Ris. 1: Vzaimodejstvie klienta i servera
V Internet domene soket - eto kombinaciya IP adresa i nomera porta, kotoraya odnoznachno opredelyaet otdel'nyj setevoj process vo vsej global'noj seti Internet. Dva soketa, odin dlya hosta-poluchatelya, drugoj dlya hosta-otpravitelya, opredelyayut soedinenie dlya protokolov, orientirovannyh na ustanovlenie svyazi, takih, kak TCP.
s = socket(domain, type, protocol);
|tot vyzov osnovyvaetsya na informacii o kommunikacionnom domene i tipe soketa. Dlya ispol'zovaniya osobennostej Internet, znacheniya parametrov dolzhny byt' sleduyushchimi:
Dlya sozdaniya soketa tipa stream s protokolom TCP, obespechivayushchim kommunikacionnuyu podderzhku, vyzov funkcii socket dolzhen byt' sleduyushchim:
s = socket(AF_INET, SOCK_STREAM, 0);
V Internet domene svyazyvanie soketa i imeni mozhet byt' ves'ma slozhnym, no, k schast'yu, obychno net neobhodimosti special'no privyazyvat' adres i nomer porta k soketu, tak kak funkcii connect i send avtomaticheski svyazhut dannyj soket s podhodyashchim adresom, esli eto ne bylo sdelano do ih vyzova.
Dlya svyazyvaniya soketa s adresom i nomerom porta ispol'zuyut sistemnyj vyzov bind:
bind(s, name, namelen);
Privyazyvaemoe imya (name) eto stroka bajt peremennoj dliny, kotoraya interpretiruetsya podderzhivaemym protokolom. Interpretaciya mozhet razlichat'sya v razlichnyh kommunikacionnyh domenah.
error = connect(s, serveraddr, serveraddrlen);
kotoraya iniciiruet ustanovlenie svyazi na sokete, ispol'zuya deskriptor soketa s i informaciyu iz struktury serveraddr, imeyushchej tip sockaddr_in, kotoraya soderzhit adres servera i nomer porta na kotoryj nado ustanovit' svyaz'. Esli soket ne byl svyazan s adresom, connect avtomaticheski vyzovet sistemnuyu funkciyu bind.
Connect vozvrashchaet 0, esli vyzov proshel uspeshno. Vozvrashchennaya velichina -1 ukazyvaet na to, chto v processe ustanovleniya svyazi proizoshla nekaya oshibka. V sluchae uspeshnogo vyzova funkcii process mozhet rabotat' s deskriptorom soketa, ispol'zuya funkcii read i write, i zakryvat' kanal ispol'zuya funkciyu close.
So storony servera process ustanovleniya svyazi slozhnee. Kogda server zhelaet predlozhit' odin iz svoih servisov, on svyazyvaet soket s obshcheizvestnym adresom, associiruyushchimsya s dannym servisom, i passivno slushaet etot soket. Dlya etih celej ispol'zuetsya sistemnyj vyzov listen:
error = listen(s, qlength);
gde s eto deskriptor soketa, a qlength eto maksimal'noe kolichestvo zaprosov na ustanovlenie svyazi, kotorye mogut stoyat' v ocheredi, ozhidaya obrabotki serverom; eto kolichestvo mozhet byt' ogranicheno osobennostyami sistemy.
Kogda server poluchaet zapros ot klienta i prinimaet reshenie ob ustanovlenii svyazi, on sozdaet novyj soket i svyazyvaet ego s associaciej, ekvivalentnoj 'slushayushchemu soketu'. Dlya Internet domena eto oznachaet tot zhe samyj nomer porta. Dlya etoj celi ispol'zuetsya sistemnyj vyzov accept:
newsock = accept(s, clientaddr, clientaddrlen);
Soket, associirovannyj klientom, i soket, kotoryj byl vozvrashchen funkciej accept, ispol'zuyutsya dlya ustanovleniya svyazi mezhdu serverom i klientom.
Process ustanovleniya svyazi pokazan na risunke 1.
write(s, buf, sizeof(buf)); read(s, buf, sizeof(buf));
Vyzovy send i recv prakticheski identichny read i write, za isklyucheniem togo, chto dobavlyaetsya argument flagov.
send(s, buf, sizeof(buf), flags); recv(s, buf, sizeof(buf), flags);
Mogut byt' ukazany odin ili bolee flagov s pomoshch'yu nenulevyh znachenij, takih, kak sleduyushchie:
Esli soket bol'she ne ispol'zuetsya, process mozhet zakryt' ego s pomoshch'yu funkcii close, vyzvav ee s sootvetstvuyushchim deskriptorom soketa:
close(s);
Esli dannye byli associirovany s soketom, obeshchayushchim dostavku (soket tipa stream), sistema budet pytat'sya osushchestvit' peredachu etih dannyh. Tem ne menee, po istechenii dovol'no taki dlitel'nogo promezhutka vremeni, esli dannye vse eshche ne dostavleny, oni budut otbrosheny. Esli pol'zovatel'skij process zhelaet prekratit' lyubuyu peredachu dannyh, on mozhet sdelat' eto s pomoshch'yu vyzova shutdown na dannom sokete dlya ego zakrytiya. Vyzov shutdown vyzyvaet "momental'noe" otbrasyvanie vseh stoyashchih v ocheredi dannyh. Format vyzova sleduyushchij:
shutdown(s, how);
gde how imeet odno iz sleduyushchih znachenij:
/*
-----------------------------------------------------------
MakeConnection
Function allocates a socket and estabishes a connection
with remote host. Default port number 80.
Input : WWW server name (with port number, if it is not 80)
Output : file descriptor on success
-1 on error
-----------------------------------------------------------
*/
int MakeConnection(unsigned char* ServerName){
int s;
struct sockaddr_in ssin;
struct hostent* hp;
int PortNum;
unsigned char strHlp[STRNGLEN], *pch;
/* use default port number - 80 or specific number from the
server name */
strcpy(strHlp,ServerName);
pch = strchr(strHlp,':');
if(pch==NULL){
PortNum = 80;
}else{
pch[0] = '\0';
pch++;
PortNum = atoi(pch);
if(PortNum==0){
PortNum = 80;
}
}
/* get host by name - resolve host name into IP address */
if( (hp=gethostbyname(strHlp)) == NULL )
{
return -1;
}
bzero(&ssin, sizeof(ssin));
bcopy(hp->h_addr, &ssin.sin_addr, hp->h_length);
ssin.sin_family = hp->h_addrtype;
ssin.sin_port = htons(PortNum);
/* allocate a socket */
if((s=socket(AF_INET, SOCK_STREAM, 0))==-1)
{
return -1;
}
/* make a connection */
if(connect(s, &ssin, sizeof(ssin), 0)==-1){
return -1;
}
return s; /* socket descriptor */
}
Last-modified: Thu, 28 May 1998 14:26:10 GMT