receive data in child process:

char data_buffer[BUFFER_SIZE];
char *ptr = "";
int data_len = 0;
int pre_len = 0;
for (;;)
{
    bzero(data_buffer, BUFFER_SIZE);
    int length = recv(client_data_socket, data_buffer, BUFFER_SIZE, 0);
    if (length == 0)
    {
        close(client_data_socket);
        break;
    }
    else if (length < 0)
    {
        if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR)
        {
            continue;
        }
        close(client_data_socket);
        printf("get data failed\n");
        exit(1);
    }
    pre_len = data_len;
    data_len += length;
    char *tmp_ptr = (char *)calloc(data_len, sizeof(char));
    memcpy(tmp_ptr, ptr, pre_len);
    memcpy(tmp_ptr + pre_len, data_buffer, length);
    if (pre_len > 0) free(ptr);
    ptr = tmp_ptr;
}
char *tmp_ptr = (char *)calloc(data_len + 1, sizeof(char));
g2u(ptr, data_len, tmp_ptr, data_len + 1);
printf("%s\n", tmp_ptr);
free(ptr);
free(tmp_ptr);
exit(0);

g2u method transforms the gbk to utf-8 and then finds that part of the read data is missing. When recv returns 0, I judge that all the data is received, but I find that there is something wrong with the result