Question

I have to memory map a file if there is not already one for that file. The code I provided is not working. I am calling this function twice from another file and each time it is creating different mmapPtr.

char *mmapPtr;

void MemoryMapFile()
{

    int fd;
    struct stat sbuf;
    if(mmapPtr==NULL) <--- why is this executed when I called MemoryMapFile() second time
    {
        // get file descriptor of file
        if ((fd = open("example.c", O_RDONLY)) == -1) 
        {
            perror("open");
            exit(1);
        }
        if (stat("example.c", &sbuf) == -1) 
        {
            perror("stat");
            exit(1);
    }
        if ((data = mmap((caddr_t)0, sbuf.st_size, PROT_READ, MAP_SHARED, fd, 0)) ==  (caddr_t)(-1)) 
        {
            perror("mmap");
            exit(1);
        }
        printf("mmap pointer %p \n",mmapPtr);
    }

Answer 1

If char* mmapPtr is not a global or static pointer the line

if(mmapPtr==NULL) 

never returns true as an uninitialized pointer has a random value, not a NULL value.

If you declare

char *mmapPtr = NULL;

it should work,( ignoring possible other errors)