< c‎ | io
Defined in header <stdio.h>
char *tmpnam( char *filename );

Creates an unique filename and stores it in 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 be in use in the filesystem and thus not suitable return values.

tmpnam is not reentrant and thus not thread-safe.


[edit] Parameters

filename - pointer to the character string to be used as a result buffer. If NULL is passed, a pointer to an internal static buffer is returned.

[edit] 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.

[edit] Notes

Although the names generated by tmpnam are difficult to guess, it is possible that a file with that name is created by another process between the moment 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.

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)

[edit] Example

tmpnam with error checking. Code generates a unique filename.

#include <stdlib.h>
#include <stdio.h>
int main(void)
    printf("TMP_MAX  = %d\n", TMP_MAX);
    printf("L_tmpnam = %d\n", L_tmpnam);
    char buffer[L_tmpnam] = {'\0'};
    char * fname = tmpnam(buffer);
    if (fname == NULL)
      fprintf(stderr,"tmpnam() failed in file %s at line # %d\n", __FILE__,__LINE__-4);
    printf("%s\n", fname);
    printf("%s\n", buffer);
    /* Argument is a null pointer. */
    char * fn1 = tmpnam(NULL);
    printf("%s\n", fn1);
    printf("%p\n", fn1);
    /* Subsequent calls to tmpnam modify the same object. */
    char * fn2 = tmpnam(NULL);
    printf("%s\n", fn2);
    printf("%p\n", fn2);


TMP_MAX  = 238328
L_tmpnam = 20

[edit] See also

returns a pointer to a temporary file
C++ documentation for tmpnam