open (system call)

<h2 id="open-call-arguments">open call arguments</h2>
<ol><li>The <a href="/facts/Pathname/F5wNAuMY">pathname</a> to the file,</li>
<li>The kind of access requested on the file (read, write, append etc.),</li>
<li>The initial file permission is requested using the third argument called mode. This argument is relevant only when a new file is being created.</li></ol>
<p>After using the file, the process should close the file using <a href="/facts/Close_(system_call)/gzzGEtKc">close</a> call, which takes the file descriptor of the file to be closed. Some filesystems include a disposition to permit releasing the file.
</p><p>Some computer languages include run-time libraries which include additional functionality for particular filesystems. The open (or some auxiliary routine) may include specifications for key size, record size, connection speed. Some open routines include specification of the program code to be executed in the event of an error.
</p>
<h3>Perl language form</h3>
open FILEHANDLE,MODE[,EXPR]

<p>for example:
</p>
open(my $fh, ">", "output.txt");

<p><a href="/facts/Perl/fx31kjlT">Perl</a> also uses the tie function of the Tie::File module to associate an array with a 
file.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> The tie::AnyDBM_File function associates a hash with a file.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p>
<h3>C library POSIX definition</h3>
<p>The  <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html">open</a> call is standardized by the <a href="/facts/POSIX/mQkSt234">POSIX</a> specification for <a href="/facts/C_(programming_language)/Ky2No763">C language</a>:
</p>
int open(const char *path, int oflag, .../*,mode_t mode */);
int openat(int fd, const char *path, int oflag, ...);
int creat(const char *path, mode_t mode);
FILE *fopen(const char *restrict filename, const char *restrict mode);

<p>The value returned is a file descriptor which is a reference to a process specific  structure which contains, among other things, a position pointer that indicates which place in the file will be acted upon by the next operation.
</p><p>Open may return <a href="/facts/%E2%88%921/WtI4msbk">−1</a> indicating a failure with errno detailing the error.
</p><p>The file system also updates a global table of all open files which is used for determining if a file is currently in use by any process.
</p>
<h4>path</h4>
<p>The name of the file to open. It includes the <a href="/facts/File_path/F5wNAuMY">file path</a> defining where, in which file system, the file is found (or should be created).
</p><p>openat expects a relative path.
</p>
<h4>oflag</h4>
<p>This argument formed by <a href="/facts/Mask_(computing)/d9Cu1A1Y">OR'ing together</a> optional parameters and  (from <<a href="/facts/Fcntl.h/Iw74UKqG">fcntl.h</a>>) one of:
</p>
O_RDONLY, O_RDWR and O_WRONLY
<p>Option parameters include:
</p>
<ul><li>O_APPEND data written will be appended to the end of the file. The file operations will always adjust the position pointer to the end of the file.</li>
<li>O_CREAT Create the file if it does not exist; otherwise it has no effect except as noted under O_EXCL below.</li>
<li>O_EXCL Used with O_CREAT if the file already exists, then fail, setting errno to EEXIST.</li>
<li>O_TRUNC If the file already exists  then discard its previous contents, reducing it to an empty file. Not applicable for a device or named pipe.</li></ul>
<p>Additional flags and errors are defined in <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html">open</a> call.
</p><p>creat() is implemented as:
</p>
int creat(const char *path, mode_t mode)
{
    return open(path, O_WRONLY|O_CREAT|O_TRUNC, mode);
}

<p><a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html">fopen</a> uses string flags such as r, w, a and + and returns a file pointer used with <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fgets.html">fgets</a>, <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fputs.html">fputs</a> and <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fclose.html">fclose</a>.
</p>
<h4>mode</h4>
<p>Optional and relevant only when creating a new file, defines the
<a href="/facts/File_permissions/DHq5SwI4">file permissions</a>. These include  read, write or execute the file by the owner, group or all users. The mode is masked by the calling process's <a href="/facts/Umask/guGm0ye9">umask</a>: bits set in the umask are cleared in the mode.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/File_descriptor/Iw74UKqG">File descriptor</a> – how it works and other functions related to open</li></ul>
<h2 id="notes">Notes</h2>

<ul><li>Advanced Programming in the UNIX Environment by W. Richard Stevens <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 81-7808-096-6</li>
<li>UNIX concept & application by Sumitabh Das</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Tie::File". perldoc.perl.org. Retrieved 2011-08-07. <a href="http://perldoc.perl.org/Tie/File.html" target="_blank">http://perldoc.perl.org/Tie/File.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"AnyDBM_File". perldoc.perl.org. Retrieved 2011-08-07. <a href="http://perldoc.perl.org/AnyDBM_File.html" target="_blank">http://perldoc.perl.org/AnyDBM_File.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

open (system call) open-in-new

open (system call)