This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The getsubopt() function shall parse suboption arguments in a flag
argument. Such options often result from the use of getopt().
The getsubopt() argument optionp is a pointer to a pointer to the
option argument string. The suboption arguments shall be separated by
<comma> characters and each may consist of either a single token, or
a token-value pair separated by an <equals-sign>.
The keylistp argument shall be a pointer to a vector of strings. The
end of the vector is identified by a null pointer. Each entry in the
vector is one of the possible tokens that might be found in *optionp.
Since <comma> characters delimit suboption arguments in optionp, they
should not appear in any of the strings pointed to by keylistp.
Similarly, because an <equals-sign> separates a token from its value,
the application should not include an <equals-sign> in any of the
strings pointed to by keylistp. The getsubopt() function shall not
modify the keylistp vector.
The valuep argument is the address of a value string pointer.
If a <comma> appears in optionp, it shall be interpreted as a
suboption separator. After <comma> characters have been processed, if
there are one or more <equals-sign> characters in a suboption string,
the first <equals-sign> in any suboption string shall be interpreted
as a separator between a token and a value. Subsequent <equals-sign>
characters in a suboption string shall be interpreted as part of the
If the string at *optionp contains only one suboption argument
(equivalently, no <comma> characters), getsubopt() shall update
*optionp to point to the null character at the end of the string.
Otherwise, it shall isolate the suboption argument by replacing the
<comma> separator with a null character, and shall update *optionp to
point to the start of the next suboption argument. If the suboption
argument has an associated value (equivalently, contains an <equals-
sign>), getsubopt() shall update *valuep to point to the value's
first character. Otherwise, it shall set *valuep to a null pointer.
The calling application may use this information to determine whether
the presence or absence of a value for the suboption is an error.
Additionally, when getsubopt() fails to match the suboption argument
with a token in the keylistp array, the calling application should
decide if this is an error, or if the unrecognized option should be
processed in another way.
The following example uses the getsubopt() function to parse a value
argument in the optarg external variable returned by a call to
#include <stdio.h>#include <stdlib.h>#include <unistd.h>
const char *type;
RO_OPTION = 0,
const char *mount_opts =
[RO_OPTION] = "ro",
[RW_OPTION] = "rw",
[READ_SIZE_OPTION] = "rsize",
[WRITE_SIZE_OPTION] = "wsize",
main(int argc, char *argv)
char *subopts, *value;
while ((opt = getopt(argc, argv, "at:o:")) != -1)
do_all = 1;
type = optarg;
subopts = optarg;
while (*subopts != ' ')
char *saved = subopts;
switch(getsubopt(&subopts, (char **)mount_opts,
read_only = 1;
read_only = 0;
if (value == NULL)
read_size = atoi(value);
if (value == NULL)
write_size = atoi(value);
/* Unknown suboption. */
printf("Unknown suboption `%s'\n", saved);
/* Do the real work. */
If the above example is invoked with:
program -o ro,rsize=512
then after option parsing, the variable do_all will be 0, type will
be a null pointer, read_size will be 512, write_size will be 0, and
read_only will be 1. If it is invoked with:
program -o oops
it will print:
"Unknown suboption `oops'"
The value of *valuep when getsubopt() returns −1 is unspecified.
Historical implementations provide various incompatible extensions to
allow an application to access the suboption text that was not found
in the keylistp array.
The keylistp argument of getsubopt() is typed as char * const * to
match historical practice. However, the standard is clear that
implementations will not modify either the array or the strings
contained in the array, as if the argument had been typed const char* const *.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 GETSUBOPT(3P)