Problem with the write function

Question

0.00/5 (No votes)

See more:

Hello everyone,

I'm a beginner's programmer and I don't understand why my write() function doesn't work with these arguments. That why i'm here.

When I execute my program, I got une error with my write function and with my read function. it's normal for my read function because it depends on the write function. I searched on Google for my arguments but I think they are good. So I don't understand why it doesn't work.

My script is on C language.

What I have tried:

C++

int main(int argc,char **argv)
{
	int sockfd,n;
	char sendline[100];
	char recvline[100];
	struct sockaddr_in servaddr;

	sockfd=socket(AF_INET,SOCK_STREAM,0);
	memset(&servaddr, 0, sizeof (servaddr));

	servaddr.sin_family=AF_INET;
	servaddr.sin_port=htons(22000);

	inet_pton(AF_INET,"127.0.0.1",&(servaddr.sin_addr));
	printf("%d\n", inet_pton(AF_INET,"198.168.1.19",&(servaddr.sin_addr)));

	connect(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));

	while(1)
	{
		memset(sendline,0, 100);
		memset( recvline,0, 100);

		fgets(sendline,100,stdin);

		if(write(sockfd,sendline,strlen(sendline)+1) < 0)
		{
			printf("The write function doesn't work !\n");
   		}
		if(read(sockfd,recvline,100) < 0)
		{
			printf("The read function doesn't work !\n");
		}
		printf("%s",recvline);
	}
}

Posted 2-Jan-19 10:13am

Member 14106234

Updated 2-Jan-19 23:21pm

Rick York

v3

Add a Solution

Comments

jsc42 2-Jan-19 16:56pm

Capture the actual error code from the 'write' and include it in your error message. Then look up the code - that should give you a clue as to what the error is.
e.g.

int errcode;
if ((errcode = write(sockfd, sendline, strlen(sendline) + 1)) < 0)
printf("The write function returned %d\n", errcode);

Also, I suspect that you might need an '&' before the recvline argument in the read function (happy to be corrected if I am wrong).

Rick York 2-Jan-19 17:43pm

No, the & is not necessary. An array without a subscript is the same as a pointer to the array so the & is not needed.

Member 14106234 2-Jan-19 17:48pm

My error code for my read and write function is -1 and the & before the recvline doesn't change anything. For me, the problem in the read function is the "sockfd" argument because we already have problem with it on the write function.

2 solutions

Solution 1

In addition to what jsc42 wrote, call strerror() to get a textual description of the error and display that. Here is an example:

C++

int errorCode;
errorCode = write( sockfd, sendline, strlen( sendline ) + 1 );
if( errorCode < 0 )
{
    fprintf( stderr, "write function failed %d : %s\n", errorCode, strerror( errorCode ) );
}

One more thing - I noticed you have used the "magic number" 100 many places. That is not a good idea. What happens if you decide to support up to 128 characters? You have a whole bunch of places where it has to change and there is a possibility of missing one. It is much better to use a definition in C or a constant value in C++. Here is the code using a macro definition :

C++

#define BUFFER_SIZE  100

int main(int argc,char **argv)
{
	int sockfd,n;
	char sendline[BUFFER_SIZE];
	char recvline[BUFFER_SIZE];
	struct sockaddr_in servaddr;

	sockfd=socket(AF_INET,SOCK_STREAM,0);
	memset(&servaddr, 0, sizeof (servaddr));

	servaddr.sin_family=AF_INET;
	servaddr.sin_port=htons(22000);

	inet_pton(AF_INET,"127.0.0.1",&(servaddr.sin_addr));
	printf("%d\n", inet_pton(AF_INET,"198.168.1.19",&(servaddr.sin_addr)));

	connect(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));

	while(1)
	{
		memset( recvline, 0, sizeof( recvline ) );
		memset( sendline, 0, sizeof( sendline ) );

		fgets( sendline, sizeof( sendline )-1, stdin );

		if( write( sockfd, sendline, strlen(sendline) + 1 ) < 0 )
		{
			printf("The write function doesn't work !\n");
		}
		if( read( sockfd, recvline, sizeof( sendline ) - 1 ) < 0 )
		{
			printf("The read function doesn't work !\n");
		}
		printf( "%s",recvline );
	}
}

Now if you want to have different maximum size you change only one thing and nothing else.

Did you notice the -1 in the fgets and read calls? They are there for the same reason you have a +1 on the write call - to account for the terminating null character.

Posted 2-Jan-19 11:43am

Rick York

Comments

Member 14106234 2-Jan-19 17:58pm

First of all, thanx for your answer. With strlen(errorCode) I get "Unknowm error". And thanx, I had not thought about the define.

Member 14106234 2-Jan-19 18:00pm

And the -1 in the fgets and the +1 on the write call is for the terminating null character.

Rick York 2-Jan-19 20:55pm

Are the write and read functions your own implementation? I believe the socket library functions are send and recv. Make sure you return the same values they return from your functions or strerror will not work correctly.

One more thing - with the last stuff I wrote I found that reading immediately following the write always failed. If I put a 1 millisecond delay in between the calls then it worked so you might want to give that a try. In the Win32 API the function is Sleep. I don't know what it's called in other OSs.

Member 14106234 3-Jan-19 5:55am

The write and read function are on stdio.h, I tried with send and recv but I get the same error. And the Sleep function, it doesn't change anything

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Richard MacCutchan · Accepted Answer · 2019-01-02T23:22:00

Solution 2

When you write to a socket file descriptor, it is similar to using the sendto function | Microsoft Docs[^] function. So you need to use the WSAGetLastError function to find out why it failed.

Posted 2-Jan-19 23:22pm

Richard MacCutchan

Comments

Member 14106234 3-Jan-19 9:59am

With WSAGetLastError, I get, for the 3 error : 10093. So I used WSAStartup() and WSACleanup() because i forget that and i remplace write and read by send and recv. With that, my program work.
Thanks you all !