---
name: "mktemp"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/mktemp/3"
---

# mktemp(3)

make temporary file name (unique)

## NAME

**mktemp**, **mkstemp**, **mkdtemp**, **mkdtempat\_np**, **mkstemps**, **mkstempsat\_np**, **mkostemp**, **mkostemps**, **mkostempsat\_np** – make temporary file name (unique)

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<unistd.h\>**

*char* *\**\

**mktemp**(*char \*template*);

*int*\

**mkstemp**(*char \*template*);

*char* *\**\

**mkdtemp**(*char \*template*);

*char* *\**\

**mkdtempat\_np**(*int dfd*, *char \*template*);

*int*\

**mkstemps**(*char \*template*, *int suffixlen*);

*int*\

**mkstempsat\_np**(*int dfd*, *char \*template*, *int suffixlen*);

*int*\

**mkostemp**(*char \*template*, *int oflags*);

*int*\

**mkostemps**(*char \*template*, *int suffixlen*, *int oflags*);

*int*\

**mkostempsat\_np**(*int dfd*, *char \*template*, *int suffixlen*, *int oflags*);

## DESCRIPTION

The **mktemp**() function takes the given file name template and overwrites a portion of it to create a file name. This file name is guaranteed not to exist at the time of function invocation and is suitable for use by the application. The template may be any file name with some number of ‘Xs’ appended to it, for example */tmp/temp.XXXXXX*. The trailing ‘Xs’ are replaced with a unique alphanumeric combination. The number of unique file names **mktemp**() can return depends on the number of ‘Xs’ provided; six ‘Xs’ will result in **mktemp**() selecting one of 56800235584 (62 \*\* 6) possible temporary file names.

The **mkstemp**() function makes the same replacement to the template and creates the template file, mode 0600, returning a file descriptor opened for reading and writing. This avoids the race between testing for a file's existence and opening it for use.

The **mkdtemp**() function makes the same replacement to the template as in **mktemp**() and creates the template directory, mode 0700.

The **mkostemp**() function is like **mkstemp**() but allows specifying additional [open(2)](/open/2.md) flags (defined in ⟨fcntl.h⟩). The permitted flags are O\_APPEND, O\_SHLOCK, O\_EXLOCK and O\_CLOEXEC.

The **mkstemps**() and **mkostemps**() functions act the same as **mkstemp**() and **mkostemp**() respectively, except they permit a suffix to exist in the template. The template should be of the form */tmp/tmpXXXXXXsuffix*. The **mkstemps**() and **mkostemps**() functions are told the length of the suffix string.

The **mkdtempat\_np**(), **mkstempsat\_np**(), and **mkostempsat\_np**() functions act the same as **mkdtemp**(), **mkstemps**(), and **mkostemps**() respectively, except in the case where the *path* specifies a relative path. In this case, path is treated as relative to the directory associated with the file descriptor *fd* instead of the current working directory.

## RETURN VALUES

The **mktemp**() and **mkdtemp**() functions return a pointer to the template on success and NULL on failure. The **mkstemp**(), **mkostemp**(), **mkstemps**() and **mkostemps**() functions return -1 if no suitable file could be created. If either call fails an error code is placed in the global variable *errno*.

## ERRORS

The **mkstemp**(), **mkostemp**(), **mkstemps**(), **mkostemps**(), and **mkdtemp**() functions may set *errno* to one of the following values:

- \[ENOTDIR\]

  The pathname portion of the template is not an existing directory.

The **mkostemp**() and **mkostemps**() functions may also set *errno* to the following value:

- \[EINVAL\]

  The *oflags* argument is invalid.

The **mkstemp**(), **mkostemp**(), **mkstemps**(), **mkostemps**() and **mkdtemp**() functions may also set *errno* to any value specified by the [stat(2)](/stat/2.md) function.

The **mkstemp**(), **mkostemp**(), **mkstemps**() and **mkostemps**() functions may also set *errno* to any value specified by the [open(2)](/open/2.md) function.

The **mkdtemp**() function may also set *errno* to any value specified by the [mkdir(2)](/mkdir/2.md) function.

## NOTES

A common problem that results in a core dump is that the programmer passes in a read-only string to **mktemp**(), **mkstemp**(), **mkstemps**() or **mkdtemp**(). This is common with programs that were developed before ISO/IEC 9899:1990 (“ISO C90”) compilers were common. For example, calling **mkstemp**() with an argument of "/tmp/tempfile.XXXXXX" will result in a core dump due to **mkstemp**() attempting to modify the string constant that was given.

## LEGACY SYNOPSIS

**#include \<unistd.h\>**

The include file ⟨unistd.h⟩ is necessary and sufficient for all functions.

## SEE ALSO

[chmod(2)](/chmod/2.md), [getpid(2)](/getpid/2.md), [mkdir(2)](/mkdir/2.md), [open(2)](/open/2.md), [stat(2)](/stat/2.md), [compat(5)](/compat/5.md)

## STANDARDS

The **mkstemp**() and **mkdtemp**() functions are expected to conform to IEEE Std 1003.1-2008 (“POSIX.1”). The **mktemp**() function is expected to conform to IEEE Std 1003.1-2001 (“POSIX.1”) and is not specified by IEEE Std 1003.1-2008 (“POSIX.1”). The **mkostemp**(), **mkstemps**() and **mkostemps**() functions do not conform to any standard.

## HISTORY

A **mktemp**() function appeared in Version 7 AT&T UNIX. The **mkstemp**() function appeared in 4.4BSD. The **mkdtemp**() function first appeared in OpenBSD 2.2, and later in FreeBSD 3.2. The **mkstemps**() function first appeared in OpenBSD 2.4, and later in FreeBSD 3.4. The **mkostemp**() and **mkostemps**() functions appeared in OS X 10.12.

## BUGS

This family of functions produces filenames which can be guessed, though the risk is minimized when large numbers of ‘Xs’ are used to increase the number of possible temporary filenames. This makes the race in **mktemp**(), between testing for a file's existence (in the **mktemp**() function call) and opening it for use (later in the user application) particularly dangerous from a security perspective. Whenever it is possible, **mkstemp**() or **mkostemp**() should be used instead, since it does not have the race condition. If **mkstemp**() cannot be used, the filename created by **mktemp**() should be created using the O\_EXCL flag to [open(2)](/open/2.md) and the return status of the call should be tested for failure. This will ensure that the program does not continue blindly in the event that an attacker has already created the file with the intention of manipulating or reading its contents.