c++ how to add a zero bits byte or null to a string
Solution 1
If you want to transfer the null across, not only must you memcpy with "strlen(filename) + 1", but you'll also need to update the destination pointer for the subsequent memcpys accordingly.
memcpy(&blk_read_request[2 + strlen(file_name) + 1], mode_netascii, strlen(mode_netascii) + 1);
Note the extra +1 in there.
For the record, being C++, you might want want to consider using a higher-level class than "char *" which can handle embedded null characters.
For example, the following prints "8", and opens up std::stringstream as a better way of forming your packets:
#include <string>
#include <iostream>
int main()
{
std::string x = "foo";
x += '\0';
x += "bar";
x += '\0';
std::cout << x.length() << std::endl;
}
Solution 2
If you need to deal with data that will (or might) include null bytes then you'll generally want to avoid using C-style strings and the functions that deal with them. Use character/byte arrays and keep a length associated with the length of the data in the buffers.
In C++ vectors of bytes (or char) would work great for this.
Also in C++, the std::string
type can contain null characters just fine; however, I'd suggest avoiding their use and sticking with std::vector<char>
since it would be all too easy to fall into some bug traps of passing the result of string::c_str()
to something that expects null terminated strings.
Solution 3
This is automatic in C++ strings.
This:
const char mode_netascii[] = "netascii";
Is equivalent to this:
const char mode_netascii[] = { 'n', 'e', 't', 'a', 's', 'c', 'i', 'i', '\0' };
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Updated on June 28, 2022Comments
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I am doing a simple client implementation of TFTP. Here i need to send read request in following format
/* send request 2 bytes string 1 byte string 1 byte ------------------------------------------------------ RRQ/ | 01/02 | Filename | 0 | Mode | 0 | WRQ ------------------------------------------------- */
in between i have to insert 1 byte zero bits value . But i am not able to add that value. Also if i add a 1 zero bits byte.. which actually means a string terminating character than how to get proper strlen value.
If any one can help me with this...
enter code here const char opcode_read[2] ={'0','1'}; const char opcode_write[2] ={'0','2'}; const char opcode_data[2] ={'0','3'}; const char opcode_acknowledge[2] ={'0','4'}; const char opcode_error[2] ={'0','5'}; const char mode_netascii[] = "netascii\0"; char blk_read_request[100]; char file_name[] = "rfc0791.txt\0"; memcpy(blk_read_request, opcode_read, 2); memcpy(&blk_read_request[2], file_name, strlen(file_name) + 1); memcpy(&blk_read_request[2 + strlen(file_name)], mode_netascii, strlen(mode_netascii) + 1); for (int i = 0; i < strlen(blk_read_request); i++) { cout << i << " : " << blk_read_request[i] << " " << std::bitset<CHAR_BIT > (blk_read_request[i]) << "\n"; }