read fails with EFAULT - c

Question

I am running the following C code, where trying to read in buffer which
is allocated on caller's stack, but fails with errno 14 (Bad Address).
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
void wrapper(int fd, char **buf)
{
int res = read(fd, *buf, 10);
printf("res: %d, errno: %d\n", res, errno);
printf("Buf: %s\n", *buf);
}
int main()
{
char buffer[10];
memset(buffer, 0, 10);
int fd = open("main.c", O_RDONLY);
wrapper(fd, (char **)&buffer);
return 0;
}
The output is
res: -1, errno: 14
Buf: (null)
I have been searching for explanation why it fails, whereas changing it to
void wrapper(int fd, char *buf)
...
wrapper(fd, (char *)buffer);
works, but without result so far.

Answer 1

why it fails
Arrays are not pointers. buffer is not a char*. Consequently, &buffer is not a char**, is not compatible with char**, and should not be cast to char**. If it is cast to char** and then dereferenced, the behaviour is undefined.

Answer 2

After analyzed your intention, of course it is possible to create something like a "wrapper" containing read string by read(2) syscall and use that buffer away from wrapper() function. You wanted to pass amount of characters which would be read from file being in a table of files whom index of the table (file descriptor) was return by open(2) syscall. But as n.m. said, arrays are not pointers and your solution cannot work properly.
Let me explain my simple fix to your code:
#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#define AMOUNT 20
#define assert_msg(x) for ( ; !(x) ; assert(x) )
void
wrapper(int fd, char **buf, size_t size)
{
ssize_t res;
char *out;
out = calloc(size + 1, sizeof(char));
assert(out != NULL);
res = read(fd, out, size);
assert_msg(res != -1) {
fprintf(stderr, "Error ocurred: %s\n", strerror(errno));
}
out[size] = '\0';
fprintf(stdout, "Inside function: %s\n", out);
fprintf(stdout, "res: %d, size: %d, errno: (%d: %s)\n", res, size,
errno, strerror(errno));
*buf = out;
}
int
main(int argc, char **argv)
{
int fd;
char *buf;
buf = NULL;
assert(argc == 2);
errno = 0;
fd = open(argv[1], O_RDONLY);
assert_msg(fd != -1) {
fprintf(stderr, "Error ocurred: %s\n", strerror(errno));
}
wrapper(fd, &buf, AMOUNT);
fprintf(stdout, "Outside function: %s\n", buf);
free(buf);
return (EXIT_SUCCESS);
}
I pass a filename as an input argument. It was a bit easier for me instead of hardcoding the name.
As you can see, inside my wrapper() implementation I allocate memory for an out buffer which size I am passing by a value of size variable. I know that the same value as AMOUNT value defined as macro but it would be easy to change in any other solution.
Then, I read given amount of characters using read(2) syscall, from a file descriptor returned by open(2) syscall in main() function which I pass to wrapper().
At the end of that function I tell that I would like to save an address to the beginning of allocated out buffer and I would like that *buf indicates on that address. It is a buffer of size + 1 char elements, allocated on heap, not on a local stack. Therefore program cannot "reuse" that addresses during his execution. Every address for variables declared like int a;, struct type name; or char tab[10]; are "freed" automatically after the end of function and you do not have an access to it. To be clear, you may have an access (e.g. print data from address saved to indicator) but you cannot be sure that you would not lose the data being saved there. Space allocated manually still exist on a heap until calling free(3) function.
So if we would do something like:
void
wrapper(int fd, char **buf, const size_t size)
{
ssize_t res;
char out[size];
(...)
*buf = out;
}
you may lost your data being saved on a local stack during continuing program execution.
Additionally, in my solution I also defined my own macro assert_msg(x) which is able to run assert(3) function and shows a text message with error. But it is only a feature but thanks to that we are able to see string corresponding to an errno number.
Of course, my program need better handling errors but it had to present the idea only.
Furthermore, you should also specify file permissions during using open(2) syscall as a third argument. It looks similar to the second argument because it is a bitwise 'or' separated list of values. Example flags: S_IRUSR, S_IRGRP, S_IWOTH etc.
In that argument, you can also just write proper value describing permissions, for example 0755.