C/C++ Programming Help Thread

[Lukeson]

np

This should be possible, shouldn't it? Errormessage: line 130: Constant expression expected.

Line 130:
artist_entry artist_list[num_artists];

I get that error when compiling with visual c++. I have done the same with gcc without problems!

[yaustar]

But headus is right there. The last part in the for loop is executed at the end, therefore it doesn't matter if the increment is prefixed or postfixed. You can easily try that by doing

for (i=0;i<3;++i) printf("%i,", i);

and it will print 0,1,2,
then you can also try that to get absolutely sure that the stuff in the last part is executed AFTER the loop, by inserting another printf("++"); before the ++i. The output will now be 0,++1,++2,++

Thanks Raphael.

[Raphael]

np

This should be possible, shouldn't it? Errormessage: line 130: Constant expression expected.

Line 130:
artist_entry artist_list[num_artists];

I get that error when compiling with visual c++. I have done the same with gcc without problems!

Is num_artists a define or a variable? The latter won't work. If the size is not known at compile time, you need to dynamically malloc an array.

[Lukeson]

That's wierd; Using gcc this alsways worked. However I got more difficult errors:

line 158: '=': 'void *' cennot be converted in 'char **'

	typedef struct {
		char * name;
		int num_albums;
		char ** albums;
	} artist_entry;

	artist_entry * artist_list;
	artist_list = (artist_entry *)malloc(num_artists*sizeof(artist_entry));

	for(i=0; i<num_artists; i++)
	{
		[...]
line 158:	artist_list[i].albums = malloc( num_albums*sizeof(char*) );
		[...]
	}

[Moonchild]

Pretty sure you'd have to write it like this:

artist_list[i].albums = (char **)malloc( num_albums*sizeof(char**) );

[Raphael]

Pretty sure you'd have to write it like this:

artist_list[i].albums = (char **)malloc( num_albums*sizeof(char**) );

Well, it should be

artist_list[i].albums = (char **)malloc( num_albums*sizeof(char*) );

but that's the same since sizeof(char*) = sizeof(char**) ;p

[Lukeson]

Ok, thanks, compiling works now, but I get wierd errors; I read a string from a file and send it to the PSP where I then decode it. Now that I found out that you can send structs via TCP I want to do the decoding-work on the server. I use exactly the same code, but it just doesn't work on the server!

Server code:

	pFile = fopen ( "artists.txt" , "rb" );
	fseek (pFile , 0 , SEEK_END);
	lSize = ftell (pFile);
	rewind (pFile);
	buffer = (char*) malloc (lSize); 
	fread (buffer,1,lSize,pFile);
	fclose (pFile);

	memcpy(num_artists_c, &buffer, 5);
	num_artists_c[5]='\0';
	offset = 5;
	printf("%s\n", num_artists_c);
	printf("num artists: %d\n", atoi(num_artists_c));
	num_artists = atoi(num_artists_c);
	artist_entry * artist_list;
	artist_list = (artist_entry *)malloc(num_artists*sizeof(artist_entry));

Output is:

êO1
num artists: 0

-= Double Post =-

I fixed it using strncopy (which sucks... but works! :)). However the serverprogram crashes with an adress-error:

	int i,j;
	for(i=0; i<num_artists; i++)
	{
		// get artist- and album-data, works
		artist_list[i].num_albums = num_albums;

		artist_list[i].name = (char*)malloc(artist_name_lenght+1);
		strcpy(artist_list[i].name, artist_name);
		artist_list[i].name[artist_name_lenght]='\0';

		artist_list[i].albums = (char **)malloc( num_albums*sizeof(char**) );

		for(j=1;j<=num_albums;j++)
		{
			// get album-name, works
			artist_list[i].albums[j] = (char*)malloc(album_name_length+1);
			strcpy(artist_list[i].albums[j], album_name);
			artist_list[i].albums[j][album_name_length]='\0';
		}
	}

Everything works when I have these lines commented.

