Function open() sets errno Post: 302827003

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Top Forums Programming Function open() sets errno Post 302827003 by Corona688 on Thursday 27th of June 2013 05:42:08 PM

06-27-2013

Registered User

From man 3 errno:

Code:

       The  <errno.h> header file defines the integer variable errno, which is
       set by system calls and some library functions in the event of an error
       to  indicate  what  went wrong.  Its value is significant only when the
       return value of the call indicated an error (i.e., -1 from most  system
       calls;  -1  or  NULL from most library functions); a function that suc-
       ceeds is allowed to change errno.

So, errno should never be used to check whether an error happened -- only which error. It's easy to picture more elaborate library calls changing the value of errno many times before they return... You must be exact about when and why you use it for what to get something meaningful.

I'm not sure why a successful system call would be changing errno, but it's allowed to. Perhaps it was a simplification -- "these first 4 cases will all return EACCESS, so set it first, and return immediately if any of them fail". And they never bother to change the error to success when it succeeds.

Another invalid way to use errno is checking the value of errno too late, after they've made another system call. This can give you the strange result 'ERROR: Success'.

Last edited by Corona688; 06-27-2013 at 06:48 PM..

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LEARN ABOUT LINUX

fdopendir

OPENDIR(3)						     Linux Programmer's Manual							OPENDIR(3)

NAME

       opendir, fdopendir - open a directory

SYNOPSIS

       #include <sys/types.h>
       #include <dirent.h>

       DIR *opendir(const char *name);
       DIR *fdopendir(int fd);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fdopendir():
	   Since glibc 2.10:
	       _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
	   Before glibc 2.10:
	       _GNU_SOURCE

DESCRIPTION

       The  opendir()  function  opens a directory stream corresponding to the directory name, and returns a pointer to the directory stream.  The
       stream is positioned at the first entry in the directory.

       The fdopendir() function is like opendir(), but returns a directory stream for the directory referred to by the open  file  descriptor  fd.
       After a successful call to fdopendir(), fd is used internally by the implementation, and should not otherwise be used by the application.

RETURN VALUE

       The  opendir()  and fdopendir() functions return a pointer to the directory stream.  On error, NULL is returned, and errno is set appropri-
       ately.

ERRORS

       EACCES Permission denied.

       EBADF  fd is not a valid file descriptor opened for reading.

       EMFILE Too many file descriptors in use by process.

       ENFILE Too many files are currently open in the system.

       ENOENT Directory does not exist, or name is an empty string.

       ENOMEM Insufficient memory to complete the operation.

       ENOTDIR
	      name is not a directory.

VERSIONS

       fdopendir() is available in glibc since version 2.4.

CONFORMING TO

       opendir() is present on SVr4, 4.3BSD, and specified in POSIX.1-2001.  fdopendir() is specified in POSIX.1-2008.

NOTES

       The underlying file descriptor of the directory stream can be obtained using dirfd(3).

       The opendir() function sets the close-on-exec flag for the file descriptor underlying the DIR *.  The fdopendir() function leaves the  set-
       ting  of  the  close-on-exec  flag  unchanged for the file descriptor, fd.  POSIX.1-200x leaves it unspecified whether a successful call to
       fdopendir() will set the close-on-exec flag for the file descriptor, fd.

SEE ALSO

       open(2), closedir(3), dirfd(3), readdir(3), rewinddir(3), scandir(3), seekdir(3), telldir(3)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2010-06-20								OPENDIR(3)