Perl 5 version 12.3 documentation
- sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This function calls the underlying operating system's
openfunction with the parameters FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module
Fcntl. See the documentation of your operating system's
opento see which values and flag bits are available. You may combine several flags using the
Some of the most common values are
O_RDONLYfor opening the file in read-only mode,
O_WRONLYfor opening the file in write-only mode, and
O_RDWRfor opening the file in read-write mode.
For historical reasons, some values work on almost every system supported by Perl: 0 means read-only, 1 means write-only, and 2 means read/write. We know that these values do not work under OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the
opencall creates it (typically because MODE includes the
O_CREATflag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to
sysopen, Perl uses the octal value
0666. These permission values need to be in octal, and are modified by your process's current
In many systems the
O_EXCLflag is available for opening files in exclusive mode. This is not locking: exclusiveness means here that if the file already exists, sysopen() fails.
O_EXCLmay not work on network filesystems, and has no effect unless the
O_CREATflag is set as well. Setting
O_CREAT|O_EXCLprevents the file from being opened if it is a symbolic link. It does not protect against symbolic links in the file's path.
Sometimes you may want to truncate an already-existing file. This can be done using the
O_TRUNCflag. The behavior of
You should seldom if ever use
0644as argument to
sysopen, because that takes away the user's option to have a more permissive umask. Better to omit it. See the perlfunc(1) entry on
umaskfor more on this.
sysopendepends on the fdopen() C library function. On many Unix systems, fdopen() is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider rebuilding Perl to use the
sfiolibrary, or perhaps using the POSIX::open() function.
See perlopentut for a kinder, gentler explanation of opening files.