Correct use of Stat on C

Question

Why does this work :

char *fd = "myfile.txt";
struct stat buf;          

stat(fd, &buf);
int size = buf.st_size;

printf("%d",size);

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But this does not work:

char *fd = "myfile.txt";
struct stat *buf;          

stat(fd, buf);
int size = buf->st_size;

printf("%d",size);

Answer 1

The reason for it not working is that buf in the first example is allocated on the stack. In the Second example you only have a pointer to a struct stat, pointing to anywhere (probably pointing to address 0x0, i.e. a NULL pointer), you need to allocate memory for it like this:

buf = malloc(sizeof(struct stat));

Then both examples should work. When using malloc(), always remember to use free() after you are finished with using the struct stat:

free(buf);

Answer 2

It is just a simple memory allocation problem.

char *fd = "myfile.txt";
struct stat *buf;          

stat(fd, buf);
int size = buf->st_size;

printf("%d",size);

The above code only declares a pointer, but in reality, there is no memory space allocated.

you should modify the code to look like this:

char *fd = "myfile.txt";
struct stat *buf;

buf = malloc(sizeof(struct stat));

stat(fd, buf);
int size = buf->st_size;
printf("%d",size);

free(buf);

This will allocate the memory, and free after it is used.