mkstemp, mkostemp - create a unique temporary file


#include <stdlib.h>

int mkstemp(char *template);

int mkostemp (char *template, int flags);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

mkstemp(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 mkostemp(): _GNU_SOURCE


The mkstemp() function generates a unique temporary filename from template, creates and opens the file, and returns an open file descriptor for the file.

The last six characters of template must be "XXXXXX" and these are replaced with a string that makes the filename unique. Since it will be modified, template must not be a string constant, but should be declared as a character array.

The file is created with permissions 0600, that is, read plus write for owner only. (In glibc versions 2.06 and earlier, the file is created with permissions 0666, that is, read and write for all users.) The returned file descriptor provides both read and write access to the file. The file is opened with the open(2) O_EXCL flag, guaranteeing that the caller is the process that creates the file.

mkostemp() is like mkstemp(), with the difference that flags as for open(2) may be specified in flags (e.g., O_APPEND, O_SYNC).


On success, these functions return the file descriptor of the temporary file. On error, -1 is returned, and errno is set appropriately.


EEXIST Could not create a unique temporary filename. Now the contents of template are undefined.
EINVAL The last six characters of template were not XXXXXX. Now template is unchanged.
These functions may also fail with any of the errors described for open(2).


mkostemp() is available since glibc 2.7.


mkstemp(): 4.3BSD, POSIX.1-2001. mkostemp(): is a glibc extension.


The old behavior of creating a file with mode 0666 may be a security risk, especially since other Unix flavors use 0600, and somebody might overlook this detail when porting programs.

More generally, the POSIX specification of mkstemp() does not say anything about file modes, so the application should make sure its file mode creation mask (see umask(2)) is set appropriately before calling mkstemp() (and mkostemp()).

The prototype for mktemp() is in <unistd.h> for libc4, libc5, glibc1; glibc2 follows POSIX.1 and has the prototype in <stdlib.h>.


mkdtemp(3), mktemp(3), tempnam(3), tmpfile(3), tmpnam(3)


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