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man pages section 3: Basic Library Functions

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Updated: Friday, August 13, 2021



tmpnam, tmpnam_r, tmpnam_s, tempnam - create a name for a temporary file


#include <stdio.h>

char *tmpnam(char *s);
char *tmpnam_r(char *s);
char *tempnam(const char *dir, const char *pfx);
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>

errno_t tmpnam_s(char *s, rsize_t maxsize);


These functions generate file names for a temporary file. While these functions check that no file currently exists with the generated name, they cannot guarantee that no file will be created with that name between the time of their check and the point at which the caller attempts to create such a file. Use of mkostemp(3C), mkstemp(3C), or tmpfile(3C) is thus recommended instead of these functions.


The tmpnam() function always generates a file name using the path prefix defined as P_tmpdir in the <stdio.h> header. The default value for P_tmpdir is /var/tmp. If that directory is not accessible, /tmp is used. If s is NULL, tmpnam() leaves its result in a thread–specific data area and returns a pointer to that area. The next call to tmpnam() by the same thread will destroy the contents of the area. If s is not NULL, it is assumed to be the address of an array of at least L_tmpnam bytes, where L_tmpnam is a constant defined through inclusion of <stdio.h>. The tmpnam() function places its result in that array and returns s.


The tmpnam_r() function has the same functionality as tmpnam() except that if s is a null pointer, the function returns NULL.


The tempnam() function allows the user to control the choice of a directory. The argument dir points to the name of the directory in which the file is to be created. If dir is NULL or points to a string that is not a name for an appropriate directory, the path prefix defined as P_tmpdir in the <stdio.h> header is used. If that directory is not accessible, /tmp is used. The environment variable TMPDIR, if set, will override the path prefix value.

Many applications prefer that temporary files have certain initial character sequences in their names. The pfx argument may be NULL or point to a string of up to five characters to be used as the initial characters of the temporary-file name.

Upon successful completion, tempnam() uses malloc(3C) to allocate space for a string, puts the generated pathname in that space, and returns a pointer to it. The pointer is suitable for use in a subsequent call to free(). If tempnam() cannot return the expected result for any reason (for example, malloc() failed), or if none of the above-mentioned attempts to find an appropriate directory was successful, a null pointer is returned and errno is set to indicate the error.


The tmpnam_s() function is part of the C11 bounds checking interfaces specified in the C11 standard, Annex K. The tmpnam_s() function behaves in a similar manner to the tmpnam() function with additional safety checks on the parameters passed. See INCITS/ISO/IEC 9899:2011.

A runtime-constraint violation (see runtime_constraint_handler(3C)) will be generated if s is a null pointer, maxsize is greater than RSIZE_MAX or if the length of the generated temporary name including the trailing NULL is greater than the value of maxsize. If a runtime-constraint violation is generated, the temporary file name is not created.


The tempnam() function will fail if:


Insufficient storage space is available.

The tempnam_s() function will fail if:


NULL pointer is passed


maxsize argument is not valid, that is, greater than RSIZE_MAX


Total length of generated string is greater than or equal to maxsize


These functions generate a different file name each time they are called.

Files created using these functions and either fopen(3C), open(2), or creat(2) are temporary only in the sense that they reside in a directory intended for temporary use, and their names are unique. It is the user's responsibility to remove the file when its use is ended.

If called more than TMP_MAX (defined in <stdio.h>) times in a single process, these functions start recycling previously used names.

Between the time a file name is created and the file is opened, it is possible for some other process to create a file with the same name. It is recommended to use functions such as mkostemp(3C), mkstemp(3C), or tmpfile(3C) to avoid this race condition by returning a file that has been created and opened. If it is not possible to use one of those functions, then the use of open(2) with O_CREAT|O_EXCL flags is recommended, followed by fdopen(3C) if a stdio stream is required.

When using the C11 Annex K functions, the tmpfile_s() function should be used in favor of the tmpnam_s() function to avoid this race condition. See INCITS/ISO/IEC 9899:2011.

The tmpnam() function is safe to use in multithreaded applications because it employs thread-specific data if it is passed a NULL pointer. However, its use is discouraged. The tempnam() function is safe in multithreaded applications and should be used instead.


See attributes(7) for descriptions of the following attributes:

Interface Stability
See below
See below

The tmpnam(), tmpnam_r(), and tempnam() functions can be used safely in multithreaded applications.

The tmpnam_s() function cannot be used safely in a multithreaded application due to the runtime constraint handler. For more information, see the runtime_constraint_handler(3C) man page.

See standards(7) for descriptions of the following standards:

  • POSIX.1-2001 through 2008,
  • SUS through SUSv4,
  • XPG1 through XPG7
  • C89 through C11
  • POSIX.1-1990 through 2008,
  • SUS through SUSv4,
  • XPG1 through XPG7
C11 Annex K

The tempnam() and tmpnam() functions are marked obsolescent in POSIX.1-2008 & XPG7, and may be removed in future versions.

See Also

open(2), unlink(2), fopen(3C), free(3C), malloc(3C), mktemp(3C), mkostemp(3C), mkstemp(3C), tmpfile(3C), tmpfile_s(3C), attributes(7), standards(7), runtime_constraint_handler(3C)