tmpnam, tmpnam_s
From cppreference.com
Defined in header <stdio.h>
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char *tmpnam( char *filename );
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(1) | |
errno_t tmpnam_s(char *filename_s, rsize_t maxsize); |
(2) | (since C11) |
#define TMP_MAX /*unspecified*/ |
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#define TMP_MAX_S /*unspecified*/ |
(since C11) | |
#define L_tmpnam /*unspecified*/ |
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#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
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:
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- 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_sis 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
Parameters
| filename | - | pointer to the character array capable of holding at least L_tmpnam |
| 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).
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Return value
1)
filename if filename
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
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
Example
Run this code
#include <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { // Note, the compiler/linker may issue a security warning, e.g. GCC: // "warning: the use of `tmpnam' is dangerous, better use `mkstemp'" 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); // POSIX offers mkstemp. The following declaration might be // necessary as mkstemp is absent in the standard C <stdlib.h>. int mkstemp(char*); char name3[] = "/tmp/fileXXXXXX"; // at least six 'X' required ^_^ int file_descriptor = mkstemp(name3); if (file_descriptor != -1) printf("temporary file name: %s\n", name3); else perror("mkstemp"); }
Possible output:
temporary file name: /tmp/file90dLlR temporary file name: /tmp/fileY9LWAg temporary file name: /tmp/filexgv8PF
References
- C23 standard (ISO/IEC 9899:2024):
- 7.21.4.4 The tmpnam function (p: TBD)
- K.3.5.1.2 The tmpnam_s function (p: TBD)
- C17 standard (ISO/IEC 9899:2018):
- 7.21.4.4 The tmpnam function (p: 222)
- K.3.5.1.2 The tmpnam_s function (p: 427-428)
- 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
See also
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(C11)
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returns a pointer to a temporary file (function) |
C++ documentation for tmpnam
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