The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group


tmpnam - create a name for a temporary file


[OB] [Option Start] #include <stdio.h>

char *tmpnam(char *
s); [Option End]


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard. [Option End]

The tmpnam() function shall generate a string that is a valid pathname that does not name an existing file. The function is potentially capable of generating {TMP_MAX} different strings, but any or all of them may already be in use by existing files and thus not be suitable return values.

The tmpnam() function generates a different string each time it is called from the same process, up to {TMP_MAX} times. If it is called more than {TMP_MAX} times, the behavior is implementation-defined.

The implementation shall behave as if no function defined in this volume of POSIX.1-2008, except tempnam(), calls tmpnam().

[CX] [Option Start] The tmpnam() function need not be thread-safe if called with a NULL parameter. [Option End]


Upon successful completion, tmpnam() shall return a pointer to a string. If no suitable string can be generated, the tmpnam() function shall return a null pointer.

If the argument s is a null pointer, tmpnam() shall leave its result in an internal static object and return a pointer to that object. Subsequent calls to tmpnam() may modify the same object. If the argument s is not a null pointer, it is presumed to point to an array of at least L_tmpnam chars; tmpnam() shall write its result in that array and shall return the argument as its value.


No errors are defined.

The following sections are informative.


Generating a Pathname

The following example generates a unique pathname and stores it in the array pointed to by ptr.

#include <stdio.h>
char pathname[L_tmpnam+1];
char *ptr;

ptr = tmpnam(pathname);


This function only creates pathnames. It is the application's responsibility to create and remove the files.

Between the time a pathname is created and the file is opened, it is possible for some other process to create a file with the same name. Applications may find tmpfile() more useful.

Applications should use the tmpfile(), mkstemp(), or mkdtemp() functions instead of the obsolescent tmpnam() function.




The tmpnam() function may be removed in a future version.


fopen, open, mkdtemp, tempnam, tmpfile, unlink

XBD <stdio.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Threads Extension.

Issue 6

Extensions beyond the ISO C standard are marked.

The normative text is updated to avoid use of the term "must" for application requirements.

The DESCRIPTION is expanded for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/142 is applied, updating the DESCRIPTION to allow implementations of the tempnam() function to call tmpnam().

Issue 7

Austin Group Interpretation 1003.1-2001 #148 is applied, clarifying that the tmpnam() function need not be thread-safe if called with a NULL parameter.

The tmpnam() function is marked obsolescent.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0669 [291] and XSH/TC1-2008/0670 [291,429] are applied.

End of informative text.


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