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setmode(3bsd) 3bsd setmode(3bsd)

getmode, setmode — modify mode bits

library “libbsd”

#include <unistd.h> (See libbsd(7) for include usage.)
void *
setmode(const char *mode_str);

mode_t
getmode(const void *set, mode_t mode);

The () 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 getmode(). The string may be an numeric (octal) or symbolic string of the form accepted by chmod(1), and may represent either an exact mode to set or a change to make to the existing mode.

The () function adjusts the file permission bits given by mode according to the compiled change representation 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.

Because some of the possible symbolic values are defined relative to the file creation mask, () may call umask(2), temporarily changing the mask. If this occurs, the file creation mask will be restored before setmode() returns. If the calling program changes the value of its file creation mask after calling setmode(), setmode() must be called again to recompile the mode string if getmode() is to modify future file modes correctly.

If the mode passed to () is invalid, setmode() returns NULL.

The value returned from () is obtained from () and should be returned to the system with () when the program is done with it, generally after a call to getmode().

The effects of the shell command ‘chmod a+x myscript.sh’ can be duplicated as follows:

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);

The setmode() function may fail and set errno for any of the errors specified for the library routines malloc(3) or strtol(3). In addition, setmode() will fail and set errno to:

[]
The mode argument does not represent a valid mode.

chmod(1), stat(2), umask(2), malloc(3)

The getmode() and setmode() functions first appeared in 4.4BSD.

The type of set should really be some opaque struct type used only by these functions rather than void *.

January 4, 2009 x86_64