< cpp‎ | io‎ | c
Input/output library
I/O manipulators
C-style I/O
(deprecated in C++98)
File I/O
String I/O
Array I/O
(deprecated in C++98)
(deprecated in C++98)
(deprecated in C++98)
Synchronized Output
Error category interface
Defined in header <cstdio>
char* tmpnam( char* filename );

Creates a unique filename that does not name a currently existing file, and stores it in the character string pointed to by filename. The function is capable of generating up to TMP_MAX of unique filenames, but some or all of them may already be in use, and thus not suitable return values.

std::tmpnam modifies static state and is not required to be thread-safe.


filename - pointer to the character array capable of holding at least L_tmpnam bytes, to be used as a result buffer. If a null pointer is passed, a pointer to an internal static buffer is returned.

Return value

filename if filename was not NULL. Otherwise a pointer to an internal static buffer is returned. If no suitable filename can be generated, NULL is returned.


Although the names generated by std::tmpnam are difficult to guess, it is possible that a file with that name is created by another process between the moment std::tmpnam returns and the moment this program attempts to use the returned name to create a file. The standard function std::tmpfile and the POSIX function mkstemp do not have this problem (creating a unique directory using only the standard C library still requires the use of tmpnam)

POSIX systems additionally define the similarly named function tempnam(), which offers the choice of a directory (which defaults to the optionally defined macro P_tmpdir)


#include <iostream>
#include <cstdio>
#include <string>
int main()
    std::string name1 = std::tmpnam(nullptr);
    std::cout << "temporary file name: " << name1 << '\n';
    char name2[L_tmpnam];
    if (std::tmpnam(name2)) {
        std::cout << "temporary file name: " << name2 << '\n';

Possible output:

temporary file name: /tmp/fileDjwifs
temporary file name: /tmp/fileEv2bfW

See also

creates and opens a temporary, auto-removing file
returns a directory suitable for temporary files