The following code (part of a request-response loop in a networked server) works most of the time, but sometimes fails, in that the client will report it has gotten some weird other string (seemingly random bytes from locations nearby in memory in this functions, or null bytes).
string res = "";
if (something) {
res = "ok";
}
if (res.length() > 0) {
send_data((void*) res.c_str(), res.length());
}
In my mind, it would seem that both "" and "ok" are constant std:strings, and res is a pointer to either one of them, and as such the whole thing should work, but apparently that's not the case, so can someone please explain to me what happens here?
You probably forgot to send the null-terminator to denote the end of the string:
send_data((void*) res.c_str(), res.length()+1);
Your code is okay, I suppose there's some memory corruption in your program.
"" and "ok" are actually zero-terminated buffers of type 'const char *', not strings. When you assign them to your string all their data is copied inside string internal buffer, not including last char which is zero, so
res = "";
will clear internal string buffer, and res.length() will become 0.
res.c_str() will return the address of that buffer, not the address of "" or "ok" literals.
Related
I am doing this IoT based project on displaying data to connected display( I've used the MAX7219 module, in this case) with the help of nodeMCU. The idea here is that the string which is stored in my firebase database is to be display on the led display.
I've had no trouble in getting the value from the database to my nodeMCU but there is this little problem with converting that string to char array since the code i am using( Max72xx_Message_serial, which was available as an example with the max72xx library) has used char array but i can only fetch the stored data in string format. I've modified that code so as to connect with firebase but the main issue is to convert the string fetched from the database to char array.
I tried toCharArray() but it still shows conversion error.
void readfromfirebase(void)
{
static uint8_t putIndex = 0;
int n=1;
while (Firebase.available())
{
newMessage[putIndex] = (char)Firebase.getString("Submit Message"); // this line produces the error
if ((newMessage[putIndex] == '\n') || (putIndex >= BUF_SIZE-3)) // end of message character or full buffer
{
// put in a message separator and end the string
newMessage[putIndex++] = ' ';
newMessage[putIndex] = '\0';
// restart the index for next filling spree and flag we have a message waiting
putIndex = 0;
newMessageAvailable = true;
}
else if (newMessage[putIndex] != '\r')
// Just save the next char in next location
{putIndex++;}
n++;
}
}
I think you are confusing the types
getString returns a String object wich can be converted to a char[] using the methods of the String class.
I assume your newMessage is of type char[] or char*.
Then I would advise you to go for the String.c_str() method, because it returns a C style null-terminated string, meaning a char*.
See https://www.arduino.cc/reference/en/language/variables/data-types/string/functions/c_str/ for reference.
It also sets the last character of the string to 0. So methods like strlen, strcmp etc will work.
! be carefull not to modify the array returned by c_str(), if you want to modify it you chould copy the char[] or use string.toCharArray(buf, len).
Your Code might then look like the following.
String msg = Firebase.getString("Submit Message");
newMessage = msg.c_str();
// rest of your code
If newMessage is a buffer storing multiple messages, meaning char* newMessage[3].
String msg = Firebase.getString("Submit Message");
newMessage[putIndex] = msg.c_str();
// rest of your code
Be careful, because you are storing multiple characters in an array, so use strcmp to compare these arrays!
If you are new to C I would recommend reading.
https://www.cprogramming.com/tutorial/c/lesson9.html
https://www.arduino.cc/reference/en/language/variables/data-types/stringobject/ (as pointed out by #gre_gor)
With reference to the code below: After calling the function CheckSMS and passing the struct * DB1, the fields are updated according the the strtok call. This function reads and parses a text message, storing its contents into char* fields of the DB struct.
In the main loop, I have called Serial.println(DB1.last_order) before and after calling the CheckSMS function. If I have received a text, the order is printed appropriately in the main loop, however on the next call to CheckSMS, DB1.last_order is cleared, replaced with a \n or a NULL or something. I cannot figure out why DB1.last_order does not retain its value, and rather, it is overwritten with every call to CheckSMS. Thanks for any help.
Note - All text messages contain "CMT+", therefore writing to DB1 only happens when a text is received. Calling CheckSMS when no text is received should simply skip through.
int CheckSMS(Robot *DB1) {
int j = 0;
char response[100];
char *pch;
unsigned long previous;
memset(response, '\0', 100);
while(Serial2.available()>0){
response[j] = Serial2.read();
j++;
Serial.println("inc");
}
delay(100);
if (strstr(response, "CMT:") != NULL){
DB1->new_message = strtok(response, " ,");
DB1->last_phone = strtok(NULL, " ,");
pch = strtok(NULL, " ,");
DB1->last_date = strtok(NULL, " ,");
DB1->last_time = strtok(NULL, " ,\n");
DB1->last_order = strtok(NULL," ,\n");
new_message = 1;
}
else{
}
return 0;
}
The strtok function returns pointers to the string you're tokenizing, the local array response in your case. When the function returns the response array goes out of scope and disappears, leaving your structure with pointers to a string that no longer exists, and giving you undefined behavior.
