problem with recvfrom

[Omega]

I have a problem with recvfrom(). I get a message and no error occurs,
but the struct sockaddr* from is not filled with sender's data.

The client's source code:

#include "reversi.h"
-------------------------------------------------------------------
int main() {
int socket_id = socket(PF_INET, SOCK_DGRAM,0);
struct sockaddr_in adres;
adres.sin_famil y = PF_INET;
adres.sin_port = htons(PORT_NR);
adres.sin_addr. s_addr = inet_addr("127. 0.0.1");
cli_msg msg;
msg.type = CMT_LOGIN;
sendto(socket_i d,&msg,sizeof(c li_msg),0,(stru ct sockaddr*)&adre s,sizeof
(struct sockaddr));
printf("wyslale m na 127.0.0.1\n");
ser_main_msg ser_msg;
unsigned int rozmiar;
recvfrom(socket _id,&ser_msg,si zeof(ser_main_m sg),0,(struct sockaddr*)
&adres,&rozmiar );
printf("otrzyma lem cos a adresu %s",inet_ntoa(a dres.sin_addr)) ;
printf("type = %d\n",ser_msg.t ype);
return 1;
}
-------------------------------------------------------------------
All needed header files are included in reversi.h

Server is rather long, but the inportant part is, the socket is datagram
socket:
-------------------------------------------------------------------
bzero(&klient1, sizeof(struct sockaddr_in));
int ret = recvfrom(sockid ,&msg_kli1,size of(cli_msg),0,( struct sockaddr*)
&klient1,&rozmi ar1);
printf("receive d %s, %d, %d\n",inet_ntoa (klient1.sin_ad dr), ret,
errno);
if (msg_kli1.type == CMT_LOGIN) {
msg.type = SMMT_WAIT;
sendto(sockid,& msg,sizeof(ser_ main_msg),0,(st ruct sockaddr*)
&klient1,rozmia r1);
printf("send SMMT_WAIT to address %s\n",inet_ntoa
(klient1.sin_ad dr));
kli_nr++;
-------------------------------------------------------------------
serwer should print:
received 127.0.0.1, 16, 0
send SMMT_WAIT to addrress 127.0.0.1

but it prints:
reveived 0.0.0.0., 16, 0
send SMMT_WAIT to address 0.0.0.0

It happens only when I run the program on my computer, when I tried on
friends computer problem doesn't occur. Any ideas?

Omega

[Keith Thompson]

Omega <Omega@NO_SPAM. pl> writes:

I have a problem with recvfrom(). I get a message and no error occurs,
but the struct sockaddr* from is not filled with sender's data.

[snip]

There is no recvfrom() function in standard C. Followups redirected.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[David Schwartz]

"Omega" <Omega@NO_SPAM. pl> wrote in message
news:Xn******** *************** ***********@213 .180.128.149...

int main() {
int socket_id = socket(PF_INET, SOCK_DGRAM,0);
struct sockaddr_in adres;
adres.sin_famil y = PF_INET;
adres.sin_port = htons(PORT_NR);
adres.sin_addr. s_addr = inet_addr("127. 0.0.1");
cli_msg msg;
msg.type = CMT_LOGIN;
sendto(socket_i d,&msg,sizeof(c li_msg),0,(stru ct sockaddr*)&adre s,sizeof
(struct sockaddr));
printf("wyslale m na 127.0.0.1\n");
ser_main_msg ser_msg;
unsigned int rozmiar;
Oops, your forgot to initialize 'rozmiar'.
recvfrom(socket _id,&ser_msg,si zeof(ser_main_m sg),0,(struct sockaddr*)
&adres,&rozmiar );
printf("otrzyma lem cos a adresu %s",inet_ntoa(a dres.sin_addr)) ;
printf("type = %d\n",ser_msg.t ype);
return 1; Server is rather long, but the inportant part is, the socket is datagram
socket:
-------------------------------------------------------------------
bzero(&klient1, sizeof(struct sockaddr_in));
int ret = recvfrom(sockid ,&msg_kli1,size of(cli_msg),0,( struct sockaddr*)
&klient1,&rozmi ar1);
Did you remember to initialize "rozmiar1"?
printf("receive d %s, %d, %d\n",inet_ntoa (klient1.sin_ad dr), ret,
errno);
if (msg_kli1.type == CMT_LOGIN) {
msg.type = SMMT_WAIT;
sendto(sockid,& msg,sizeof(ser_ main_msg),0,(st ruct sockaddr*)
&klient1,rozmia r1);
printf("send SMMT_WAIT to address %s\n",inet_ntoa
(klient1.sin_ad dr));
kli_nr++;
-------------------------------------------------------------------
serwer should print:
received 127.0.0.1, 16, 0
send SMMT_WAIT to addrress 127.0.0.1

but it prints:
reveived 0.0.0.0., 16, 0
send SMMT_WAIT to address 0.0.0.0

It happens only when I run the program on my computer, when I tried on
friends computer problem doesn't occur. Any ideas?

