CODEWITHSUNDEEP
cipcmmap
iadducchio3
iadducchio3

Reputation: 75

storing strings in a mmap shared array? (C)

I am trying to store all filenames in the current directory in a shared array using mmap I can print all 9 files in the directory to the screen but when I try to store them in the array (shared_array) and print the array, all entries contain the same string (file.txt). Thanks in advance!

char **shared_array; 
shared_array= mmap(0,100*sizeof(char*),PROT_READ | PROT_WRITE,MAP_SHARED | MAP_ANON,-1,0);  
char * filename;

const MAXLINE = 80; 
char line [MAXLINE];

FILE *fp = popen("/bin/ls","r");

int i = 0; 
while(fgets(line, MAXLINE,fp) !=NULL){    
    filename = line;
    shared_array[i] = filename; 
    i++;        
}
pclose(fp);

int j;
for(j=0;j<i;j++){
    printf("\n%s",shared_array[j]);
}

Upvotes: 3

Views: 2257

Answers (2)

nitin pundir
nitin pundir

Reputation: 41

If MAXLINE denotes total number of strings each of size N :-

You cannot do this. It will give a very long string of size MAXLINE X N and will have to use offset to access substrings.

char (*shared_memory)[MAXLINE]  -> returns pointer to a string of size MAXLINE.
shared_array= mmap(0,100*MAXLINE,PROT_READ | PROT_WRITE,MAP_SHARED | MAP_ANON,-1,0);

You need to do this

char *shared_memory[MAXLINE] - This returns array of char pointers

for(int i=0; i<MAXLINE;i++){
shared_memory[i] = mmap(NULL,sizeof(char)*N,PROT_READ | PROT_WRITE,MAP_SHARED | MAP_ANONYMOUS,-1,0);
}

By doing this now you can access the strings by doing shared_memory[i]. Even though if each string is less then page granularity you will be wasting space but it will be easier to access.

or simply use

char shared_memory[100][MAXLINE];

Upvotes: 1

dbush
dbush

Reputation: 223699

When you do this:

filename = line;
shared_array[i] = filename;

You're not actually copying the contents of your line array to shared_array. You're assigning the address of line to each array element. As a result, all array elements point to the same place, i.e. line, which will only contain the most recently stored value.

You need to allocate memory for each string you want to copy. You could do that with strdup:

shared_array[i] = strdup(line);

But the problem with this is that you have shared memory containing pointers to non-shared memory.

You'll need to allocate space in shared memory for an array of strings:

const MAXLINE = 80;
char (*shared_array)[MAXLINE]; 
shared_array= mmap(0,100*MAXLINE,PROT_READ | PROT_WRITE,MAP_SHARED | MAP_ANON,-1,0);

Then you can use strcpy to copy into each array element:

strcpy(shared_array[i], line);

Upvotes: 4

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