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Contents
Application Usage
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.
Copyright
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 GETSUBOPT(3POSIX)
Description
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
value.
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.
Errors
No errors are defined.
Thefollowingsectionsareinformative.Examples
ParsingSuboptions
The following example uses the getsubopt() function to parse a value argument in the optarg external
variable returned by a call to getopt().
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int do_all;
const char *type;
int read_size;
int write_size;
int read_only;
enum
{
RO_OPTION = 0,
RW_OPTION,
READ_SIZE_OPTION,
WRITE_SIZE_OPTION
};
const char *mount_opts[] =
{
[RO_OPTION] = "ro",
[RW_OPTION] = "rw",
[READ_SIZE_OPTION] = "rsize",
[WRITE_SIZE_OPTION] = "wsize",
NULL
};
int
main(int argc, char *argv[])
{
char *subopts, *value;
int opt;
while ((opt = getopt(argc, argv, "at:o:")) != -1)
switch(opt)
{
case 'a':
do_all = 1;
break;
case 't':
type = optarg;
break;
case 'o':
subopts = optarg;
while (*subopts != ' ')
{
char *saved = subopts;
switch(getsubopt(&subopts, (char **)mount_opts,
&value))
{
case RO_OPTION:
read_only = 1;
break;
case RW_OPTION:
read_only = 0;
break;
case READ_SIZE_OPTION:
if (value == NULL)
abort();
read_size = atoi(value);
break;
case WRITE_SIZE_OPTION:
if (value == NULL)
abort();
write_size = atoi(value);
break;
default:
/* Unknown suboption. */
printf("Unknown suboption `%s'\n", saved);
abort();
}
}
break;
default:
abort();
}
/* Do the real work. */
return 0;
}
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'"
before aborting.
Future Directions
None.
Name
getsubopt — parse suboption arguments from a string
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.
Rationale
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 constchar*const*.
Return Value
The getsubopt() function shall return the index of the matched token string, or -1 if no token strings
were matched.
See Also
getopt()
The Base Definitions volume of POSIX.1‐2017, <stdlib.h>Synopsis
#include <stdlib.h>
int getsubopt(char **optionp, char * const *keylistp, char **valuep);
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