Values change after calling function in C

Question

I'm trying to make a file system in C. I have trouble with this portion of my code when I'm printing my values in the code below:

for (int i = 0; i<NUM_POINTERS; i++) {
            printf("before SB->root[%d]=%d\n", i, SB->root->pointers[i]);
        }
        write_blocks(0, 1, SB);
        for (int i = 0; i<NUM_POINTERS; i++) {
            printf("after SB->root[%d]=%d\n", i, SB->root->pointers[i]);
        }

my write_blocks method:

    int write_blocks(int start_address, int nblocks, void *buffer)
{
    int i, e, s;
    e = 0;
    s = 0;

    void* blockWrite = (void*) malloc(BLOCK_SIZE);

    /*Checks that the data requested is within the range of addresses of the disk*/
    if (start_address + nblocks > MAX_BLOCK)
    {
        printf("out of bound error\n");
        return -1;
    }

    /*Goto where the data is to be written on the disk*/
    fseek(fp, start_address * BLOCK_SIZE, SEEK_SET);

    /*For every block requested*/        
    for (i = 0; i < nblocks; ++i)
    {
        /*Pause until the latency duration is elapsed*/
        usleep(L);

        memcpy(blockWrite, buffer+(i*BLOCK_SIZE), BLOCK_SIZE);
        fwrite(blockWrite, BLOCK_SIZE, 1, fp);
        fflush(fp);
        s++;
    }
    free(blockWrite);

    /*If no failure return the number of blocks written, else return the negative number of failures*/
    if (e == 0)
        return s;
    else
        return e;
}

And here's what gets printed:

before SB->root[0]=1

before SB->root[1]=2

before SB->root[2]=3

before SB->root[3]=4

before SB->root[4]=5

before SB->root[5]=6

before SB->root[6]=7

before SB->root[7]=8

before SB->root[8]=9

before SB->root[9]=10

before SB->root[10]=11

before SB->root[11]=12

before SB->root[12]=13

before SB->root[13]=14

after SB->root[0]=1234344888

after SB->root[1]=32688

after SB->root[2]=3

after SB->root[3]=4

after SB->root[4]=5

after SB->root[5]=6

after SB->root[6]=7

after SB->root[7]=8

after SB->root[8]=9

after SB->root[9]=10

after SB->root[10]=11

after SB->root[11]=12

after SB->root[12]=13

after SB->root[13]=14

I don't understand why my first and second pointer value change?

Some additional information: SB is a superBlock here's my structures:

    typedef struct iNode
{
    int id;
    int size;
    int pointers[NUM_POINTERS];
} iNode;

typedef struct superBlock
{
    int magic_number;
    int block_size;
    int num_blocks;
    int num_inodes;
    iNode *root;
    iNode jNodes[20];
} superBlock;

Answer 1

Is this single threaded? Does the modified SB->root[0,1] contain the data you are trying to write? What is your BLOCK_SIZE?

I suspect the problem is outside of write_blocks(). My best guess would be that you accidentally freed SB somewhere and malloc gave you the same address. After the malloc check (print or debugger) both buffer and blockWrite and make sure they are different and valid.

Unrelated Issues:

• printf has more % than params
• You should check the return of malloc
• e is never set
• s and i are equal. AKA redundant.
• Out of bounds error causes a memory leak (since it is after the malloc)
• usleep is strange perhaps you want fsync?