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NAME
getsubopt — parse suboption arguments from a string
SYNOPSIS
#include <stdlib.h>
int getsubopt(char **optionp, char * const *keylistp, char **valuep);
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.
RETURN VALUE
The getsubopt() function shall return the index of the matched token string, or -1 if no
token strings were matched.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Parsing Suboptions
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.
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.
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 const char
* const *.
FUTURE DIRECTIONS
None.
SEE ALSO
getopt()
The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
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 .