GETSUBOPT(3P) POSIX Programmer's Manual GETSUBOPT(3P)PROLOG
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.
NAMEgetsubopt — parse suboption arguments from a string
int getsubopt(char **optionp, char * const *keylistp, char **valuep);
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. Simi‐
larly, 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
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 separa‐
tor between a token and a value. Subsequent <equals-sign> characters in
a suboption string shall be interpreted as part of the value.
If the string at *optionp contains only one suboption argument (equiva‐
lently, 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 asso‐
ciated value (equivalently, contains an <equals-sign>), getsubopt()
shall update *valuep to point to the value's first character. Other‐
wise, 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 getsubopt() function shall return the index of the matched token
string, or −1 if no token strings were matched.
No errors are defined.
The following sections are informative.
The following example uses the getsubopt() function to parse a value
argument in the optarg external variable returned by a call to
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. His‐
torical 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 imple‐
mentations will not modify either the array or the strings contained in
the array, as if the argument had been typed const char * const *.
The Base Definitions volume of POSIX.1‐2008, <stdlib.h>
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 Electri‐
cal 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 https://www.ker‐
IEEE/The Open Group 2013 GETSUBOPT(3P)