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?

Solution 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.

Solution 2

You shouldn't define errno variable by yourself. errno it is global varibale (aclually it's more complex than just varibale) defined in errno.h So remove you int errno = 0; and run again. Dont forget to include errno.h

Solution 3

You're declaring a local errno variable, effectively masking the global errno. You need to include errno.h, and declare the extern errno, e.g.:

#include <errno.h>
...

extern int    errno;

...
fd = open( "/dev/tty0", O_RDWR | O_SYNC );
if ( fd < 0 ) {
    fprintf( stderr, "errno is %d\n", errno );
    ... error handling goes here ...
}

You can also use strerror() to turn the errno integer into a human readable error message. You'll need to include string.h for that:

#include <errno.h>
#include <string.h>

fprintf( stderr, "Error is %s (errno=%d)\n", strerror( errno ), errno );

Solution 4

Please add this to your module: #include <errno.h> instead of int errno;

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Comments

[ArjunShankar]

Also, don't do errno = 0. open will set it correctly anyway.

[cnicutar]

@Mr.32 That open call seems to be directly opening a tty device, typically associated with a console. I suspect the error message is EPERM.