const char ack_msg[] = "ack: received your msg\n";
In the code file server.c i got:
Code:
extern const char ack_msg[];
And else it is only used in a function call:
Code:
user$ grep ack_msg *c *h
server.c:extern const char ack_msg[];
server.c: n = write(clientsfd, ack_msg, strlen(ack_msg));
data.h:const char ack_msg[] = "ack: received your msg\n";
user$
But i get this error when compiling:
Code:
user$ ls
checkr.c checkr.h client client.c data.h server.c
user$ gcc checkr.c server.c -Wall -Wextra --pedantic -o server
/tmp/ccdKHyIr.o:(.rodata+0x0): multiple definition of `ack_msg'
/tmp/ccUnDvRc.o:(.rodata+0x0): first defined here
collect2: ld returned 1 exit status
user$
Thanks so much, it seems clear now. I got another question about header files. Say i got checkr.h with:
Code:
void check_argc(int, int);
and the function in checkr.c:
Code:
void check_argc(int argcount, int n) { if ( argcount != n ) { fprintf(stderr, "ERROR: wrong argument count\n"); usage(); exit(EXIT_FAILURE); } }
From what you say i think i would need to use "extern" in checkr.h ? Can i also use it in the main file (called server.c) ? I know that i don't have to, but it helps me a bit to keep track of the functions without opening and closing files all the time (or using :vsplit). I think it has to do with definition and declaration? I read about it quite often, but never really understand it and usually forget it soon.
Thanks. Your explanation and example makes it more clear (also thanks for pointing out it may not be best praxis, but for now i just try to understand the general idea).
I think you say that: header files are (or can be ) used _instead of declaring the variable foo as extern in File2 and File3. foo is defined in File1 (and storage is set aside for it). File1 includes myheader.h too, and if foo is declared (File2 and File3) or defined (File1) is decided by the #ifndef Mainfile and #define Mainfile in File1.
I read in §The C Programming langauge"; (As "C A modern approach" was not clear on the subject) and in chapter 1 "External variables and scope" (page 33 for me) it says the following (or that is what i understand):
- a definition creates a variable and sets aside storage
- a declaration only announces the nature of a variable
- extern says that the definition is elsewhere and only declares
- header files include declarations, not definitions
Is both correct (What i understood of your post and what i understood of the book)?
I think i am getting closer to understand it. Let me rethink and read again.Really thanks.
From what you say i think i would need to use "extern" in checkr.h ?
If you don't define a function's contents with { } right then and there, it'll be assumed to be extern. This is very unlike variables, which never assume they're external unless you say so.
extern used to be a bigger deal for functions when there were many methods of linking -- there could be several kinds of extern functions, and you'd use extern to tell the compiler which they are with extern dllexport functionname(); or what have you. Sometimes this is still important when mixing C with other languages, to guarantee your function calls are generated in a way foreign code can understand.
I've been struggling with this one for quite a while and cannot seem to find a solution for this find/replace scenario. Perhaps I'm getting rusty.
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09-09-2013