Passing an uninitalized value to 'recvfrom' results in unpredictable
behavior. Initialize it to the allocated length of the structure.

DS

[clayne]

Omega wrote:

I have a problem with recvfrom(). I get a message and no error occurs,
but the struct sockaddr* from is not filled with sender's data.

The client's source code:

ser_main_msg ser_msg;
unsigned int rozmiar;
recvfrom(socket _id,&ser_msg,si zeof(ser_main_m sg),0,(struct sockaddr*)
&adres,&rozmiar );

You're not initializing 'rozmiar' to the length of 'adres'.

ssize_t recvfrom(int s, void *buf, size_t len, int flags,
struct sockaddr *from, int *fromlen);

'fromlen' is value-result parameter meaning that recvfrom() uses it's
passed value and before returning modifies the contents of 'fromlen' to
indicate the actual size of the address stored there.

What this means to you is:

unsigned int rozmiar;
rozmiar = sizeof(adres); /* or rozmiar = sizeof(struct sockaddr_in); */
recvfrom(socket _id, &ser_msg, sizeof(ser_main _msg), 0, (struct
sockaddr*)&adre s, &rozmiar);

You have to initialize 'fromlen' (in your case rozmiar) to the proper
length before each call to recvfrom(). The reason it works sometimes is
because of any number of random possibilities as to what the contents
of 'rozmiar' are (since it's an auto variable) without you explicitly
initializing it.

[clayne]

Omega wrote:

I have a problem with recvfrom(). I get a message and no error occurs,
but the struct sockaddr* from is not filled with sender's data.

The client's source code:

ser_main_msg ser_msg;
unsigned int rozmiar;
recvfrom(socket _id,&ser_msg,si zeof(ser_main_m sg),0,(struct sockaddr*)
&adres,&rozmiar );

You're not initializing 'rozmiar' to the length of 'adres'.

ssize_t recvfrom(int s, void *buf, size_t len, int flags,
struct sockaddr *from, int *fromlen);

'fromlen' is value-result parameter meaning that recvfrom() uses it's
passed value and before returning modifies the contents of 'fromlen' to
indicate the actual size of the address stored there.

What this means to you is:

unsigned int rozmiar;
rozmiar = sizeof(adres); /* or rozmiar = sizeof(struct sockaddr_in); */
recvfrom(socket _id, &ser_msg, sizeof(ser_main _msg), 0, (struct
sockaddr*)&adre s, &rozmiar);

You have to initialize 'fromlen' (in your case rozmiar) to the proper
length before each call to recvfrom(). The reason it works sometimes is
because of any number of random possibilities as to what the contents
of 'rozmiar' are (since it's an auto variable) without you explicitly
initializing it.

[Rainer Temme]

Omega wrote:

Hi Alpha,

unsigned int rozmiar;
/* dont forget this */
rozmiar=sizeof( adres);
recvfrom(socket _id,&ser_msg,si zeof(ser_main_m sg),0,(struct sockaddr*)
&adres,&rozmiar );

Regards ... Rainer

[Maxim Yegorushkin]

Omega wrote:

I have a problem with recvfrom(). I get a message and no error occurs,
but the struct sockaddr* from is not filled with sender's data.
[]
unsigned int rozmiar;
recvfrom(socket _id,&ser_msg,si zeof(ser_main_m sg),0,(struct sockaddr*)
&adres,&rozmiar );

rozmiar must be of type socklen_t and have to be initialized with the
length of your address structure prior the call to recvfrom

socklen_t rozmiar = sizeof adres;

see
http://www.opengroup.org/onlinepubs/.../recvfrom.html

[clayne]

Maxim Yegorushkin wrote:

rozmiar must be of type socklen_t and have to be initialized with the
length of your address structure prior the call to recvfrom

socklen_t rozmiar = sizeof adres;

To be 100% portable it should be socklen_t - but of course that's not
why it's failing here.

[pete]

Omega wrote:

I have a problem with recvfrom(). Any ideas?

Yes.
This is off topic for comp.lang.c.
Followup set.

--
pete

[loic-dev]

Hi,

I have a problem with recvfrom(). I get a message and no error occurs,
but the struct sockaddr* from is not filled with sender's data.
ssize_t recvfrom(int socket, void *restrict buffer, size_t length,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
The 6th parameter /addrlen_len/ is an IN/OUT argument. It should be
initialized to the size of the structure pointed by the /address/
argument prior to calling recvfrom().

HTH,
Loic.


The client's source code:

#include "reversi.h"
-------------------------------------------------------------------
int main() {
int socket_id = socket(PF_INET, SOCK_DGRAM,0);
struct sockaddr_in adres; [snip] unsigned int rozmiar;
this line should be:
| unsigned int rozmiar = sizeof (adres);

Similarly in the server: -------------------------------------------------------------------
bzero(&klient1, sizeof(struct sockaddr_in));
the following line is missing:
| rozmiar1 = sizeof (klient1);
int ret = recvfrom(sockid ,&msg_kli1,size of(cli_msg),0,( struct sockaddr*)
&klient1,&rozmi ar1);

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