11-06-2002
umask
in this unix book that i have, it says:
the statement:
filedes = open(pathname, O_CREAT, mode);
is actually
filedes = open(pathname, O_CREAT, (~mask)&mode); /* ~ is the negation symbol */
like it's doing some type of masking.
for example,
fd = open("newfile", O_CREAT, 0644);
will actually given the mode of 0640.
how did it come up with that? what value does it mask to my mode set to come up with 0640?
why is it doing that?
thanks
Last edited by bb00y; 11-06-2002 at 04:51 AM..
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LEARN ABOUT OSF1
sem_open
sem_open(3) Library Functions Manual sem_open(3)
NAME
sem_open - Opens/creates a named semaphore for use by a process (P1003.1b)
LIBRARY
Realtime Library (librt.so, librt.a)
SYNOPSIS
#include <semaphore.h>
sem_t *sem_open (
const char *name,
int oflag,
mode_t mode,
unsigned int value);
PARAMETERS
name Points to a string naming the semaphore object.
oflag Specifies whether the semaphore is to be created (O_CREAT flag bit set) or only opened (O_CREAT flag bit clear). If O_CREAT is
set, the O_EXCL flag bit may additionally be set to specify that the call should fail if a semaphore of the same name already
exists. The O_CREAT and O_EXCL flags are defined in the <fcntl.h> header file.
mode Specifies the semaphore's permission bits. This argument is used only when the semaphore is being created.
value Specifies the initial value to be given to the semaphore. This argument is used only when the semaphore is being created.
DESCRIPTION
Use the sem_open function to establish the connection between a named semaphore and a process. Subsequently, the calling process can ref-
erence the semaphore by using the address returned from the call. The semaphore is available in subsequent calls to sem_wait, sem_trywait,
sem_post, and sem_getvalue functions. The semaphore remains usable by the process until the semaphore is closed by a successful call to
the sem_close function.
The O_CREAT flag bit in the oflag parameter controls whether the semaphore is created or only opened by the call to sem_open.
A created semaphore's user ID is set to the user ID of the calling process and its group ID is set to a system default group or to the
group ID of the process. The semaphore's permission bits are set to the value of the mode argument, except for those set in the file mode
creation mask of the process.
After a semaphore is created, other processes can open the semaphore by calling sem_open with the same value for the name argument.
RETURN VALUES
On successful completion, this function opens the semaphore and returns the semaphore's descriptor. Otherwise, the function returns -1 and
sets errno to indicate the error.
ERRORS
The sem_open function fails under the following conditions:
[EACCES] The named semaphore exists and the permissions specified by oflag are denied, or the named semaphore does not exist and
the permissions specified by oflag are denied.
[EEXIST] O_CREAT and O_EXCL are set and the named semaphore already exists.
[EINTR] The sem_open operation was interrupted by a signal.
[EINVAL] The sem_open operation is not supported for the given name. Or, O_CREAT was specified in oflag and value was greater
than {SEM_VALUE_MAX}.
[EMFILE] Too many semaphore descriptors or file descriptors are currently in use by this process.
[ENAMETOOLONG] The length of the name string exceeds {PATH_MAX}, or a pathname component is longer than {NAME_MAX} while
{_POSIX_NO_TRUNC} is in effect.
[ENFILE] Too many semaphores are currently open in the system.
[ENOENT] O_CREAT is not set and the named semaphore does not exist.
[ENOSPC] Insufficient space exists for the creation of a new named semaphore.
RELATED INFORMATION
Functions: sem_close(3), sem_post(3), sem_trywait(3), sem_unlink(3), sem_wait(3)
Guide to Realtime Programming delim off
sem_open(3)