Tuesday, July 8, 2014

C program to dump shared memory segments to disk on Linux.

The following program was written to help investigate Oracle database shared memory on Linux.  It dumps the contents of existing shared memory segments to files on disk.  Note that it won't work against Oracle 11g and 12C databases as they use mmap instead of shmat for managing shared memory.  Sample program for reading from 11g and 12C here (mmap example )

Compile it using "gcc -o shared shared.c"  It is free for anyone to copy or modify as they wish, but I do not guarantee the functionality.

Check the format of the include listings below as I had to remove hashes and greater-than/less-than symbols to keep blogger happy.

include stdio.h
include stdlib.h
include sys/shm.h

int main (int argc, char *argv[]) {
    int maxkey, id, shmid = 0;
    struct shm_info shm_info;
    struct shmid_ds shmds;
    void * shared_data;
    FILE * outfile;
    maxkey = shmctl(0, SHM_INFO, (void *) &shm_info);
    for(id = 0; id <= maxkey; id++) {
        shmid = shmctl(id, SHM_STAT, &shmds);
        char shmidchar[16];
        snprintf(shmidchar, sizeof(shmidchar), "%d", shmid);
        if (shmid < 0)
        if(shmds.shm_segsz > 0) {
            printf("Shared memory segment %s found.\n",shmidchar);
            shared_data = shmat(shmid, NULL, 0666);
            if(shared_data != NULL) {
                outfile = fopen(shmidchar, "wb");
                if(outfile == NULL) {
                    printf("Could not open file %s for writing.", shmidchar);
                else {
                    fwrite(shared_data, shmds.shm_segsz, 1, outfile);
                    printf("Dumped to file %s\n\n", shmidchar);