That's not the preferred way to specify file modes in UNIX. By not giving it the O_CREAT file it means that the file exists and the open call will have no affect on it and instead of giving 0644 for the file mode bits use...
That's for creating and opening a file that does not exist because if it does then the open call fails so should check its return value.
What does the system call "dup" do?
What is the difference between dup and dup2
I have a fair idea of what it does but I am confused when its coming down to the exact details...
Please help me!:confused: (2 Replies)
open, creat, read, write, lseek and close
Are they all primitive?
:confused:
*Another Question: is there a different between a system call, and an i/o system call? (2 Replies)
Hi,
I'm new to UNIX system calls. Can someone share your knowledge as to how exactly system calls should be executed?
Can they be typed like commands such as mkdir on the terminal itself? Also, are there any websites which will show me an example of the output to expect when a system call like... (1 Reply)
Hello,
how would i be able to call ps in C programming?
thanks,
---------- Post updated at 01:39 AM ---------- Previous update was at 01:31 AM ----------
here's the complete system call, ps -o pid -p %d, getpit() (2 Replies)
I'm currently studying for my exam, and is practicing with sample exam questions.
However there is a question asking "Name THREE UNIX Directory system calls" and the answer given is "opendir, closedir and readdir", however the next question ask "Why is a write directory system call not included... (1 Reply)
Hello...
I'm trying to setup a cronjob to record system data using glance at certain times of the day. My question is, how would one export the "Global System Calls" information to a file? Below is the command I have been using and it works to export CPU information.
glance -f ... (0 Replies)
Hi i am very new to programming in UNIX and don't understand the difference between a system call and a normal function call. Also can I implement system calls from within a program? If so could someone please give me an example of a system call from within a program. Lastly, when creating a... (1 Reply)
Discussion started by: bjhum33
1 Replies
LEARN ABOUT MOJAVE
shm_open
SHM_OPEN(2) BSD System Calls Manual SHM_OPEN(2)NAME
shm_open -- open a shared memory object
SYNOPSIS
#include <sys/mman.h>
#include <fcntl.h>
int
shm_open(const char *name, int oflag, ...);
The parameter "mode_t mode" is optional.
DESCRIPTION
The shared memory object referenced by name is opened for reading and/or writing as specified by the argument oflag and the file descriptor
returned to the calling process. The returned file descriptor will be the lowest non-open file descriptor for the calling process, and is
not shared with any other processes, as it is a new file descriptor. The new file descriptor will have the FD_CLOEXEC flag set. Repeated
calls to shm_open with the same string value for name() will return a file descriptor referring to the same shared memory object, provided
that the object has not been unlinked by a call to shm_unlink(). The oflag argument may indicate the file is to be created if it does not
exist (by specifying the O_CREAT flag), in which case the file is created with mode mode as described in chmod(2) and modified by the
process' umask value (see umask(2)).
The value of oflag is formed by or'ing the following values:
O_RDONLY open for reading only
O_RDWR open for reading and writing
O_CREAT create object if it does not exist
O_EXCL error if create and object exists
O_TRUNC truncate size to 0
Exactly one of O_RDONLY or O_RDWR must be specified.
If O_TRUNC is specified and the file exists, the file is truncated to zero length. If O_EXCL is set with O_CREAT and the file already
exists, shm_open() returns an error. This may be used to implement a simple exclusive access locking mechanism.
If successful, shm_open() returns a non-negative integer, termed a file descriptor. It returns -1 and sets errno on failure. The file
pointer used to mark the current position within the memory object is set to the beginning of the object.
When a new shared memory object is created it is given the owner and group corresponding to the effective user and group of the calling
process. There is no visible entry in the file system for the created object in this implementation.
When a shared memory object is created, it persists until it it unlinked and all other references are gone. Objects do not persist across a
system reboot.
The system imposes a limit on the number of file descriptors open simultaneously by one process. getdtablesize(2) returns the current system
limit.
ERRORS
The named object is opened unless:
[EACCES] The required permissions (for reading and/or writing) are denied for the given flags.
[EACCES] O_CREAT is specified, the object does not exist, and permission to create the object is denied.
[EEXIST] O_CREAT and O_EXCL were specified and the object exists.
[EINTR] The shm_open() operation was interrupted by a signal.
[EINVAL] The shm_open() operation is not supported.
[EMFILE] The process has already reached its limit for open file descriptors.
[ENAMETOOLONG] name exceeded the name size limit. This is currently PSHMNAMLEN characters (defined in <sys/posix_shm.h>), but this may
change in the future.
[ENFILE] The system file table is full.
[ENOENT] O_CREAT is not set and the named object does not exist.
[ENOSPC] O_CREAT is specified, the file does not exist, and there is insufficient space available to create the object.
SEE ALSO chmod(2), close(2), getdtablesize(2), mmap(2), shm_unlink(2), umask(2)HISTORY
shm_open() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).
Darwin August 29, 2008 Darwin