man3/tempnam.3

   1 .\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl)
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  23 .TH TEMPNAM 3  1999-06-14 "" "Linux Programmer's Manual"
  24 .SH NAME
  25 tempnam \- create a name for a temporary file
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <stdio.h>
  29 .sp
  30 .BI "char *tempnam(const char *" dir ", const char *" pfx );
  31 .fi
  32 .SH DESCRIPTION
  33 The
  34 .BR tempnam ()
  35 function returns a pointer to a string that is a valid filename,
  36 and such that a file with this name did not exist when
  37 .BR tempnam ()
  38 checked.
  39 The filename suffix of the pathname generated will start with
  40 .I pfx
  41 in case
  42 .I pfx
  43 is a non-NULL string of at most five bytes.
  44 The directory prefix part of the pathname generated is required to
  45 be `appropriate' (often that at least implies writable).
  46
  47 Attempts to find an appropriate directory go through the following
  48 steps:
  49 .TP
  50 a)
  51 In case the environment variable TMPDIR exists and
  52 contains the name of an appropriate directory, that is used.
  53 .TP
  54 b)
  55 Otherwise, if the
  56 .I dir
  57 argument is non-NULL and appropriate, it is used.
  58 .TP
  59 c)
  60 Otherwise,
  61 .I P_tmpdir
  62 (as defined in
  63 .IR <stdio.h> )
  64 is used when appropriate.
  65 .TP
  66 d)
  67 Finally an implementation-defined directory may be used.
  68 .PP
  69 The string returned by
  70 .BR tempnam ()
  71 is allocated using
  72 .BR malloc (3)
  73 and hence should be freed by
  74 .BR free (3).
  75 .SH "RETURN VALUE"
  76 The
  77 .BR tempnam ()
  78 function returns a pointer to a unique temporary
  79 filename, or NULL if a unique name cannot be generated.
  80 .SH ERRORS
  81 .TP
  82 .B ENOMEM
  83 Allocation of storage failed.
  84 .SH "CONFORMING TO"
  85 SVr4, 4.3BSD, POSIX.1-2001
  86 .SH NOTES
  87 Although
  88 .BR tempnam (3)
  89 generates names that are difficult to guess,
  90 it is nevertheless possible that between the time that
  91 .BR tempnam (3)
  92 returns a pathname, and the time that the program opens it,
  93 another program might create that pathname using
  94 .BR open (2),
  95 or create it as a symbolic link.
  96 This can lead to security holes.
  97 To avoid such possibilities, use the
  98 .BR open (2)
  99 .B O_EXCL
 100 flag to open the pathname.
 101 Or better yet, use
 102 .BR mkstemp (3)
 103 or
 104 .BR tmpfile (3).
 105
 106 SUSv2 does not mention the use of TMPDIR; glibc will use it only
 107 when the program is not set-user-ID.
 108 On SVr4, the directory used under \fBd)\fP is
 109 .IR /tmp
 110 (and this is what glibc does).
 111 .LP
 112 Because it dynamically allocates memory used to return the pathname,
 113 .BR tempnam ()
 114 is reentrant, and thus thread safe, unlike
 115 .BR tmpnam (3).
 116 .LP
 117 The
 118 .BR tempnam ()
 119 function generates a different string each time it is called,
 120 up to TMP_MAX (defined in
 121 .IR <stdio.h> )
 122 times.
 123 If it is called more than TMP_MAX times,
 124 the behaviour is implementation defined.
 125 .LP
 126 .BR tempnam ()
 127 uses at most the first five bytes from
 128 .IR pfx .
 129
 130 The glibc implementation of
 131 .BR tempnam ()
 132 will fail with the error
 133 .B EEXIST
 134 upon failure to find a unique name.
 135 .SH BUGS
 136 The precise meaning of `appropriate' is undefined;
 137 it is unspecified how accessibility of a directory is determined.
 138
 139 Never use this function.
 140 Use
 141 .BR mkstemp (3)
 142 or
 143 .BR tmpfile (3)
 144 instead.
 145 .SH "SEE ALSO"
 146 .BR mkstemp (3),
 147 .BR mktemp (3),
 148 .BR tmpfile (3),
 149 .BR tmpnam (3)