Send C struct over TCP socket

Question

I have seen a lot of answers that are related to my question, but I really cannot use it under my condition.

I am building a network module using socket programming with C language under Linux domain. I have to implement a function that could send a struct that consists of int, char, char * and some other structures (nested struct).

struct EnQuery
{
   char  type[6]; // insert, select , update , delete
   char * columns; //note here, it's an array of big, {name, age, sex, position, email}  not in string
   struct Values * values; //an array of values(another struct), {tom, 23, male, student, [email protected]}  is represented by the second struct values, not in string
   struct condition * enCondition; //  array of condition,  "name=tom and age>30" is represnted by the third struct condition
   short len;
   short rn; 
};

struct Values
{
    char * doc;
    char  key[2];
    char  s;
};

struct condition
{
     short k;              
     struct condition * children;
};

Above is the structure that I am trying to send. I am declaring the variables and sending over through socket using send() function.

How would I have to send char * through the socket? Or would there be a way to resize the char array length conveniently?

P.S I can't use external libraries

Just decide on the wire format you want to use and write code to convert to and from that wire format. Look at existing specification (BER, XML, JSON, etcetera) for ideas. — David Schwartz
Your strcts has pointers, and that cannot be transmitted as they will not be valid at the receiving end -- you need to serialize and deserialize the data -- how you do that has a million good answers. — Soren
Everything with a star (*) needs to be serialized, not only the chars. If you didn't have any pointers you could just send the struct as is (assuming receiver have the same endianness and memory padding etc). — Soren
Don't use structs as network protocols. Use network protocols as network protocols. First define the protocol in octets, then write yourself an API to send and receive it. — user207421

Jeremy Friesner Jeremy Friesner · Accepted Answer · 2015-10-14T00:16:04

How would I have to send char * through the socket?

Obviously, sending the pointer value isn't a useful thing to do, since the pointer wouldn't point to anything valid on the receiving computer.

So instead of sending the pointer, you have to send the data that it points to. A typical method for doing that would be to first send the number of bytes the pointer points to:

uint32_t sLen = strlen(doc)+1;  // +1 because I want to send the NUL byte also
uint32_t bigEndianSLen = htonl(sLen);
if (send(sock, &bigEndianSLen, sizeof(bigEndianSLen), 0) != sizeof(bigEndianSLen)) perror("send(1)");

.... and then follow by sending the bytes of the string:

if (send(sock, doc, sLen, 0) != sLen) perror("send(2)");

On the receiving side, you'd do the opposite: first receive the string's length:

uint32_t bigEndianSLen;
if (recv(sock, &bigEndianSLen, sizeof(bigEndianSLen), 0) != sizeof(bigEndianSLen)) perror("recv(1)");
uint32_t sLen = ntohl(bigEndianSLen);

... and then receive the string's data:

char * doc = malloc(sLen+1);
if (recv(sock, doc, sLen, 0) != sLen) perror("recv(2)");
doc[sLen] = '\0';  // paranoia:  make sure the string is terminated no matter what

Note that this example code is a bit naive, in that it doesn't handle correctly the case where send() or recv() return a value other than the byte-count that was passed in to them. Production-quality code would handle errors properly (e.g. by closing the connection), and would also handle properly the case where send() or recv() send/receive only transferred some of the bytes that were requested (by calling send()/recv() again later for the remaining unsent/unreceived bytes).

Send C struct over TCP socket

4 Answers