How to use shared memory with Linux in C

Question

I have a bit of an issue with one of my projects    I have been trying to find a well documented example of using shared memory with fork   but to no success   Basically the scenario is that when the user starts the program  I need to store two values in shared memory  current path which is a char  and a file name which is also char    Depending on the command arguments  a new process is kicked off with fork   and that process needs to read and modify the current path variable stored in shared memory while the file name variable is read only   Is there a good tutorial on shared memory with example code  if possible  that you can direct me to

User · Answer

These are includes for using shared memory   include lt sys ipc h gt   include lt sys shm h gt   int shmid  int shmkey   12222   u can choose it as your choice  int main         now your main starting   shmid   shmget shmkey 1024 IPC CREAT        1024   your preferred size for share memory      IPC CREAT  its a flag to create shared memory      now attach a memory to this share memory   char  shmpointer   shmat shmid NULL        do your work with the shared memory      read -write will be done with the  shmppointer     after your work is done deattach the pointer    shmdt  amp shmpointer  NULL

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There are two approaches  shmget and mmap  I ll talk about mmap  since it s more modern and flexible  but you can take a look at man shmget  or this tutorial  if you d rather use the old-style tools   The mmap   function can be used to allocate memory buffers with highly customizable parameters to control access and permissions  and to back them with file-system storage if necessary   The following function creates an in-memory buffer that a process can share with its children    include  lt stdio h gt   include  lt stdlib h gt   include  lt sys mman h gt   void  create shared memory size t size         Our memory buffer will be readable and writable    int protection   PROT READ   PROT WRITE        The buffer will be shared  meaning other processes can access it   but      anonymous  meaning third-party processes cannot obtain an address for it        so only this process and its children will be able to use it    int visibility   MAP SHARED   MAP ANONYMOUS        The remaining parameters to  mmap    are not important for this use case       but the manpage for  mmap  explains their purpose    return mmap NULL  size  protection  visibility  -1  0       The following is an example program that uses the function defined above to allocate a buffer  The parent process will write a message  fork  and then wait for its child to modify the buffer  Both processes can read and write the shared memory    include  lt string h gt   include  lt unistd h gt   int main       char parent message      hello       parent process will write this message   char child message      goodbye      child process will then write this one    void  shmem   create shared memory 128      memcpy shmem  parent message  sizeof parent message       int pid   fork       if  pid    0        printf  Child read   s n   shmem       memcpy shmem  child message  sizeof child message        printf  Child wrote   s n   shmem        else       printf  Parent read   s n   shmem       sleep 1       printf  After 1s  parent read   s n   shmem

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try this code sample  I tested it  source  http   www makelinux net alp 035   include  lt stdio h gt    include  lt sys shm h gt    include  lt sys stat h gt    int main         int segment id     char  shared memory     struct shmid ds shmbuffer     int segment size     const int shared segment size   0x6400         Allocate a shared memory segment         segment id   shmget  IPC PRIVATE  shared segment size                    IPC CREAT   IPC EXCL   S IRUSR   S IWUSR         Attach the shared memory segment         shared memory    char   shmat  segment id  0  0      printf   shared memory attached at address  p n   shared memory         Determine the segment s size        shmctl  segment id  IPC STAT   amp shmbuffer      segment size                  shmbuffer shm segsz     printf   segment size   d n   segment size         Write a string to the shared memory segment         sprintf  shared memory   Hello  world           Detach the shared memory segment         shmdt  shared memory          Reattach the shared memory segment  at a different address         shared memory    char   shmat  segment id   void   0x5000000  0      printf   shared memory reattached at address  p n   shared memory         Print out the string from shared memory         printf    s n   shared memory         Detach the shared memory segment         shmdt  shared memory          Deallocate the shared memory segment         shmctl  segment id  IPC RMID  0       return 0