[Raphael]

I fixed it using strncopy (which sucks... but works! :)). However the serverprogram crashes with an adress-error:

	int i,j;
	for(i=0; i<num_artists; i++)
	{
		// get artist- and album-data, works
		artist_list[i].num_albums = num_albums;

		artist_list[i].name = (char*)malloc(artist_name_lenght+1);
		strcpy(artist_list[i].name, artist_name);
		artist_list[i].name[artist_name_lenght]='\0';

		artist_list[i].albums = (char **)malloc( num_albums*sizeof(char**) );

		for(j=1;j<=num_albums;j++)
		{
			// get album-name, works
			artist_list[i].albums[j] = (char*)malloc(album_name_length+1);
			strcpy(artist_list[i].albums[j], album_name);
			artist_list[i].albums[j][album_name_length]='\0';
		}
	}

Everything works when I have these lines commented.

for(j=1;j<=num_albums;j++ ) counts from 1 to num_albums, but the albums array goes from 0 to num_albums-1

[SG57]

I keep getting mixed answers on this, but you can assign multiple variables to one value with use of multiple = signs...

int var1, var2, var3;

var1=var2=var3=123;

printf("%i%i%i",var1,var2,var3

That returns each var as '123'. Why is it some say it will only set var1 to 123?

Just thought Id ask why some may get confused, is it differ in C++ or are the people just... out of it.

[Moonchild]

That will set var3 to 123, var2 to var3(123) and var1 to var2(123).
Nothing strange about that...

[SG57]

I already know that ;) Im saying, Zetta thought other wise... Not to point fingers I mean, im just saying, does C++ not allow this or what? Im just curious.

[Raphael]

It does. So what's the problem if some don't know this? Some at some point don't know how to avoid a loop being infinitely allocating memory, but they will learn. It would be different if there were a huge group of people claiming that & is not an operator, but a conjunctor and therefore insisting that C and C++ have bugs.
Also giving names with that is hugely childish (even when saying your not wanting to point fingers... sarcasm is no protection). You better edit your post.

[SG57]

Ya, a simple yes or no answer was all I needed Raphael. Excuse me for wanting to clairfy an answer to a question and getting bashed for it. Sometimes I think you act childish Raphael... You think I'm annoying so you find any little hole in a statement and exploit it... I highly doubt ZettBlade cares that I used his name as we had a simple 5 word discussion on it last night, and I just wanted to know from a reliable source.

[Raphael]

It was no bash, it was a statement that I find it inappropriate to talk names with abviously wrong assumptions to ridicule those persons, although the wrong assumption is pretty meaningless.
As I said, I just suggest you edit your post to comment out the name. It has nothing to do with the original question, which was already answered anyway.

EDIT-PS: And yes, I may sometimes act childish in some way too, I admit. That's most likely pretty much human nature to do every then and when, but I try not to ridicule people without a reason (I hope everyone sees enough reason for my signature though :P)- and therefore I didn't say a name with my allusion, because the person in charge should know he is being talked about.

[Access_Denied]

Is there a 'dofile' sort of command in C? Like if I had a .txt file with this:

variable1 = 7
variable2 = 4
variable3 = 9

How would I call these variables?

[Insert_Witty_Name]

You'd have to parse the files and set the variables yourself.

I believe Dark_Alex's custom firmware POC had some code in it to parse a config file like this. Check it out.

[FreePlay]

That will set var3 to 123, var2 to var3(123) and var1 to var2(123).
Nothing strange about that...

Err?
Wouldn't

int x,y,z;
x=y=z=3

result in z==3, y==1, x==1? IIRC, 'var=value' resolves to '1', not 'value'. Or... am I in desperate need of a C refresher here?

[Samstag]

Err?
Wouldn't

int x,y,z;
x=y=z=3

result in z==3, y==1, x==1? IIRC, 'var=value' resolves to '1', not 'value'. Or... am I in desperate need of a C refresher here?

x, y, and z will all equal 3.

I'm not sure I've ever seen that in Kernighan and Ritchie, but it does work that way.

Edit: Try resolving 'x=0' and see if you get a '1' out of that. ;)

[Lukeson]

for(j=1;j<=num_albums;j++ ) counts from 1 to num_albums, but the albums array goes from 0 to num_albums-1

That was it! Thank you!

[Raphael]

Err?
Wouldn't

int x,y,z;
x=y=z=3

result in z==3, y==1, x==1? IIRC, 'var=value' resolves to '1', not 'value'. Or... am I in desperate need of a C refresher here?

Refresh I would say ;) == is the comparison operator resolving to 1 if var is equal to value and zero else. x=y=z=1 is best read from right to left, ie z gets set to 1, then y gets set to z (=1) and x gets set to y (=z=1).

[Lukeson]

As for the send-struct-via-tcp-socket I have a whole lotta questions:

1.) How do I send a struct via a socket?

send(sock, (char)myStruct, sizeof(myStruct), 0);

?

