The tmpnam() function generates a unique filename that can be used to create a temporary file without overwriting any existing one i.e. it returns a string containing a file name different from the name of any existing file, and thus suitable to safely create a temporary file without risking to overwrite an existing file.
It 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.
If str is a null pointer, the resulting string is stored in an internal static array that can be accessed by the return value. The content of this string is preserved at least until a subsequent call to this same function, which may overwrite it.
If str is not a null pointer, it shall point to an array of at least L_tmpnam characters that will be filled with the proposed temporary file name.
The file name returned by this function can be used to create a regular file using fopen to be used as a temporary file. The file created this way, unlike those created with tmpfile is not automatically deleted when closed. A program shall call remove to delete this file once closed.
char* tmpnam (char* filename);
The tmpnam() function takes a single argument which is a character string and returns a unique filename. This function is capable of generating up to TMP_MAX unique filenames.
It is defined in the <cstdio> header file.
Pointer to an array of characters where the proposed temporary name will be stored as a C string. The suggested size of this array is at least L_tmpnam characters.
Alternatively, a null pointer can be specified to use an internal static array to store the proposed temporary name, whose pointer is returned by the function.
On success, a pointer to the C string containing the proposed name for a temporary file:
1. If str was a null pointer, this points to an internal buffer (whose content is preserved at least until the next call to this function).
2. If str was not a null pointer, str is returned. If filename is not null, it returns filename.
3. If the function fails to create a suitable filename, it returns a null pointer. If any error occurs, 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).
Program 1: Example of tmpnam() function:
std::string name1 = std::tmpnam(nullptr);
std::cout << "temporary file name: " << name1 << '\n';
std::cout << "temporary file name: " << name2 << '\n';
Output: temporary file name: \salk.
temporary file name: \salk.1
Program 2: Example of tmpnam() function:
int main ()
char buffer [L_tmpnam];
char * pointer;
printf ("Tempname #1: %s\n",buffer);
pointer = tmpnam (NULL);
printf ("Tempname #2: %s\n",pointer);
Output: Tempname #1: \shpo.
Tempname #2: \shpo.1
Program 3: Example of tmpnam() function:
using namespace std;
int main ()
cout << "Temporary filenames:" << endl;
cout << "1. " << filename1 << endl;
cout << "2. " << filename2 << endl;
char* filename3 = tmpnam(NULL);
cout << "3. " << filename3;
Output: Temporary filenames: