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.\"     @(#)setmode.3	8.2 (Berkeley) 4/28/95
.\"
.Dd January 4, 2009
.Dt SETMODE 3
.Os
.Sh NAME
.Nm getmode ,
.Nm setmode
.Nd modify mode bits
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft void *
.Fn setmode "const char *mode_str"
.Ft mode_t
.Fn getmode "const void *set" "mode_t mode"
.Sh DESCRIPTION
The
.Fn setmode
function accepts a string representation of a file mode change,
compiles it to binary form, and returns an abstract representation
that may be passed to
.Fn getmode .
The string may be an numeric (octal) or symbolic string of the form
accepted by
.Xr chmod 1 ,
and may represent either an exact mode to set or a change to make to
the existing mode.
.Pp
The
.Fn getmode
function
adjusts the file permission bits given by
.Fa mode
according to the compiled change representation
.Fa set ,
and returns the adjusted mode.
While only the permission bits are altered, other parts of the file
mode, particularly the type, may be examined.
.Pp
Because some of the possible symbolic values are defined relative to
the file creation mask,
.Fn setmode
may call
.Xr umask 2 ,
temporarily changing the mask.
If this occurs, the file creation mask will be restored before
.Fn setmode
returns.
If the calling program changes the value of its file creation mask
after calling
.Fn setmode ,
.Fn setmode
must be called again to recompile the mode string if
.Fn getmode
is to modify future file modes correctly.
.Pp
If the mode passed to
.Fn setmode
is invalid,
.Fn setmode
returns
.Dv NULL .
.Sh EXAMPLES
The effects of the shell command
.Ql "chmod a+x myscript.sh"
can be duplicated as follows:
.Bd -literal -offset indent
const char *file = "myscript.sh";
struct stat st;
mode_t newmode;

stat(file, &st);
newmode = getmode(setmode("a+x"), st.st_mode);
chmod(file, newmode);
.Ed
.Sh ERRORS
The
.Fn setmode
function
may fail and set
.Va errno
for any of the errors specified for the library routines
.Xr malloc 3
or
.Xr strtol 3 .
In addition,
.Fn setmode
will fail and set
.Va errno
to:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa mode
argument does not represent a valid mode.
.El
.Sh SEE ALSO
.Xr chmod 1 ,
.Xr stat 2 ,
.Xr umask 2 ,
.Xr malloc 3
.Sh HISTORY
The
.Fn getmode
and
.Fn setmode
functions first appeared in
.Bx 4.4 .
.Sh BUGS
Each call to
.Nm setmode
allocates a small amount of memory that there is no correct way to
free.
.Pp
The type of
.Fa set
should really be some opaque struct type used only by these functions
rather than
.Ft void * .