# <unistd.h>

The <unistd.h> header defines the symbolic constants and structures
which are not already defined or declared in some other header.

> 
> 
> int access(const char \*path, int mode); [POSIX] unsigned int
> alarm(unsigned int seconds); [POSIX] char \*getcwd(char \*buf, size\_t
> size); [POSIX] pid\_t getpid(void); [POSIX]
> 
> 
> char \*getwd(char \*path\_name); [POSIX]
> 
> 
> int isatty(int fd); [POSIX]
> 
> 
> void swab(const void \*src, void \*dest, size\_t len); [POSIX]
> 
> 
> long int sysconf(int name); [POSIX]
> 
> 
> int unlink(const char \*path); [POSIX]
> 
> 
> extern char \*optarg; extern int optind; extern int optopt; extern
> int opterr; extern int optreset;
> 
> 
> int getopt(int argc, char \* const argv[], const char \*optstring);
> 
> 
> [POSIX]
> 
> 
> access
> 
> 
> int access(const char \*path, int mode); [POSIX]

The access function checks the accessibility of the file named by path
for the access permissions indicated by mode. The value of mode is the
bitwise inclusive OR of the access permissions to be checked (R\_OK for
read permission, W\_OK for write permission and X\_OK for execute/search
permission) or the existence test, F\_OK. All components of the pathname
path are checked for access permissions (including F\_OK).

When Hexagon standalone programs execute on a simulator that is running
on the Windows Operating system, the check for X\_OK is not supported due
to restrictions in the Windows platform.

The real user ID is used in place of the effective user ID and the real
group access list (including the real group ID) are used in place of the
effective ID for verifying permission.

If a process has super-user privileges and indicates success for R\_OK or
W\_OK, the file may not actually have read or write permission bits set.
If a process has super-user privileges and indicates success for X\_OK,
at least one of the user, group, or other execute bits is set. (However,
the file may still not be executable.)

If path cannot be found or if an access modes would not be granted, a -1
value is returned; otherwise a 0 value is returned.

## alarm

> 
> 
> unsigned int alarm(unsigned int seconds); [POSIX]

The alarm function sets a timer to deliver the signal SIGALRM to the
calling process seconds after the call to alarm. If an alarm has already
been set with alarm but has not been delivered, another call to alarm
will supersede the prior call. The request alarm(0) voids the current
alarm and the signal SIGALRM will not be delivered. The maximum number
of seconds allowed is 2,147,483,647.

The return value of alarm is the amount of time left on the timer from a
previous call to alarm. If no alarm is currently set, the return value
is 0. If there is an error setting the timer, alarm returns ((unsigned
int) -1).

## getcwd

> 
> 
> char \*getcwd(char \*buf, size\_t size); [POSIX]

The function copies the absolute pathname of the current working
directory into the memory referenced by buf and returns a pointer to
buf. The size argument is the size, in bytes, of the array referenced by
buf.

If buf is NULL, space is allocated as necessary to store the pathname.
This space can later be freed with free.

Upon successful completion, a pointer to the pathname is returned.
Otherwise a NULL pointer is returned and the global variable errno is
set to indicate the error. In addition, getwd copies the error message
associated with errno into the memory referenced by buf.

## getopt

> 
> 
> extern char \*optarg; extern int optind; extern int optopt; extern
> int opterr; extern int optreset;
> 
> 
> int getopt(int argc, char \* const argv[], const char \*optstring);
> 
> 
> [POSIX]

The function incrementally parses a command line argument list argv and
returns the next known option character. An option character is known if
it has been specified in the string of accepted option characters,
optstring.

The option string optstring may contain the following elements:
individual characters, and characters followed by a colon to indicate an
option argument is to follow. For example, an option string “x”
recognizes an option “-x”, and an option string “x:” recognizes an
option and argument “-x argument”. It does not matter to getopt if a
following argument has leading whitespace.

On return from getopt, optarg points to an option argument, if it is
anticipated, and the variable optind contains the index to the next argv
argument for a subsequent call to getopt. The variable optopt saves the
last known option character returned

The variables opterr and optind are both initialized to 1. The optind
variable can be set to another value before a set of calls to getopt in
order to skip over more or less argv entries.

To use getopt to evaluate multiple sets of arguments, or to evaluate a
single set of arguments multiple times, the variable optreset must be
set to 1 before the second and each additional set of calls to getopt,
and the variable optind must be reinitialized.

The getopt function returns -1 when the argument list is exhausted. The
interpretation of options in the argument list can be canceled by the
option – (double dash) which causes getopt to signal the end of
argument processing and return -1. When all options have been processed
(up to the first non-option argument), getopt returns -1.

