mktemp, mkstemp, mkstemps, mkdtemp - Construct a unique
file name or directory
#include <stdlib.h>
char *mktemp(
char *template ); int mkstemp(
char *template ); int mkstemps(
char *template,
int suffixlen ); char *mkdtemp(
char *template );
Standard C Library (libc)
System V Compatibility Library (libsys5)
Interfaces documented on this reference page conform to
industry standards as follows:
mktemp(), mkstemp(): XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Points to a string to be modified to form a unique file
name. The string should be composed of characters from the
portable character set. Elements in the string consist of
an optional prefix, 6 or more X characters (which are
replaced to make the file name unique), and -- in the case
of the mkstemps() function -- a suffix. The template
string prior to the suffix element should look like a file
name with six or more trailing X characters. The string
has the following forms: For mktemp(), mkstemp(), and
mkdtemp() functions:
[prefix]XXXXXXX For the mkstemps() function:
[prefix]XXXXXXsuffix
The prefix portion of the template string (often a
directory specification and/or file name prefix) is
optional, the string of 6 or more Xs is replaced
when the file name is generated, and the suffix
portion is the string to be appended when generating
the file name. The template string prior to
suffix should look like a file name with six or
more trailing X characters. The length of the suffix
in the template string.
The mktemp() function replaces, with a unique file name,
the contents of the template string pointed to by the template
parameter. The application should initialize the
template string to be a file name with six or more trailing
X characters. To generate a unique file name, mktemp()
replaces the Xs in the template string with characters
from the portable character set. Note that a possible race
condition can exist when using the mktemp() function. The
mkstemp(), mkstemps(), and mkdtemp() functions avoid this
problem; see APPLICATION USAGE for details.
For compatibility with SVID2, the libsys5 version of
mktemp() uses the getpid() function when generating the
file name. In addition to suffering from the same race
condition mentioned above, the required use of getpid() by
the libsys5 version of mktemp() results in significantly
more predictable file name generation, and this predictability
could be misused by a hostile process. To
avoid these problems, use the mkstemp(), mkstemps(), or
mkdtemp() functions (not specified by SVID2).
The mkstemp() function performs the same substitution to
the template name as mktemp(), but it also creates the
file with mode 0600 and returns, for the file, a file
descriptor that is open for reading and writing. The function
thus prevents any possible race condition that could
exist in the interval between testing whether the file
exists and opening it for use.
The mkstemps() function is similar to the mkstemp() function,
except that it permits the inclusion of a trailing
suffix in the template string. The suffix is appended to
the generated file name.
The mkdtemp() function is similar to the mkstemp() function,
except that it creates a mode 0700 directory instead
of a file name and returns a pointer to a string containing
the resulting directory path that has been created.
The mkdtemp() and mkstemps() functions first appeared in
OpenBSD.
Upon successful completion, the mktemp() function returns
the address of the string pointed to by the template
parameter. If the string pointed to by the template parameter
does not contain trailing Xs or if the mktemp() function
is unable to construct a unique file name, the first
character of the template string is replaced with a null
character and a null pointer is returned.
Upon successful completion, the mkstemp() function returns
an open file descriptor. If the string pointed to by the
template parameter does not contain trailing Xs or if no
suitable file can be created, -1 is returned.
Upon successful completion, the mkstemps() function
returns an open file descriptor. Under any of the following
conditions, -1 is returned and errno is set to indicate
the error: If the string pointed to by the template
parameter does not contain trailing Xs prior to the start
of the suffix If the suffix contains a slash (/) If suffixlen
is larger than the length of the template If no
suitable file can be created
Upon successful completion, the mkdtemp() function returns
the address of the string pointed to by the template
parameter. If the string pointed to by the template parameter
does not contain trailing Xs or if no suitable directory
can be created, a null pointer is returned and errno
is set to indicate the error.
No errors are defined for the mktemp() or mkstemp() functions.
The mkstemps() function may set errno to one of the following
values or to any value specified by the open() or
stat() function: The string pointed to by the template
parameter does not contain trailing Xs prior to the start
of the suffix, the suffix contains a slash (/), or suffixlen
is larger than the length of the template. The
pathname portion of the template is not an existing directory.
The mkdtemp() function may set errno to one of the following
values or to any value specified by the stat() and
mkdir() functions: The string pointed to by the template
parameter does not contain trailing Xs. The pathname portion
of the template is not an existing directory.
The use of mktemp() is discouraged due to a race condition
that may exist between the time a file name is created and
the time the file is opened. During this interval, it is
possible for some other (potentially hostile) process to
create a file with the same name. The mkstemp(),
mkstemps(), and mkdtemp() functions avoid this problem by
creating the file name or directory in a way that ensures
that duplication cannot occur.
It is important that the template parameter passed to
these functions does not point to a read-only string. For
example, a call such as:
int retval; retval = mkstemp("/tmp/temp.XXXXXX");
may result in a core dump when the mkstemp() function
attempts to modify a string constant.
The following example uses mkstemp() to create and open a
temp file:
#include <string.h> #include <stdlib.h> #include
<unistd.h> #include <stdio.h> main() {
char template[16];
int fd = -1;
strcpy(template, "/tmp/tempXXXXXX");
if ((fd = mkstemp(template)) != -1) {
/* use 'fd' here */
unlink(template);
close(fd);
} else
fprintf(stderr, "Failed to create temp file.\n");
}
The following example uses mkstemps() to create and open a
temp file with a suffix:
#include <string.h> #include <stdlib.h> #include
<unistd.h> #include <stdio.h> #include <errno.h> main() {
char template[20];
int fd = -1;
strcpy(template, "/tmp/tempXXXXXXX.xyz");
if ((fd = mkstemps(template, 4)) != -1) {
/* use 'fd' here */
unlink(template);
close(fd);
} else
fprintf(stderr, "Failed to create temp
file.\n%s\n",
strerror(errno)); }
The following example uses mkdtemp() to create a temp
directory and, if successful, then uses mkstemp() to create
a temp file within the directory:
#include <string.h> #include <stdlib.h> #include
<unistd.h> #include <stdio.h> #include <errno.h> main() {
char template[35], dirpath[22], *dp;
int fd = -1;
strcpy(dirpath, "/tmp/tempdir.XXXXXXXX");
if ((dp = mkdtemp(dirpath)) != NULL) {
/* begin - example use of 'dp' */
sprintf(template, "%s/%s", dp, "tempXXXXXXXX");
if ((fd = mkstemp(template)) != -1) {
/* use 'fd' here */
unlink(template);
close(fd);
} else
fprintf(stderr, "Failed to create temp
file.\n%s\n",
strerror(errno));
/* end - example use of 'dp' */
rmdir(dirpath);
} else
fprintf(stderr, "Failed to create temp directory.\n%s\n",
strerror(errno)); }
Functions: tmpfile(3), tmpnam(3), mkdir(2), open(2),
stat(2)
Standards: standards(5)
mktemp(3)
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