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Full Discussion: C header file and extern
Top Forums Programming C header file and extern Post 302851859 by tornow on Tuesday 10th of September 2013 12:11:34 AM
Old 09-10-2013
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.
 

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sip_create_OKack(3SIP)				   Session Initiation Protocol Library Functions			    sip_create_OKack(3SIP)

NAME
sip_create_OKack - create an ACK request for a final response SYNOPSIS
cc [ flag ... ] file ... -lsip [ library ... ] #include <sip.h> int sip_create_OKack(sip_msg_t response, sip_msg_t ack_msg, char *transport, char *sent_by, int sent_by_port, char *via_params); DESCRIPTION
The sip_create_OKack() function constructs an ACK request in ack_msg for the final 2XX SIP response. The request line is created using the URI in the CONTACT header from the response. The SIP-Version in the request line is "2.0". The VIA header for the ACK request is created using transport, sent_by, sent_by_port (if non-zero), and via_params (if non-null). The following headers are copied to ack_msg from response: FROM TO CALL-ID MAX_FORWARDS The CSEQ header is created using the method as ACK and the sequence number from the CSEQ header in response. RETURN VALUES
The sip_create_OKack() function returns 0 on success and the appropriate error value in case of failure. The value of errno is not changed by these calls in the event of an error. ERRORS
On failure, the sip_create_OKack() function could return one of the following errors: EINVAL If mandatory input is not provided or if the input is invalid. The sip_create_OKack() function can return this error if it does not find a CONTACT header or if it is unable to obtain the URI from the CONTACT header for the request line. ENOTSUP If the input SIP message cannot be modified. ENOMEM If memory allocation fails when creating the request/response line or when creating headers in the ACK request. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Committed | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ SEE ALSO
libsip(3LIB) SunOS 5.11 25 Jan 2007 sip_create_OKack(3SIP)
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