The getopt function returns the next known option character in
optstring. If getopt encounters a character not found in optstring or if
it detects a missing option argument, it returns “?” (question mark). If
optstring has a leading “:”. a missing option argument causes “:” to be
returned instead of “?”. In either case, the variable optopt is set to
the character that caused the error. The getopt function returns -1 when
the argument list is exhausted.

For example:

> 
> 
> extern char \*optarg; extern int optind; int bflag, ch, fd;
> 
> 
> bflag = 0;
> 
> 
> while ((ch = getopt(argc, argv, “bf:”)) != -1) { switch (ch) {
> 
> 
> case ‘b’: bflag = 1; break;
> 
> 
> case ‘f’:
> 
> 
> if ((fd = open(optarg, O\_RDONLY, 0)) < 0) { (void)fprintf(stderr,
> 
> 
> “myname: %s: %s\n”, optarg, strerror(errno)); exit(1);
> 
> 
> }
> 
> 
> break;
> 
> 
> case ‘?’: default:
> 
> 
> usage();
> 
> 
> }
> 
> 
> }
> 
> 
> argc -= optind; argv += optind;

If the getopt function encounters a character not found in the string
optstring or detects a missing option argument it writes an error
message to stderr and returns “?”. Setting opterr to a zero disables
these error messages. If optstring has a leading “:”, a missing option
argument causes a “:” to be returned in addition to suppressing any
error messages.

Option arguments are allowed to begin with “-”; this is reasonable but
reduces the amount of error checking possible.

Bugs

The getopt function was once specified to return EOF instead of -1. This
was changed by IEEE Std. 1003.2-1992 (“POSIX.2”) to decouple getopt from
<stdio.h>.

A single dash (“-”) can be specified as a character in optstring,
however it should never have an argument associated with it. This allows
getopt to be used with programs that expect “-” as an option flag.

This practice is wrong, and should not be used in any current
development. It is provided for backwards compatibility only. Care
should be taken not to use “-” as the first character in optstring to
avoid a semantic conflict with GNU getopt, which assigns different
meaning to an optstring that begins with a “-”. By default, a single
dash causes getopt to return -1.

It is also possible to handle digits as option letters. This allows
getopt to be used with programs that expect a number (“-3”) as an
option. This practice is wrong, and should not be used in any current
development. It is provided for backward compatibility only. The
following code fragment works in most cases.

> 
> 
> int ch;
> 
> 
> long length; char \*p;
> 
> 
> while ((ch = getopt(argc, argv, “0123456789”)) != -1) { switch (ch) {
> 
> 
> case ‘0’: case ‘1’: case ‘2’: case ‘3’: case ‘4’:
> 
> 
> case ‘5’: case ‘6’: case ‘7’: case ‘8’: case ‘9’: p = argv[optind -
> 1];
> 
> 
> if (p[0] == ‘-’ && p[1] == ch && !p[2]) length = ch - ‘0’;
> 
> 
> else
> 
> 
> length = strtol(argv[optind] + 1, NULL, 10); break;
> 
> 
> }
> 
> 
> }

## getpid

> 
> 
> pid\_t getpid(void); [POSIX]

The function returns the process ID of the calling process. The ID is
guaranteed to be unique and is useful for constructing temporary file
names.

When programs are compiled in standalone mode there is no job control.
When the getpid

routine is called the return value will be the thread number the program
is executing on.

## getwd

> 
> 
> char \*getwd(char \*path\_name); [POSIX]
> 
> 
> The function is a compatibility routine that calls getcwd with its
> buf argument and a size of MAXPATHLEN (as defined in the include file
> <sys/param.h>). Obviously, buf should be at least MAXPATHLEN bytes in
> length.

These routines have traditionally been used by programs to save the name
of a working directory for the purpose of returning to it. A much faster
and less error-prone method of accomplishing this is to open the current
directory (.) and use the fchdir function to return.

Upon successful completion, a pointer to the pathname is returned.
Otherwise a NULL pointer is returned and the global variable errno is
set to indicate the error. In addition, getwd copies the error message
associated with errno into the memory referenced by buf.

Because getwd does not know the length of the supplied buffer, it is
possible for a long (but valid) path to overflow the buffer and provide
a means for an attacker to exploit the caller. getcwd should be used in
place of getwd (the latter is only provided for compatibility purposes).

## isatty

> 
> 
> int isatty(int fd); [POSIX]

The function determines if the file descriptor fd refers to a valid
terminal type device.

The function returns 1 if fd is associated with a terminal device;
otherwise it returns 0 and

errno is set to indicate the error.

## swab

> 
> 
> void swab(const void \*src, void \*dest, size\_t len); [POSIX]

The function copies len bytes from the location referenced by src to the
location referenced by dest, swapping adjacent bytes. The argument len
must be even number.