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Here s a mmap example    include  lt sys mman h gt   include  lt sys types h gt   include  lt sys stat h gt   include  lt fcntl h gt         pvtmMmapAlloc - creates a memory mapped file area       The return value is a page-aligned memory value  or NULL if there is a failure     Here s the list of arguments      mmapFileName - the name of the memory mapped file     size - the size of the memory mapped file  should be a multiple of the system page for best performance      create - determines whether or not the area should be created      void  pvtmMmapAlloc  char   mmapFileName  size t size  char create              void   retv   NULL                                                                                                  if  create                                                                                                                                                                                                      mode t origMask   umask 0                                                                              int mmapFd   open mmapFileName  O CREAT O RDWR  00666                                                  umask origMask                                                                                         if  mmapFd  lt  0                                                                                                                                                                                                  perror  open mmapFd failed                                                                             return NULL                                                                                                                                                                                                 if   ftruncate mmapFd  size     0                                                                                                                               int result   lseek mmapFd  size - 1  SEEK SET                        if  result    -1                                                                                                                                                                                                perror  lseek mmapFd failed                                                                            close mmapFd                                                                                           return NULL                                                                                                                                                                                                     Something needs to be written at the end of the file to                                                have the file actually have the new size                                                               Just writing an empty string at the current file position will do                                      Note                                                                                                    - The current position in the file is at the end of the stretched                                        file due to the call to lseek                       - The current position in the file is at the end of the stretched                                 file due to the call to lseek                                                                        - An empty string is actually a single   0  character  so a zero-byte                                    will be written at the last byte of the file                                                                                                                                                            result   write mmapFd      1                                                                           if  result    1                                                                                                                                                                                                 perror  write mmapFd failed                                                                            close mmapFd                                                                                           return NULL                                                                                                                                                                                                 retv     mmap NULL  size                       PROT READ   PROT WRITE  MAP SHARED  mmapFd  0                                               if  retv    MAP FAILED    retv    NULL                                                                                                                                                                          perror  mmap                                                                                           close mmapFd                                                                                           return NULL                                                                                                                                                                                                                                                                                                                                                                                                           else                                                                                                                                                                                                            int mmapFd   open mmapFileName  O RDWR  00666                                                          if  mmapFd  lt  0                                                                                                                                                                                                  return NULL                                                                                                                                                                                                 int result   lseek mmapFd  0  SEEK END                                                                 if  result    -1                                                                                                                                                                                                perror  lseek mmapFd failed                            close mmapFd                                                                                           return NULL                                                                                                                                                                                                 if  result    0                                                                                                                                                                                                 perror  The file has 0 bytes                                     close mmapFd                                                                                           return NULL                                                                                                                                                                                              retv     mmap NULL  size                       PROT READ   PROT WRITE  MAP SHARED  mmapFd  0                                               if  retv    MAP FAILED    retv    NULL                                                                                                                                                                          perror  mmap                                                                                           close mmapFd                                                                                           return NULL                                                                                                                                                                                                  close mmapFd                                                                                                                                                                                                 return retv

User · Answer

Here is an example for shared memory     include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   include  lt sys types h gt   include  lt sys ipc h gt   include  lt sys shm h gt    define SHM SIZE 1024     make it a 1K shared memory segment     int main int argc  char  argv          key t key      int shmid      char  data      int mode       if  argc  gt  2            fprintf stderr   usage  shmdemo  data to write  n            exit 1                 make the key         if   key   ftok  hello txt    R       -1    Here the file must exist               perror  ftok            exit 1                  create the segment         if   shmid   shmget key  SHM SIZE  0644   IPC CREAT      -1            perror  shmget            exit 1                 attach to the segment to get a pointer to it         data   shmat shmid  NULL  0       if  data     char    -1             perror  shmat            exit 1                 read or modify the segment  based on the command line         if  argc    2            printf  writing to segment     s   n   argv 1            strncpy data  argv 1   SHM SIZE         else         printf  segment contains     s   n   data           detach from the segment         if  shmdt data     -1            perror  shmdt            exit 1              return 0      Steps       Use ftok to convert a pathname and a project identifier to a System V IPC key Use shmget which allocates a shared memory segment Use shmat to  attache the shared memory segment identified by shmid to the address space of the calling process Do the operations on the memory area  Detach using shmdt

[c] How to use shared memory with Linux in C

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