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# tmpnam, tmpnam_s

From cppreference.com
< c‎ | io

Defined in header `<stdio.h>` |  |
char *tmpnam( char *filename ); | (1) |
errno_t tmpnam_s(char *filename_s, rsize_t maxsize); | (2) | (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 he 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 all bounds-checked functions, `tmpnam_s` is only guaranteed to be available of __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.

### [edit] Parameters

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).

### [edit] 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).

### [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 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)`.

### [edit] Example

Run this code

    #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

### [edit] References

- C11 standard (ISO/IEC 9899:2011):

  - 7.21.4.4 The tmpnam function (p: 303-304)

  - K.3.5.1.2 The tmpnam_s function (p: 587-588)

- C99 standard (ISO/IEC 9899:1999):

  - 7.19.4.4 The tmpnam function (p: 269-270)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.9.4.4 The tmpnam function

### [edit] See also

tmpfile tmpfile_s (C11) | returns a pointer to a temporary file (function) [edit]
C++ documentation for tmpnam

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