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I have a program that needs to share a string between two processes. I have declared a struct that contains an array of *char. This struct is allocated with shmget and shmat before the main process is forked. 

typedef struct Queue
{
    int index;
    char *directory[10];
} Queue;

In one of the processes, I try to set the value: (data->dir_name is a *char to a string such as "/data1") 

queue->directory[i] = data->dir_name; // Option 1
queue->directory[i] = "foo";          // Option 2

My question is, what is the difference between the first and second statements above? When setting the queue->directory[i] to "foo", the other process sees it. However, passing the value data->dir_name, it does not.

Thanks in advance!

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  • there shouldn't be a difference, are you declaring/setting them in different points of the program? the only way I can see this happening is if one process reads the data before it has been written to file by the other process. Commented Mar 20, 2013 at 21:22
  • 1
    Maybe you make free(data->dir_name) somewhere before using queue->directory[i]? Also data->dir_name maybe an automatic variable. Commented Mar 20, 2013 at 21:25
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    could you share more of your code, where the process is forked? Commented Mar 20, 2013 at 21:27
  • They are set in the same part of the program. I change 'code'(data->dir_name) to 'code'"foo" in the source, recompile it, and see that "foo" shows up but the value or data->dir_name does not. Commented Mar 20, 2013 at 21:29

1 Answer 1

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The problem is you are only assigning a pointer, not copying the string data. In the first case you are setting the value to point to memory that the second process can't see. When you do the first line, the pointer data->dir_name is put into queue->directory[i], but when the other process looks at that memory address within its own memory space, the data is not there. On the other hand, the second line puts the address of the static string "foo" into the variable. Since the processes are compiled from the same source, that string is in the same place in each process's memory, so the second one can see it.

What you want to be doing is having a buffer in the struct that you strcpy the directory name into. You'll need

char directory[10][200];

and

strcpy (queue->directory[i], data->dir_name);

You'll want to check that the string length is less than 200 (in this case), and report an appropriate error if it's too long. I'm not familiar with shared memory functions to know exactly how to do a malloc equivalent; if you could do that; then you would copy the string into the malloced shared memory and put the pointer to it in an array like you have in your code. From a very quick Google search though, it appears that mallocing shared memory like this might not work very well.

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4 Comments

+1 excellent point, any immediate strings are placed in the data segment of your program at compile time, even though they don't have declarations. when you fork(), the entire memory of your program is copied, including the data segment
I see what you mean. What I'd like to do is have a data structure (2D array possibly?) that will contain an index to a string value. Similar to a Map<Integer,String> in java. I don't want the char *directory[10]; to be limited to 10 characters, I want there to only be 10 entries allowed.
strncpy is not the best solution. It can easily leave an array without a terminating '\0' null character (i.e., not a string).
@KeithThompson You're quite right, and since there's nothing useful to do if the string is too long, I've updated my answer to reflect this.

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