вот что-то в этом роде получилось, поругайте:

typedef struct {
 char signature[8];
 int command;
 size_t msgsize;
} ctrl_header;

int operate(int sock, sq_command command, void * response, size_t len_buff)
{
 char *recvbuf;
 int n;
 ctrl_header hdr;
 memcpy(hdr.signature,"SQ1.0.00\n",10);
 hdr.command=1;
 hdr.msgsize=0;
 n = send(sock, &hdr, sizeof(hdr),0);
 n = recv(sock, &hdr, sizeof(hdr),MSG_WAITALL);
 if ((n==sizeof(hdr)) && (strncmp(hdr.signature,"SQ1.0.00\n",10)==0)){
  if (hdr.msgsize>0){
   if ((recvbuf=malloc(hdr.msgsize))!=NULL){
    if ((n = recv(sock,recvbuf, hdr.msgsize, MSG_WAITALL))==hdr.msgsize){
     if ((response!=NULL) && (len_buff>=0)) {
      memcpy(response,recvbuf,len_buff);
     }
    } else {
     //maybe socket error, count of received data<than promised
    }
    free(recvbuf);
   } else {
    // allocation error
   };
  }
 } else {
  // maybe again socket error, or there is no squash on that side...
 }
 return(1);
}