Why does open() fail and errno is not set?

Question

In my code open() fails with a return code of -1 but somehow errno is not getting set.

int fd;
int errno=0;
fd = open("/dev/tty0", O_RDWR | O_SYNC);
printf("errno is %d and fd is %d",errno,fd);

output is

errno is 0 and fd is -1

Why is errno not being set? How can i determine why open() fails?

Answer 1

int errno=0;

The problem is you redeclared errno, thereby shadowing the global symbol (which need not even be a plain variable). The effect is that what open sets and what you're printing are different things. Instead you should include the standard errno.h.