How to parse the Port Number from SDP using C language?

[joan fruchtalle]

Hi,
I am working on a VoIP project, in that there is a small task of extracting Port Number from the incoming SDP which is stored in a Buffer.


Let me say it in another way, I have a text which is stored in a Buffer (lets say 'string21[]' ) and from that I need to extract a Port Number and assign it to an Integer variable (lets say 'audio_port' ).

For example the text which is coming from the Server is "m = audio 17764 RTP/AVP 0 8" [this is just a part of the Text, the whole Text is very big.......], in this case "17764" should be extracted.

One more point in "m = audio 17764 RTP/AVP 0 8" , m = audio RTP/AVP 0 8 are constant, only the port number will be changing randomly, so that each time my code the port number will be changed and therefore I need this to be extracted each time.

Any idea of how I can do it, I can only use Header Files and Functions from C not from C++(no C++ libraries).

Regards,
Joan

[hype261]

I would look into using the strstr function in string.h

char * strstr ( const char *, const char * );

I would also look at the atoi function in stdlib.h

int atoi ( const char * str );

[joan fruchtalle]

my problem is the I need to search this port number which is not constant and keeps on changing every time I run it new and also it should be extracted from a whole bunch of text which looks something like this.......

SIP/2.0 200 OK
Call-ID: 058E40CC-5AD9-40C7-A919-07758E503BA0@13 0.237.250.244
Content-Type: application/sdp
Content-Length: 134
CSeq: 1 INVITE
From: "bob"<sip:127.0 .0.1>;tag=12778 715617702
To: <sip:2@127.0.0. 1:5000>;tag=127 7871561770
Via: SIP/2.0/UDP 127.0.0.1;rport ;branch=z9hG4bK 82edfaf40000001 7461e8e8a000055 8d00000001

v=0
o=bob 3385396490 3385396490 IN IP4 130.237.250.244
s=SJphone
c=IN IP4 130.237.250.244
t=0 0
m=audio 3456 RTP/AVP 0 8
a=rtpmap: 0 PCMU/8000

so out of this I need to extract the port number(which is not a constant) and you can find it in this line "m=audio 3456 RTP/AVP 0 8"

I hope somebody got my point what I am intend to do.

Regards,
Joan

[hype261]

Okay so you have all this text and I am assuming that you can get this text into a char *. So first I would use the strstr to find the string "m=audio". The strstr function will return a pointer to the first occurance of "m=audio" or NULL if "m=audio" doesn't exist. I would then advance the return pointer 7 to get it past "m=audio" and at the beginning of the port number. I would then pass this pointer to the atoi function to convert the string into an integer.

There are some degenerate conditions that I haven't mentioned here that you will need to account for. Hopefully this gets you started.

[joan fruchtalle]

Hi Hype261,
thanks for your reply that helped me!

Joan