why int main(void) is used.

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That is a good simplifeid explanation. Technically speaking, main isn't known to the OS - the executable file contains a value called "start address", which is where the application is supposed to start. In most cases, that will not be main() itself, but some C runtime startup code, that happens inside the startup code, after doing various steps to set the environment so that C code can be run (setting up a stack pointer, sorting out the arguments to main: argc, argv; and such things).

The return address from main is then given to the OS in some way - usually, there is a call to the OS to say "exit now, and my 'result' is X". This allows OTHER programs to figure out if it went OK or not - you can then use the reuslt to say "well, that worked, so lets do the next step" or "that didn't work, so lets tell the user and quit". This is what the compiler does when compiling your code in an IDE - the IDE will know how to compile and link, but it will only do the linking step if you got through the compile step. [Linking is what compines your code with library functions such as printf and the startup code that sets the environment up for the application].

If this is confusing you, please just ignore it.

--
Mats

@Elysia
it means we can use any value as return value.for eg there's no problem in return 1,return 2 etc but it has to be int or it can be float,char also.
@matsp
can you give me some link where it is explained in detail.

It must be int because the standard says so.

Quote:

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Originally Posted by Elysia

It must be int because the standard says so.

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i just executed a simple "hello world" code with different return values(return 2,return 2.0,return '2') and in all the cases the program ran successfully.does the standard also says that returned value has to be 0.or return 1 is also according to the standards.

As mats says, the OS doesn't really care what you return. It's like a message for any program that wants to know the state of your program (ie how it ran), if it ran well or bad.

plz be definite.is return 1 according to the standards or not? i just wanna know it although i'll use return 0.

Quote:

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Originally Posted by BEN10

plz be definite.is return 1 according to the standards or not? i just wanna know it although i'll use return 0.

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no - return value is application defined. your Helloworld program could decide just to return return value of printf

in this case 0 will be failure, while non-negative value indicating success...


But more general situation is when program indicates successful end with 0 and error with negative value... in most cases it will be specified in the man pages for tools where the return value could be of interest for the caller

Quote:

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Originally Posted by BEN10

@Elysia
it means we can use any value as return value.for eg there's no problem in return 1,return 2 etc but it has to be int or it can be float,char also.
@matsp
can you give me some link where it is explained in detail.

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I don't know of any document that describes the "pre-main" startup, but I have worked at [debugged and modified] the "before main" section of code more than once, so I have some idea of how it works. You could try looking at the source code for your C library startup code, if you like - run in the debugger and check out the call-stack at the start of main, and check where that came from.

The return type from main is an int because the C standard says so. If you use something else, the C runtime code that calls main is STILL going to look at the place where integers are returned and expect to find an integer. If you happen to pass a float, then it's likely that your actual value is a "random" number (either e.g. 0x3F800000 instead of 1.0 if int and float are passed in the same register, or whatever happens to be in the register used to return integers, so completely unpredictable from reading the code, but not reliably random in the sense you could use it for random number generation) - same applies to void. If it's char, then you are likely to have random content in the bits covering the upper bits beyond the size of char - of course, that DOES depend on how the value was loaded into the return value register. In some architectures, there is no way to load a byte into a register, in which case it may appear to work. If you then change to another processor architecture, that code will break! That's the annoying bit about "undefined behaviour" - it sometimes appear to work perfectly fine, only to fail when something changes.

--
Mats

Quote:

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Originally Posted by matsp

I don't know of any document that describes the "pre-main" startup, but I have worked at [debugged and modified] the "before main" section of code more than once, so I have some idea of how it works. You could try looking at the source code for your C library startup code, if you like - run in the debugger and check out the call-stack at the start of main, and check where that came from.

Mats

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i'm using visual studio 2005.how can i look at the source code for C library startup code in that?

Set a breakpoint in main, then use the callstack to go the startup code and place a breakpoint there. Now restart the debugger.
Of course, if you don't want to step through it, then you don't need the final breakpoint or restart the debugger.

Quote:

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Originally Posted by BEN10

i'm using visual studio 2005.how can i look at the source code for C library startup code in that?

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Depending on which distribution of Visual Studio it is, you may not have the source code for the C library startup code. If it's a commercial version (not Express Edition), then it probably does have the C library source code. If so, set a breakpoint at the start of main in a project, run in the debugger, and just look at the call-stack (bottom right corner of the screen in the debugger if you haven't moved your debug windows around).

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Mats