You have a couple of solutions:
Allocate the string dynamically using malloc, but then you must save it in the structure so you can free it when you're done with the structure
Make the response array static, but then the next call to the function will have the same array leading the old data to be updated
Pass in a string to store the response and use that string, the string must have a lifetime at least as long as the structure and don't change contents. The string can of course be a member of the structure itself
The answer Joachim gave is correct, I just want to add that you could also change the Robot structure to contain char arrays (like this: char new_message[MAX_BUF_CHARS]; and so on). Be sure to have enough space in them. Then instead of assigning the pointers returned from strtok, copy the strings in there.
I'm having a string is not null terminated error, though I'm not entirely sure why. The usage of std::string in the second part of the code is one of my attempt to fix this problem, although it still doesn't work.
My initial codes was just using the buffer and copy everything into client_id[]. The error than occurred. If the error is correct, that means I've got either client_ id OR theBuffer does not have a null terminator. I'm pretty sure client_id is fine, since I can see it in debug mode. Strange thing is buffer also has a null terminator. No idea what is wrong.
char * next_token1 = NULL;
char * theWholeMessage = &(inStream[3]);
theTarget = strtok_s(theWholeMessage, " ",&next_token1);
sendTalkPackets(next_token1, sizeof(next_token1) + 1, id_clientUse, (unsigned int)std::stoi(theTarget));
Inside sendTalkPackets is. I'm getting a string is not null terminated at the last line.
void ServerGame::sendTalkPackets(char * buffer, unsigned int buffersize, unsigned int theSender, unsigned int theReceiver)
{
std::string theMessage(buffer);
theMessage += "0";
const unsigned int packet_size = sizeof(Packet);
char packet_data[packet_size];
Packet packet;
packet.packet_type = TALK;
char client_id[MAX_MESSAGE_SIZE];
char theBuffer[MAX_MESSAGE_SIZE];
strcpy_s(theBuffer, theMessage.c_str());
//Quick hot fix for error "string not null terminated"
const char * test = theMessage.c_str();
sprintf_s(client_id, "User %s whispered: ", Usernames.find(theSender)->second.c_str());
printf("This is it %s ", buffer);
strcat_s(client_id, buffersize , theBuffer);
Methinks that problem lies in this line:
sendTalkPackets(next_token1, sizeof(next_token1) + 1, id_clientUse, (unsigned int)std::stoi(theTarget));
sizeof(next_token1)+1 will always gives 5 (on 32 bit platform) because it return size of pointer not size of char array.
One thing which could be causing this (or other problems): As
buffersize, you pass sizeof(next_token1) + 1. next_token1 is
a pointer, which will have a constant size of (typically) 4 or 8. You
almost certainly want strlen(next_token1) + 1. (Or maybe without the
+ 1; conventions for passing sizes like this generally only include
the '\0' if it is an output buffer. There are a couple of other
places where you're using sizeof, which may have similar problems.
But it would probably be better to redo the whole logic to use
std::string everywhere, rather than all of these C routines. No
worries about buffer sizes and '\0' terminators. (For protocol
buffers, I've also found std::vector<char> or std::vector<unsigned char>
quite useful. This was before the memory in std::string was
guaranteed to be contiguous, but even today, it seems to correspond more
closely to the abstraction I'm dealing with.)
You can't just do
std::string theMessage(buffer);
theMessage += "0";
This fails on two fronts:
The std::string constructor doesn't know where buffer ends, if buffer is not 0-terminated. So theMessage will potentially be garbage and include random stuff until some zero byte was found in the memory beyond the buffer.
Appending string "0" to theMessage doesn't help. What you want is to put a zero byte somewhere, not value 0x30 (which is the ascii code for displaying a zero).
The right way to approach this, is to poke a literal zero byte buffersize slots beyond the start of the buffer. You can't do that in buffer itself, because buffer may not be large enough to accomodate that extra zero byte. A possibility is:
char *newbuffer = malloc(buffersize + 1);
strncpy(newbuffer, buffer, buffersize);
newbuffer[buffersize] = 0; // literal zero value
Or you can construct a std::string, whichever you prefer.
I'm trying to understand why a segmentation fault (SIGSEGV) occurs during the execution of this piece of code. This error occurs when testing the condition specified in the while instruction, but it does not occur at the first iteration, but at the second iteration.
