man3/mkstemp.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright (C) 2008, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 18:48:48 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified 980310, aeb
.\" Modified 990328, aeb
.\" 2008-06-19, mtk, Added mkostemp(); various other changes
.\"
.TH MKSTEMP 3  2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
mkstemp, mkostemp, mkstemps, mkostemps \- create a unique temporary file
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.BI "int mkstemp(char *" template );
.BI "int mkostemp(char *" template ", int " flags );
.BI "int mkstemps(char *" template ", int " suffixlen );
.BI "int mkostemps(char *" template ", int " suffixlen ", int " flags );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR mkstemp ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
        || /* Glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
.fi
.PP
.BR mkostemp ():
.nf
    _GNU_SOURCE
.fi
.PP
.BR mkstemps ():
.nf
    /* Glibc since 2.19: */ _DEFAULT_SOURCE
        || /* Glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
.fi
.PP
.BR mkostemps ():
.nf
    _GNU_SOURCE
.fi
.SH DESCRIPTION
The
.BR mkstemp ()
function generates a unique temporary filename from
.IR template ,
creates and opens the file,
and returns an open file descriptor for the file.
.PP
The last six characters of
.I template
must be "XXXXXX" and these are replaced with a string that makes the
filename unique.
Since it will be modified,
.I template
must not be a string constant, but should be declared as a character array.
.PP
The file is created with
permissions 0600, that is, read plus write for owner only.
The returned file descriptor provides both read and write access to the file.
The file is opened with the
.BR open (2)
.B O_EXCL
flag, guaranteeing that the caller is the process that creates the file.
.PP
The
.BR mkostemp ()
function is like
.BR mkstemp (),
with the difference that the following bits\(emwith the same meaning as for
.BR open (2)\(emmay
be specified in
.IR flags :
.BR O_APPEND ,
.BR O_CLOEXEC ,
and
.BR O_SYNC .
Note that when creating the file,
.BR mkostemp ()
includes the values
.BR O_RDWR ,
.BR O_CREAT ,
and
.BR O_EXCL
in the
.I flags
argument given to
.BR open (2);
including these values in the
.I flags
argument given to
.BR mkostemp ()
is unnecessary, and produces errors on some
.\" Reportedly, FreeBSD
systems.
.PP
The
.BR mkstemps ()
function is like
.BR mkstemp (),
except that the string in
.I template
contains a suffix of
.I suffixlen
characters.
Thus,
.I template
is of the form
.IR "prefixXXXXXXsuffix" ,
and the string XXXXXX is modified as for
.BR mkstemp ().
.PP
The
.BR mkostemps ()
function is to
.BR mkstemps ()
as
.BR mkostemp ()
is to
.BR mkstemp ().
.SH RETURN VALUE
On success, these functions return the file descriptor
of the temporary file.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EEXIST
Could not create a unique temporary filename.
Now the contents of \fItemplate\fP are undefined.
.TP
.B EINVAL
For
.BR mkstemp ()
and
.BR mkostemp ():
The last six characters of \fItemplate\fP were not XXXXXX;
now \fItemplate\fP is unchanged.
.IP
For
.BR mkstemps ()
and
.BR mkostemps ():
.I template
is less than
.I "(6 + suffixlen)"
characters long, or the last 6 characters before the suffix in
.I template
were not XXXXXX.
.PP
These functions may also fail with any of the errors described for
.BR open (2).
.SH VERSIONS
.BR mkostemp ()
is available since glibc 2.7.
.BR mkstemps ()
and
.BR mkostemps ()
are available since glibc 2.11.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR mkstemp (),
.BR mkostemp (),
.BR mkstemps (),
.BR mkostemps ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
.BR mkstemp ():
4.3BSD, POSIX.1-2001.
.PP
.BR mkstemps ():
unstandardized, but appears on several other systems.
.\" mkstemps() appears to be at least on the BSDs, Mac OS X, Solaris,
.\" and Tru64.
.PP
.BR mkostemp ()
and
.BR mkostemps ():
are glibc extensions.
.SH NOTES
In glibc versions 2.06 and earlier, the file is created with permissions 0666,
that is, read and write for all users.
This old behavior may be
a security risk, especially since other UNIX flavors use 0600,
and somebody might overlook this detail when porting programs.
POSIX.1-2008 adds a requirement that the file be created with mode 0600.
.PP
More generally, the POSIX specification of
.BR mkstemp ()
does not say anything
about file modes, so the application should make sure its
file mode creation mask (see
.BR umask (2))
is set appropriately before calling
.BR mkstemp ()
(and
.BR mkostemp ()).
.\"
.\" The prototype for
.\" .BR mkstemp ()
.\" is in
.\" .I <unistd.h>
.\" for libc4, libc5, glibc1; glibc2 follows POSIX.1 and has the prototype in
.\" .IR <stdlib.h> .
.SH SEE ALSO
.BR mkdtemp (3),
.BR mktemp (3),
.BR tempnam (3),
.BR tmpfile (3),
.BR tmpnam (3)