shared memory of std::string give segmentation fault (linux)

Question

I am currently trying the put structures in a shared memory between 2 process on linux. I have no problem sharing bool or int but when trying to share a string, std::string or char i have a segmentation fault error.

Right now my code is :

#include <iostream>
#include <sys/types.h> //shmat
#include <sys/shm.h>
#include <sys/stat.h> //open
#include <fcntl.h>
#include <unistd.h> //close

using namespace std;

struct Prises{

int numero;
int transactionId;
bool reservation;
bool charge;
bool disponibilite;
bool defaut;
bool verrouillage;
bool trappe;
int LEDverte;
int LEDrouge;
std::string carte;
std::string etat;

};

int main()
{
const char *keyFile = "/tmp/key.dat";
/* Make sure the file exists. */
int descriptor = open(keyFile, O_CREAT | O_RDWR, S_IRWXU);

/* Only wanted to make sure that the file exists. */
close(descriptor);

/* Generate memory key. */
key_t sharedKey = ftok(keyFile, 1);

/* Get the shared memory segment id. Note we include
   the permissions. */
int sharedSpaceId = shmget(sharedKey, 2*sizeof(Prises),
    IPC_CREAT | S_IRUSR | S_IWUSR);

/* Attach the shared memory segment. */
Prises *PrisesArray = (Prises *) shmat(sharedSpaceId, NULL, 0);

PrisesArray[1].defaut=true;
PrisesArray[2].defaut=false;

int ok;
std::cin>>ok;
return 0;
}

In this example sharing the 2 bool from the 2 structures is working well but if i try to input a data or read a data from the std::string (etat, carte) like this :

PrisesArray[1].etat="hello";

It gives me a segmentation fault in debug (and clear don't work in release), i tried with simple string and char (even one char) and it still gives me a segmentation fault.

Am i missing something when it comes to text sharing or making a mistake here ?

It should work fine if you used a C string (a null-terminated character array) instead of std::string. — paulsm4
– paulsm4, Commented Sep 15, 2015 at 8:31
I finally got it to work using char, it seems the lack of lenght of the char when i defined my structure was the error. — Wowy
– Wowy, Commented Sep 15, 2015 at 8:51
Also note that PrisesArray[2] accesses the array past its end. — chi
– chi, Commented Sep 15, 2015 at 10:33

Werner Erasmus · Accepted Answer · 2015-09-15 20:01:47Z

It gives me a segmentation fault in debug (and clear don't work in release), i tried with simple string and char (even one char) and it still gives me a segmentation fault.

This is because std::string is not a POD (Plain Old Data) type. std::string performs dynamic memory allocation (using new) behind the scenes, and when serializing it (e.g to shared memory or file), only the pointers are serialized. When deserializing it (from shared memory or file) memory to the original pointers are likely to not exist anymore, and this renders the deserialized string unusable.

You would have to write a function that specifically serializes the string explicitly to your shared memory, as is the case with the standard operator std::ostream operator >>(std::ostream& stream, const std::string&)

One could also write an allocator that takes memory from shared memory, and let the string use that to allocate it's memory.

Adrian · Accepted Answer · 2018-05-16 00:38:48Z

You have to share the data associated with the std::string, i.e.

//server part:

struct ShmData{
`...`
char chAux[1024];
};

int main()
{
...
shared_memory_object shm (create_only, "MyMem", read_write);
shm.truncate(offset_t(sizeof(ShmData)) );
mapped_region region(shm, read_write);
ShmData *shmData = (ShmData*)(region.get_address());
...
std::string str( "some stuff over the server" );
...
strcpy( shmData->chAux, str.c_str() );
...
}
//on client, the same struct and:
int main()
{
shared_memory_object shm (open_only, "MyMem", read_write); 
mapped_region region(shm, read_write);
ShmData *shmData = (ShmData*)(region.get_address());
std::string strLocal(shmData->chAux); //assign to a local string the  shared data
std::cout << "show strLocal\n";
std::cout << strLocal << std::endl;
....
}

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