## sysconf

> 
> 
> long int sysconf(int name); [POSIX]

The function provides a method for applications to determine the current
value of a configurable system limit or option variable. The name
argument specifies the system variable to be queried. Symbolic constants
for each name value are found in the include file

> 
> 
> <unistd.h>.
> 
> 
> [Table 11-12](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_posix_unistd.html#_bookmark790) lists the sysconf values.
> 
> 
> **Table 11-12 sysconf values**

| **Value** | **Description** |
| --- | --- |
| \_SC\_ARG\_MAX | The maximum bytes of argument to execve(2). |
| \_SC\_ATEXIT\_MAX | - The maximum number of functions that can be registered with<br>    - atexit(3). |
| \_SC\_BARRIERS | - The version of IEEE Std. 1003.1 (“POSIX.1”) and its Barriers<br>    - option to which the system attempts to conform, otherwise -1. |
| \_SC\_CLOCK\_SELECTION | - Return the POSIX version the implementation of the Clock<br>    - Selection Option on this system conforms to, or -1 if<br>unavailable. |
| \_SC\_CHILD\_MAX | The maximum number of simultaneous processes per user ID. |
| \_SC\_CLK\_TCK | The number of clock ticks per second. |
| \_SC\_FSYNC | - Return 1 if the File Synchronization Option is available on this<br>    - system, otherwise -1. |
| \_SC\_HOST\_NAME\_MAX | The maximum size of a hostname, including NULL. |
| \_SC\_IOV\_MAX | - The maximum number of iovec structures that a process has<br>    - available for use with preadv(2), pwritev(2), readv(2),<br>recvmsg(2), sendmsg(2) or writev(2). |
| \_SC\_JOB\_CONTROL | - Return 1 if job control is available on this system, otherwise<br>    - -1. |
| \_SC\_LOGIN\_NAME\_MAX | - Returns the size of the storage required for a login name, in<br>    - bytes, including the terminating NUL. |
| \_SC\_MAPPED\_FILES | - Return 1 if the Memory Mapped Files Option is available on this<br>    - system, otherwise -1. |
| \_SC\_MEMLOCK | - Return 1 if the Process Memory Locking Option is available on<br>    - this system, otherwise -1. |
| \_SC\_MEMLOCK\_RANGE | - Return 1 if the Range Memory Locking Option is available on this<br>    - system, otherwise -1. |
| \_SC\_MEMORY\_PROTECTION | - Return 1 if the Memory Protection Option is available on this<br>    - system, otherwise -1. |
| \_SC\_MONOTONIC\_CLOCK | - Return the POSIX version the implementation of the Monotonic<br>    - Clock Option on this system conforms to, or -1 if unavailable. |
| \_SC\_NGROUPS\_MAX | The maximum number of supplemental groups. |
| \_SC\_OPEN\_MAX | The maximum number of open files per process. |
| \_SC\_PAGESIZE | The size of a system page in bytes. |
| \_SC\_PASS\_MAX | The maximum length of the password, not counting NULL. |
| \_SC\_READER\_WRITER\_LOCKS | - The version of IEEE Std. 1003.1 (“POSIX.1”) and its Read-Write<br>    - Locks option to which the system attempts to conform, otherwise<br>-1. |
| \_SC\_REGEXP | - Return 1 if POSIX regular expressions are available on this<br>    - system, otherwise -1. |
| \_SC\_SEMAPHORES | - The version of IEEE Std. 1003.1 (“POSIX.1”) and its Semaphores<br>    - option to which the system attempts to conform, otherwise -1.<br><br><br>Availability of the Semaphores option depends on the<br>P1003\_1B\_SEMAPHORE kernel option. |
| \_SC\_SHELL | - Return 1 if POSIX shell is available on this system, otherwise<br>    - -1. |

