freebsd-dev/lib/libc/stdio/mktemp.3
Mike Pritchard d428a98d6d Added a little NOTES section explaining that passing in a string that
resides in read-only memory is going to cause the program to core dump,
and this is commmon with older pre-ANSI C programs.

(I've scratched my head over this one at 3 in the morning before
while trying to port some ancient program)

Suggested by:	Gary Kline <kline@tera.com>
1996-02-15 05:02:16 +00:00

159 lines
4.3 KiB
Groff

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.\" @(#)mktemp.3 8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt MKTEMP 3
.Os
.Sh NAME
.Nm mktemp
.Nd make temporary file name (unique)
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft char *
.Fn mktemp "char *template"
.Ft int
.Fn mkstemp "char *template"
.Sh DESCRIPTION
The
.Fn mktemp
function
takes the given file name template and overwrites a portion of it
to create a file name.
This file name is unique and suitable for use
by the application.
The template may be any file name with some number of
.Ql X Ns s
appended
to it, for example
.Pa /tmp/temp.XXXX .
The trailing
.Ql X Ns s
are replaced with the current process number and/or a
unique letter combination.
The number of unique file names
.Fn mktemp
can return depends on the number of
.Ql X Ns s
provided; six
.Ql X Ns s
will
result in
.Fn mktemp
testing roughly 26 ** 6 combinations.
.Pp
The
.Fn mkstemp
function
makes the same replacement to the template and creates the template file,
mode 0600, returning a file descriptor opened for reading and writing.
This avoids the race between testing for a file's existence and opening it
for use.
.Sh RETURN VALUES
The
.Fn mktemp
function
returns a pointer to the template on success and
.Dv NULL
on failure.
The
.Fn mkstemp
function
returns \-1 if no suitable file could be created.
If either call fails an error code is placed in the global variable
.Va errno .
.Sh ERRORS
The
.Fn mktemp
and
.Fn mkstemp
functions
may set
.Va errno
to one of the following values:
.Bl -tag -width [ENOTDIR]
.It Bq Er ENOTDIR
The pathname portion of the template is not an existing directory.
.El
.Pp
The
.Fn mktemp
and
.Fn mkstemp
functions
may also set
.Va errno
to any value specified by the
.Xr stat 2
function.
.Pp
The
.Fn mkstemp
function
may also set
.Va errno
to any value specified by the
.Xr open 2
function.
.Sh NOTES
A common problem that results in a core dump is that the programmer
passes in a read-only string to
.Fn mktemp
or
.Fn mkstemp .
This is common with programs that were developed before
.St -ansiC
compilers were common..
For example, calling
.Fn mkstemp
with an argument of
.Qq /tmp/tempfile.XXXXXX
will result in a core dump due to
.Fn mkstemp
attempting to modify the string constant that was given.
If the program in question makes heavy use of that type
of function call, you do have the option of compiling the program
so that it will store string constants in a writable segment of memory.
See
.Xr gcc 1
for more information.
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr getpid 2 ,
.Xr open 2 ,
.Xr stat 2
.Sh HISTORY
A
.Nm mktemp
function appeared in
.At v7 .