2.) Can I also send an array of myStructs?

3.) If so, can I send structs that contain strings (char-arrays) of variable lenght? Like

typedef struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;

artist_entry * artist_list;
artist_list = (artist_entry *)malloc(num_artists*sizeof(artist_entry));

for(i=0; i<num_artists; i++)
{
	[...]
	artist_list[ i ].name = (char*)malloc(artist_name_lenght);
	[...]
}

send(???);

4.) If all that sending-jazz really works; How do I convert the sent string back to a struct-array?

5.) What was the point of the universe, again?

Thank you

Luke

[Raphael]

As for the send-struct-via-tcp-socket I have a whole lotta questions:

1.) How do I send a struct via a socket?

send(sock, (char)myStruct, sizeof(myStruct), 0);

?

send(sock, myStruct, sizeof(myStruct), 0);

more likely. Typecasting a struct into a char is a bad idea generally. I'm not sure about the second parameter of the send command, but if it expects a pointer, you'd need to use &myStruct.

2.) Can I also send an array of myStructs?

Yes, as long as the array is one single allocation (as in objection to an array of pointers to structs).

3.) If so, can I send structs that contain strings (char-arrays) of variable lenght? Like

typedef struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;

artist_entry * artist_list;
artist_list = (artist_entry *)malloc(num_artists*sizeof(artist_entry));

for(i=0; i<num_artists; i++)
{
	[...]
	artist_list[ i ].name = (char*)malloc(artist_name_lenght);
	[...]
}

send(???);

You can, but you will not submit the contents of the array, but only the pointer which in turn is useless on the client. So actually no, you cant. You need to make your structs non dynamic, without pointers for this to work as you want.

4.) If all that sending-jazz really works; How do I convert the sent string back to a struct-array?

As said in the prior post, as long as the above requirements fullfill (well, even if they don't fullfill, it would work the same way, but when you'd try to use the struct on the client, you would get a crash).

5.) What was the point of the universe, again?

42

Thank you

Luke

No Problem :P

[FreePlay]

Buh. I'm going insane. Time to re-teach myself C. Sigh.

[Lukeson]

Yes, as long as the array is one single allocation (as in objection to an array of pointers to structs).

Yes it is (using malloc).

You can, but you will not submit the contents of the array, but only the pointer which in turn is useless on the client. So actually no, you cant. You need to make your structs non dynamic, without pointers for this to work as you want.

I was afraid that was the case. Isn't there a bad 'hacking'-way to actually copy the strings to where they are needed? Aren't there functions that allocate the memory where it's needed, to create strings in the place?
I can't even use char name[length_of_the_longes_arti stname] in windows (can I?) so I'll probalby have cut some names. That sucks big time.

42

oh, I forgot about that. Thanks for enlighting me
-= Double Post =-

Buh. I'm going insane.

That's not neccissarily the worst thing to do, do what makes you happy...
-= Double Post =-

You can, but you will not submit the contents of the array, but only the pointer which in turn is useless on the client. So actually no, you cant. You need to make your structs non dynamic, without pointers for this to work as you want.

I think I'll go with my "send and parse a formatted string"-solution. Even though the memory allocation takes an ugly second.

[Raphael]

I was afraid that was the case. Isn't there a bad 'hacking'-way to actually copy the strings to where they are needed? Aren't there functions that allocate the memory where it's needed, to create strings in the place?
I can't even use char name[length_of_the_longes_arti stname] in windows (can I?) so I'll probalby have cut some names. That sucks big time.

Only possibility would be to make your name and albums list a fixed array of maximum size (depending on how many artists, albums per artist max you want to support) and then transfer that. It will have a big overhead though (memory and therefore transfer size), especially when a lot of artists only have 1 or 2 albums actually so the rest of the array(s) is filled with zeros.

I think I'll go with my "send and parse a formatted string"-solution. Even though the memory allocation takes an ugly second.

In your case that's prolly the best way to go.