> 
> 
> **Table 11-12 sysconf values**

| **Value** | **Description** |
| --- | --- |
| \_SC\_SPIN\_LOCKS | - The version of IEEE Std. 1003.1 (“POSIX.1”) and its Spin Locks<br>    - option to which the system attempts to conform, otherwise -1. |
| \_SC\_STREAM\_MAX | - The minimum maximum number of streams that a process may have<br>    - open at any one time. |
| \_SC\_SYMLOOP\_MAX | - The maximum number of symbolic links that can be expanded in a<br>    - path name. |
| \_SC\_SYNCHRONIZED\_IO | - Return 1 if the Synchronized I/O Option is available on this<br>    - system, otherwise -1. |
| \_SC\_THREADS | - The version of IEEE Std. 1003.1 (“POSIX.1”) and its Threads<br>    - option to which the system attempts to conform, otherwise -1. |
| \_SC\_TIMERS | - The version of IEEE Std. 1003.1 (“POSIX.1”) and its Timers<br>    - option to which the system attempts to conform, otherwise -1. |
| \_SC\_TZNAME\_MAX | - The minimum maximum number of types supported for the name of a<br>    - timezone. |
| \_SC\_SAVED\_IDS | - Returns 1 if saved set-group and saved set-user ID is available,<br>    - otherwise -1. |
| \_SC\_VERSION | - The version of ISO/IEC 9945 (POSIX 1003.1) with which the system<br>    - attempts to comply. |
| \_SC\_XOPEN\_SHM | - Return 1 if the X/Open Portability Guide Issue 4, Version 2<br>    - (“XPG4.2”) Shared Memory option is available on this system,<br>otherwise -1. Availability of the Shared Memory option depends<br>on the SYSVSHM kernel option. |
| \_SC\_BC\_BASE\_MAX | The maximum ibase/obase values in the bc(1) utility. |
| \_SC\_BC\_DIM\_MAX | The maximum array size in the bc(1) utility. |
| \_SC\_BC\_SCALE\_MAX | The maximum scale value in the bc(1) utility. |
| \_SC\_BC\_STRING\_MAX | The maximum string length in the bc(1) utility. |
| \_SC\_COLL\_WEIGHTS\_MAX | - The maximum number of weights that can be assigned to any entry<br>    - of the LC\_COLLATE order keyword in the locale definition file. |
| \_SC\_EXPR\_NEST\_MAX | - The maximum number of expressions that can be nested within<br>    - parenthesis by the expr(1) utility. |
| \_SC\_LINE\_MAX | - The maximum length in bytes of a text-processing utility’s input<br>    - line. |
| \_SC\_RE\_DUP\_MAX | - The maximum number of repeated occurrences of a regular<br>    - expression permitted when using interval notation. |
| \_SC\_2\_VERSION | - The version of POSIX 1003.2 with which the system attempts to<br>    - comply. |
| \_SC\_2\_C\_BIND | - Return 1 if the system’s C-language development facilities sup-<br>    - port the C-Language Bindings Option, otherwise -1. |
| \_SC\_2\_C\_DEV | - Return 1 if the system supports the C-Language Development<br>    - Utilities Option, otherwise -1. |
| \_SC\_2\_CHAR\_TERM | - Return 1 if the system supports at least one terminal type<br>    - capable of all operations described in POSIX 1003.2, otherwise<br>-1. |
| \_SC\_2\_FORT\_DEV | - Return 1 if the system supports the FORTRAN Development<br>    - Utilities Option, otherwise -1. |

> 
> 
> **Table 11-12 sysconf values**

| **Value** | **Description** |
| --- | --- |
| \_SC\_2\_FORT\_RUN | - Return 1 if the system supports the FORTRAN Runtime Utilities<br>    - Option, otherwise -1. |
| \_SC\_2\_LOCALEDEF | - Return 1 if the system supports the creation of locales, other-<br>    - wise -1. |
| \_SC\_2\_SW\_DEV | - Return 1 if the system supports the Software Development<br>    - Utilities Option, otherwise -1. |
| \_SC\_2\_UPE | - Return 1 if the system supports the User Portability Utilities<br>    - Option, otherwise -1. |
| \_SC\_GETGR\_R\_SIZE\_MAX | - The minimum size of the buffer passed to getgrgid\_r(3) and<br>    - getgrnam\_r(3). |
| \_SC\_GETPW\_R\_SIZE\_MAX | - The minimum size of the buffer passed to getpwnam\_r(3) and<br>    - getpwuid\_r(3). |
| \_SC\_NPROCESSORS\_CONF | The number of processors configured. |
| \_SC\_NPROCESSORS\_ONLN | The number of processors online (capable of running processes). |

If the call to sysconf is not successful, -1 is returned and errno is
set appropriately. Otherwise, if the variable is associated with
functionality that is not supported, -1 is returned and errno is not
modified. Otherwise, the current variable value is returned.

## unlink

> 
> 
> int unlink(const char \*path); [POSIX]

The function removes a link to a file. If path names a symbolic link,
unlink removes the symbolic link and does not affect any file or
directory named by the contents of the symbolic link. Otherwise, unlink
removes the link named by path and decrements the link count of the file
referenced by the link.

When the file’s link count becomes 0 and no process has the file open,
the space occupied by the file will be freed and the file is no longer
accessible. If one or more processes have the file open when the last
link is removed, the link is removed before unlink returns, but the
removal of the file contents is postponed until all references to the
file are closed.

The path argument must not name a directory unless the process has
appropriate privileges and the implementation supports using unlink on
directories.

Upon successful completion, unlink marks for update the st\_ctime and
st\_mtime fields of the parent directory. If the file’s link count is not
0, the st\_ctime field of the file is marked for update. Upon successful
completion, 0 is returned. Otherwise, -1 is returned, errno is set to
indicate the error, and the file is not unlinked.

Last Published: Oct 06, 2026

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