# double_t(3) - phpMan [SYSTEM_DATA_TYPES(7)] Linux Programmer's Manual [SYSTEM_DATA_TYPES(7)] **NAME** system_data_types - overview of system data types **DESCRIPTION** _aiocb_ _Include_: __. struct aiocb { int aio_fildes; /* File descriptor */ off_t aio_offset; /* File offset */ volatile void *aio_buf; /* Location of buffer */ size_t aio_nbytes; /* Length of transfer */ int aio_reqprio; /* Request priority offset */ struct sigevent aio_sigevent; /* Signal number and value */ int aio_lio_opcode;/* Operation to be performed */ }; For further information about this structure, see [**aio**(7)]. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **aio**___**[cancel**(3)], **aio**___**[error**(3)], **aio**___**[fsync**(3)], **aio**___**[read**(3)], **aio**___**[return**(3)], **aio**___**[suspend**(3)], **aio**___**[write**(3)], **lio**___**[listio**(3)] _clock_t_ _Include_: __ or __. Alternatively, __. Used for system time in clock ticks or **CLOCKS**___**PER**___**SEC** (defined in __). According to POSIX, it shall be an integer type or a real-floating type. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**times**(2)], [**clock**(3)] _clockid_t_ _Include_: __. Alternatively, __. Used for clock ID type in the clock and timer functions. Ac- cording to POSIX, it shall be defined as an arithmetic type. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **clock**___**[adjtime**(2)], **clock**___**[getres**(2)], **clock**___**[nanosleep**(2)], **timer**___**[create**(2)], **clock**___**[getcpuclockid**(3)] _dev_t_ _Include_: __. Alternatively, __. Used for device IDs. According to POSIX, it shall be an integer type. For further details of this type, see [**makedev**(3)]. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**mknod**(2)], [**stat**(2)] _div_t_ _Include_: __. typedef struct { int quot; /* Quotient */ int rem; /* Remainder */ } div_t; It is the type of the value returned by the [**div**(3)] function. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**div**(3)] _double_t_ _Include_: __. The implementation's most efficient floating type at least as wide as _double_. Its type depends on the value of the macro **FLT**___**EVAL**___**METHOD** (defined in __): 0 _double_t_ is _double_. 1 _double_t_ is _double_. 2 _double_t_ is _long_ _double_. For other values of **FLT**___**EVAL**___**METHOD**, the type of _double_t_ is im- plementation-defined. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _float_t_ type in this page. _fd_set_ _Include_: __. Alternatively, __. A structure type that can represent a set of file descriptors. According to POSIX, the maximum number of file descriptors in an _fd_set_ structure is the value of the macro **FD**___**SETSIZE**. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**select**(2)] _fenv_t_ _Include_: __. This type represents the entire floating-point environment, in- cluding control modes and status flags; for further details, see [**fenv**(3)]. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**fenv**(3)] _fexcept_t_ _Include_: __. This type represents the floating-point status flags collec- tively; for further details see [**fenv**(3)]. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**fenv**(3)] _FILE_ _Include_: __. Alternatively, __. An object type used for streams. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**fclose**(3)], [**flockfile**(3)], [**fopen**(3)], [**fprintf**(3)], [**fread**(3)], [**fscanf**(3)], [**stdin**(3)], [**stdio**(3)] _float_t_ _Include_: __. The implementation's most efficient floating type at least as wide as _float_. Its type depends on the value of the macro **FLT**___**EVAL**___**METHOD** (defined in __): 0 _float_t_ is _float_. 1 _float_t_ is _double_. 2 _float_t_ is _long_ _double_. For other values of **FLT**___**EVAL**___**METHOD**, the type of _float_t_ is im- plementation-defined. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _double_t_ type in this page. _gid_t_ _Include_: __. Alternatively, __, __, __, __, __, __, or __. A type used to hold group IDs. According to POSIX, this shall be an integer type. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**chown**(2)], [**getgid**(2)], [**getegid**(2)], [**getgroups**(2)], **getres-** [**gid**(2)], [**getgrnam**(2)], [**credentials**(7)] _id_t_ _Include_: __. Alternatively, __. A type used to hold a general identifier. According to POSIX, this shall be an integer type that can be used to contain a _pid_t_, _uid_t_, or _gid_t_. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**getpriority**(2)], [**waitid**(2)] _imaxdiv_t_ _Include_: __. typedef struct { intmax_t quot; /* Quotient */ intmax_t rem; /* Remainder */ } imaxdiv_t; It is the type of the value returned by the [**imaxdiv**(3)] function. