Question

Update:
I add a while to get the remain data, problem solved. thanks you guys.

while(res != rcvread->size + 4) {/* do not get full data */
    tmp = recv(connfd, (void *)rcvread + res, rcvread->size + 4 - res, 0);
    if(tmp == -1)
        printf("error:%s\n",(char *)strerror(errno));
    res+=tmp;
}

I want to send a structure mfsio through socket, the return value of write is 37772, but when I got it from the other program, the return value of read is 32768, that’s really odd.

The definition of this structure is here

struct mfsio{
    size_t size;
    char buf[];
};

The send code is here

struct mfsio *sndread;
sndread = malloc(sizeof(struct mfsio) + rcvcmd->size);
res = pread(fd, &(sndread->buf), rcvcmd->size, rcvcmd->offset);
sndread->size = res;
res = write(connfd, sndread, sizeof(struct mfsio) + res);

The receive code is here

struct mfsio *rcvread;
rcvread = malloc(sizeof(struct mfsio) + size);
res = 0;    
res = read(connfd, rcvread, sizeof(struct mfsio) + size);

size equals to rcvcmd->size, and the res of pread is 32678, the res of write is 32772.
but res of read is 32678.

How can such thing happened? Does I do something wrong here?

If there is no input from write, the read function will just waiting for some data and just hanging there

If I use a loop to avoid such problem, how can I finally get the entire data structure, I mean if I do loop reading, I will get the remain bytes from the socket, but how can I combine these clips I get?

Answer 1

You need to do your reads and writes in a loop until all of the expected data is read/written. For the read, you have to read until it returns 0, indicating nothing else is available. Or -1 indicating an error.

Your more recent problem is that you are using pointer arithmetic with your struct pointer:

tmp = recv(connfd, rcvread + res, rcvread->size + 4 - res, 0);

In C when you do pointer arithmetic you are not working with bytes, but you are working with the size of the object (rvcread for example). You should define a char * which is the byte location of where you will read into and then update this as you read through the data.