CODEWITHSUNDEEP
clinux
Groosha
Groosha

Reputation: 3067

Strange behaviour with arrays in C

sorry for my bad English. I'm new to Linux system programming and new to C programming as well.
At the moment I'm trying to unlink files, to do this, I must store file path somewhere. I'm using 1024-element char array for each file. If I decrease the size of array, file descriptors become a mess. Sounds stupid, but it is.
Here's some code:

char path[1024], path2[1024];
const char *file_name = "myfile_1", *file_name2 = "myfile_2",*working_directory="/home/Alexander/lab01/";
strcpy(path, working_directory);
strcat(path, file_name);
strcpy(path2, working_directory);
strcat(path2, file_name);

then I open some files, read\write and so on. If path & path2 are 1024 bytes long, everything goes well..all_ok
but when I decrease path & path2 size to 512 or 256, some strange thing happens with memory and other file descriptors...
all_crap
I can't understand what's going on, please, help.

Code, where I read file:

fdesc_input = open("/dev/urandom",O_RDONLY);
if (fdesc_input < 0 ) {
    perror("Error opening /dev/urandom: ");
}
fdesc_output = open(path, O_RDWR|O_CREAT|O_TRUNC, 0777);
if (fdesc_output < 0 ) {
        perror("Error opening my file: ");
    }
buffer = (unsigned char*) malloc(buffer_size); // make 1kb buffer

desired_filesize = 1024*10; // 10 kilobytes

int curr_size = 0;
while (curr_size < desired_filesize) {

           //AFTER NEXT LINE STRANGE HAPPENS
    ssize_t result = read(fdesc_input, &buffer, buffer_size);
    if (result < 0) {
        perror ("Error reading /dev/urandom: ");
        exit(1);
    }
    curr_size += result;
    write(fdesc_output, &buffer, buffer_size);
}

Upvotes: 0

Views: 88

Answers (1)

Elmar Peise
Elmar Peise

Reputation: 15413

You are passing &buffer to read, however, buffer itself is a pointer. You pass the address of buffer which is a local variable. As a result, read reads data into the memory starting at this local variable and thus overwriting the other local variables.

Pass buffer instead of &buffer and you should be fine.

Upvotes: 3

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