lseek() and read() proper usage - c

Question

I am reading in a "non human readable file" full of structs. I have declared my struct that is a known member of the file. I want to read the file from the begging to the sizeof the stuct populating the fields. I don't understand why my out put is zero for all the struct fields. I have looked at the man pages of these functions and I believe I am using them correctly, so am I just not accessing the fields correctly from the buffer, or? please see below code snippet.
int main (int argc, char * argv[])
{
// make sure you were given an image
if (argc != 2)
{
printf("no image\n");
return -1;
}
int fs_fd = open(argv[1], O_RDWR);
// make sure you can open the image
if(fs_fd == -1)
{
printf("image is busted\n");
return -1;
}
struct superblock *mySB;
mySB = malloc(sizeof(struct superblock));
mySB->size = 0;
mySB->nblocks = 0;
mySB->ninodes = 0;
char buffer[4096];
if(lseek(fs_fd, 0, SEEK_SET));
read(fs_fd, &buffer, sizeof(struct superblock));
mySB->size = buffer[0];
mySB->nblocks = buffer[4];
mySB->ninodes = buffer[8];
printf("Size: %u, nblocks: %u, ninodes: %u\n", mySB->size, mySB->nblocks, mySB->ninodes);
}
ok, suggestions taken into consideration, though in truth I'm not concerned withe good error checking at this point. the following results in a seg fault as i'm sure this is not reading into the struct correctly, but I am trying to use the suggestions.
int main (int argc, char * argv[])
{
// make sure you were given an image
if (argc != 2)
{
printf("usage: file.img");
return -1;
}
int fd = open(argv[1], O_RDWR);
// make sure you can open the image
if(fd == -1)
{
perror("Error> ");
return -1;
}
struct superblock *mySB;
mySB = malloc(sizeof(struct superblock));
read(fd, &mySB, sizeof(struct superblock));
printf("Size: %u, nblocks: %u, ninodes: %u\n", mySB->size, mySB->nblocks, mySB->ninodes);
}

Answer 1

Why no compilable testcase along with an example file?
int fs_fd = open(argv[1], O_RDWR);
// make sure you can open the image
if(fs_fd == -1)
{
printf("image is busted\n");
return -1;
}
Wrong. At the very least use perror to tell what the user what the error is. What's up fs_fd name anyway? Standard name is mere fd.
struct superblock *mySB;
mySB = malloc(sizeof(struct superblock));
Consider sizeof(*mySB);. Null-checks can arguably be ignored.
mySB->size = 0;
mySB->nblocks = 0;
mySB->ninodes = 0;
What's the point of this if you initialize same fields below?
char buffer[4096];
if(lseek(fs_fd, 0, SEEK_SET));
What? Not only teh offset is already at the beginning, you are just ignoring the error (if any) in the worst possible way. If you really need to call this and ignore the error, do (void)lseek(fd_fd, 0, SEEK_SET).
read(fs_fd, &buffer, sizeof(struct superblock));
How about some error checking. If you read only teh size of the struct, why having the buffer be 4096? What if the struct is larger?
mySB->size = buffer[0];
mySB->nblocks = buffer[4];
mySB->ninodes = buffer[8];
Nobody knows how this struct looks like. It seems you have 4-byte sized fields. But you only read ONE byte for each field here.
You likely can just read into the structure instead. If you really want to have an intermediate buffer you have to properly calculate offsets as padding could have screwed you over. Google for offsetof and padding.