LPTSTR arrayStr[STR_COUNT];
LPTSTR inputStr;
LPTSTR str;
// calls a function from external library
// in order to set the inputStr string
set_input_str(param1, (char*)&inputStr, param3);
str = inputStr;
while( *str != '\0' )
{
if( debug )
printf("String[%d]: %s\n", i, (char*)str);
arrayStr[i] = str;
str = str + strlen((char*)str) + 1;
i++;
}
After reading this answer, I have done some research on the internet and found this article, so I tried to modify the above code, using this piece of code read in this article (see below). However, this change did not solve the problem.
for (LPTSTR pszz = pszzStart; *pszz; pszz += lstrlen(pszz) + 1) {
... do something with pszz ...
}
As assumed in this answer, it seems that the code expects double null terminated arrays of string. Therefore, I wonder how I could check the contents of the inputStr string, in order to check if it actually contains only one null terminator char.
NOTE: the number of characters in the string printed from printf instruction is twice the value returned by the lstrlen(str) function call at the first iteration.
OK, now that you've included the rest of the code it is clear that it is indeed meant to parse a set of consecutive strings. The problem is that you're mixing narrow and wide string types. All you need to do to fix it is change the variable definitions (and remove the casts):
char *arrayStr[STR_COUNT];
char *inputStr;
char *str;
// calls a function from external library
// in order to set the inputStr string
set_input_str(param1, &inputStr, param3);
str = inputStr;
while( *str != '\0' )
{
if( debug )
printf("String[%d]: %s\n", i, str);
arrayStr[i] = str;
str = str + strlen(str) + 1;
i++;
}
Specifically, the issue was occurring on this line:
while( *str != '\0' )
since you hadn't cast str to char * the comparison was looking for a wide nul rather than a narrow nul.
str = str + strlen(str) + 1;
You go out of bounds, change to
str = str + 1;
or simply:
str++;
Of course you are inconsistently using TSTR and strlen, the latter assuming TCHAR = char
In any case, strlen returns the length of the string, which is the number of characters it contains not including the nul character.
Your arithmetic is out by one but you know you have to add one to the length of the string when you allocate the buffer.
Here however you are starting at position 0 and adding the length which means you are at position len which is the length of the string. Now the string runs from offset 0 to offset len - 1 and offset len holds the null character. Offset len + 1 is out of bounds.
Sometimes you might get away with reading it, if there is extra padding, but it is undefined behaviour and here you got a segfault.
This looks to me like code that expects double null terminated arrays of strings. I suspect that you are passing a single null terminated string.
So you are using something like this:
const char* inputStr = "blah";
but the code expects two null terminators. Such as:
const char* inputStr = "blah\0";
or perhaps an input value with multiple strings:
const char* inputStr = "foo\0bar\0";
Note that these final two strings are indeed double null terminated. Although only one null terminator is written explicitly at the end of the string, the compiler adds another one implicitly.
Your question edit throws a new spanner in the works? The cast in
strlen((char*)str)
is massively dubious. If you need to cast then the cast must be wrong. One wonders what LPTSTR expands to for you. Presumably it expands to wchar_t* since you added that cast to make the code compile. And if so, then the cast does no good. You are lying to the compiler (str is not char*) and lying to the compiler never ends well.
The reason for the segmentation fault is already given by Alter's answer. However, I'd like to add that the usual style of parsing a C-style string is more elegant and less verbose
while (char ch = *str++)
{
// other instructions
// ...
}
The scope of ch is only within in the body of the loop.
Aside: Either tag the question as C or C++ but not both, they're different languages.
In my project,I have met these strange problem with strncpy. I have checked the reference. But the function strncpy behavior make me confused.
In this function, when it runs to strncpy(subs,target,term_len);
While I don't know why there is two blanks after the string?!!! It is a big project, I cannot paste all the code here. Following is just a piece. All my code is here.
char* subs = new char[len];
while(top<=bottom){
char* term = m_strTermTable[bottom].strterm;
int term_len = strlen(term);
memset(subs,'\0',len);
strncpy(subs,target,term_len);
int subs_len = strlen(subs);
int re = strcmp(subs,term);
if (re == 0)
{
return term_len;
}
bottom--;
}
delete[] subs;
strncpy does not add a terminating null byte if the source string is longer than the maximum number of characters (i.e. in your case, that would be if strlen(target) > term_len holds). If that happens, subs may or may not be null terminated correctly.
Try changing your strncpy call to
strncpy(subs, target, term_len-1);
so that even if strncpy doesn't add a terminating null byte, subs will still be null-terminated correctly due to the previous memset call.
Now, that being said - you could avoid using a separate subs buffer altogether (which leaks anyway in case the control flow gets to the return statement) by just using strncmp as in
while(top<=bottom) {
char* term = m_strTermTable[bottom].strterm;
int term_len = strlen(term);
if (strncmp(term, target, term_len) == 0) {
return term_len;
}
bottom--;
}