[Lukeson]

Ok, does anyone have an idea why this only goes till artist 516 of 517 and, only at the 516th artist work only 4 of 5 albums?

memcpy(num_artists, &artist_data, 5);
num_artists[5]='\0';
offset = 5;
printf("num artists: %d\n", atoi(num_artists)); // num artists: 517


int i,j;
for(i=1; i<=atoi(num_artists); i++)
{
	memcpy(artist_name_lenght_c, &artist_data[offset], 3);
	artist_name_lenght_c[3]='\0';
	artist_name_lenght = atoi(artist_name_lenght_c);
	offset += 3;
	artist_name = (char *)realloc(artist_name, artist_name_lenght+1);
	memcpy(artist_name, &artist_data[offset], artist_name_lenght);
	artist_name[artist_name_lenght]='\0';
	offset += artist_name_lenght;

	memcpy(num_albums_c, &artist_data[offset], 4);
	num_albums_c[4]='\0';
	num_albums = atoi(num_albums_c);
	offset += 4;

	printf("artist #%d: %s (%d), %d albums:\n", i, artist_name, artist_name_lenght, num_albums);

	if(num_albums==0) num_albums=1;
	for(j=1;j<=num_albums;j++)
	{
		memcpy(album_name_length_c, &artist_data[offset], 3);
		album_name_length_c[3]='\0';
		album_name_length = atoi(album_name_length_c);
		offset += 3;
		album_name = (char *)realloc(album_name, album_name_length+1);
		memcpy(album_name, &artist_data[offset], album_name_length);
		album_name[album_name_length]='\0';
		offset += album_name_length;

		printf("     %s (%d)\n", album_name, album_name_length);
	}
}

----------------------------------------= Double Post =----------------------------------------

I'm having more troubles:
Errormessage: line 104: pspwamp_commands.c:104: error: invalid use of undefined type 'struct artist_entry'

Spoiler for Whole error-list:

pspwamp_commands.c:104: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:104: error: dereferencing pointer to incomplete type
pspwamp_commands.c:105: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:105: error: dereferencing pointer to incomplete type
pspwamp_commands.c:106: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:106: error: dereferencing pointer to incomplete type
pspwamp_commands.c:106: error: syntax error before ';' token
pspwamp_commands.c:113: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:113: error: dereferencing pointer to incomplete type
pspwamp_commands.c:117: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:117: error: dereferencing pointer to incomplete type
pspwamp_commands.c:129: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:129: error: dereferencing pointer to incomplete type
pspwamp_commands.c:130: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:130: error: dereferencing pointer to incomplete type
pspwamp_commands.c:131: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:131: error: dereferencing pointer to incomplete type
pspwamp_commands.c:134: error: invalid use of undefined type 'struct artist_entr
y'
pspwamp_commands.c:134: error: dereferencing pointer to incomplete type

Code:
pspwamp_datatypes.c

[...]
typedef struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;
[...]

main.c

	[...]
	artist_entry* artist_list;
	artist_list = (artist_entry *)malloc(sizeof(artist_entry));
	get_artists_from_server(sock, artist_list);
	[...]

pspwamp_commands.c

int get_artists_from_server(int socket, struct artist_entry * artist_list)
{
	[...]
	artist_list = (artist_entry *)realloc(artist_list, atoi(num_artists)*sizeof(artist_entry));

	[...]

	int i,j;
	for(i=0; i<atoi(num_artists); i++)
	{
		[...]
		artist_list[i].name = (char*)malloc(artist_name_lenght+1);	//l.104
		[...]
	}
}

Does anyone have an idea?

[yaustar]

Try from

typedef struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;

to

typedef struct artist_entry_struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;

http://www.netalive.org/codersguild/posts/1750.shtml

[Lukeson]

typedef struct artist_entry_struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;

http://www.netalive.org/codersguild/posts/1750.shtml

Hm, doesn't work, I get the same errors. Why are there actually 2 names (artist_entry_struct and artist_entry)? That kinda confuses me...

[yaustar]

Hm, doesn't work, I get the same errors. Why are there actually 2 names (artist_entry_struct and artist_entry)? That kinda confuses me...

Which was why I left a link ;).. You have the error because the datatype artist_entry is not defined in pspwamp_commands.c. You have defined it in pspwamp_datatypes.c but pspwamp_commands.c knows nothing about this file.

Put the artist_entry defintion in a header file with header guards and include it where you need to use that datatype.

[Lukeson]

Did I write pspwamp_datatypes.c up there? It's pspwamp_datatypes.h! The whole code is

#ifndef PSPWAMP_DATATYPES
#define PSPWAMP_DATATYPES

typedef struct artist_entry_struct {
	char * name;
	int num_albums;
	char ** albums;
} artist_entry;

#endif

and it's included in main.c and in pspwamp_commands.c