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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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GETSUBOPT(3P) POSIX Programmer's Manual GETSUBOPT(3P)
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.
getsubopt — parse suboption arguments from a string
#include <stdlib.h>
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. 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.
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.
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.
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 *.
None.
getopt(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)
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(3P)
Pages that refer to this page: stdlib.h(0p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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