tmpnam, tmpnam_s

Defined in header <stdio.h>
char *tmpnam( char *filename );
errno_t tmpnam_s(char *filename_s, rsize_t maxsize);
(2) (since C11)
#define TMP_MAX        /*unspecified*/
#define TMP_MAX_S      /*unspecified*/
(since C11)
#define L_tmpnam       /*unspecified*/
#define L_tmpnam_s     /*unspecified*/
(since C11)
1) Creates a unique valid file name (no longer than L_tmpnam in length) 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.
2) Same as (1), except that up to TMP_MAX_S names may be generated, no longer than L_tmpnam_s in length, and the following errors are detected at runtime and call the currently installed constraint handler function:
  • filename_s is a null pointer
  • maxsize is greater than RSIZE_MAX
  • maxsize is less than the generated file name string
As with all bounds-checked functions, tmpnam_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdio.h>.

tmpnam and tmpnam_s modify static state (which may be shared between these functions) and are 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 null pointer is passed, a pointer to an internal static buffer is returned.
filename_s - pointer to the character array capable of holding at least L_tmpnam_s bytes, to be used as a result buffer.
maxsize - maximum number of characters the function is allowed to write (typically the size of the filename_s array).

Return value

1) filename if filename was not a null pointer. Otherwise a pointer to an internal static buffer is returned. If no suitable filename can be generated, null pointer is returned.
2) Returns zero and writes the file name to filename_s on success. On error, returns non-zero and writes the null character to filename_s[0] (only if filename_s is not null and maxsize is not zero and is not greater than RSIZE_MAX).


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 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 <stdio.h>
#include <string.h>
int main(void)
    char* name1 = tmpnam(NULL);
    printf("temporary file name: %s\n", name1);
    char name2[L_tmpnam];
    if (tmpnam(name2))
        printf("temporary file name: %s\n", name2);

Possible output:

temporary file name: /tmp/fileRZHMwL
temporary file name: /tmp/file420gSN


  • C17 standard (ISO/IEC 9899:2018):
    • The tmpnam function (p: 222)
    • K. The tmpnam_s function (p: 427-428)
  • C11 standard (ISO/IEC 9899:2011):
    • The tmpnam function (p: 303-304)
    • K. The tmpnam_s function (p: 587-588)
  • C99 standard (ISO/IEC 9899:1999):
    • The tmpnam function (p: 269-270)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The tmpnam function

See also

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

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