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**imaxdiv**(3)] _intmax_t_ _Include_: __. Alternatively, __. A signed integer type capable of representing any value of any signed integer type supported by the implementation. According to the C language standard, it shall be capable of storing val- ues in the range [**INTMAX**___**MIN**, **INTMAX**___**MAX**]. The macro **INTMAX**___**C**() expands its argument to an integer constant of type _intmax_t_. The length modifier for _intmax_t_ for the [**printf**(3)] and the [**scanf**(3)] families of functions is **j**; resulting commonly in **%jd** or **%ji** for printing _intmax_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _Bugs_: _intmax_t_ is not large enough to represent values of type ___int128_ in implementations where ___int128_ is defined and _long_ _long_ is less than 128 bits wide. _See_ _also_: the _uintmax_t_ type in this page. _int_N__t_ _Include_: __. Alternatively, __. _int8_t_, _int16_t_, _int32_t_, _int64_t_ A signed integer type of a fixed width of exactly N bits, N be- ing the value specified in its type name. According to the C language standard, they shall be capable of storing values in the range [**INT**N___**MIN**, **INT**N___**MAX**], substituting N by the appropri- ate number. According to POSIX, _int8_t_, _int16_t_, and _int32_t_ are required; _int64_t_ is only required in implementations that provide integer types with width 64; and all other types of this form are op- tional. The length modifiers for the _int_N__t_ types for the [**printf**(3)] fam- ily of functions are expanded by macros of the forms **PRId**N and **PRIi**N (defined in __); resulting for example in **%"PRId64"** or **%"PRIi64"** for printing _int64_t_ values. The length modifiers for the _int_N__t_ types for the [**scanf**(3)] family of func- tions are expanded by macros of the forms **SCNd**N and **SCNi**N, (de- fined in __); resulting for example in **%"SCNd8"** or **%"SCNi8"** for scanning _int8_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _intmax_t_, _uint_N__t_, and _uintmax_t_ types in this page. _intptr_t_ _Include_: __. Alternatively, __. A signed integer type such that any valid (_void_ _*_) value can be converted to this type and back. According to the C language standard, it shall be capable of storing values in the range [**INTPTR**___**MIN**, **INTPTR**___**MAX**]. The length modifier for _intptr_t_ for the [**printf**(3)] family of functions is expanded by the macros **PRIdPTR** and **PRIiPTR** (defined in __); resulting commonly in **%"PRIdPTR"** or **%"PRIiPTR"** for printing _intptr_t_ values. The length modifier for _intptr_t_ for the [**scanf**(3)] family of functions is expanded by the macros **SCNdPTR** and **SCNiPTR,** (defined in __); resulting com- monly in **%"SCNdPTR"** or **%"SCNiPTR"** for scanning _intptr_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _uintptr_t_ and _void_ _*_ types in this page. _lconv_ _Include_: __. struct lconv { /* Values in the "C" locale: */ char *decimal_point; /* "." */ char *thousands_sep; /* "" */ char *grouping; /* "" */ char *mon_decimal_point; /* "" */ char *mon_thousands_sep; /* "" */ char *mon_grouping; /* "" */ char *positive_sign; /* "" */ char *negative_sign; /* "" */ char *currency_symbol; /* "" */ char frac_digits; /* CHAR_MAX */ char p_cs_precedes; /* CHAR_MAX */ char n_cs_precedes; /* CHAR_MAX */ char p_sep_by_space; /* CHAR_MAX */ char n_sep_by_space; /* CHAR_MAX */ char p_sign_posn; /* CHAR_MAX */ char n_sign_posn; /* CHAR_MAX */ char *int_curr_symbol; /* "" */ char int_frac_digits; /* CHAR_MAX */ char int_p_cs_precedes; /* CHAR_MAX */ char int_n_cs_precedes; /* CHAR_MAX */ char int_p_sep_by_space; /* CHAR_MAX */ char int_n_sep_by_space; /* CHAR_MAX */ char int_p_sign_posn; /* CHAR_MAX */ char int_n_sign_posn; /* CHAR_MAX */ }; Contains members related to the formatting of numeric values. In the "C" locale, its members have the values shown in the com- ments above. _Conforming_ _to_: C11 and later; POSIX.1-2001 and later. _See_ _also_: [**setlocale**(3)], [**localeconv**(3)], [**charsets**(5)], [**locale**(7)] _ldiv_t_ _Include_: __. typedef struct { long quot; /* Quotient */ long rem; /* Remainder */ } ldiv_t; It is the type of the value returned by the [**ldiv**(3)] function. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**ldiv**(3)] _lldiv_t_ _Include_: __. typedef struct { long long quot; /* Quotient */ long long rem; /* Remainder */ } lldiv_t; It is the type of the value returned by the [**lldiv**(3)] function. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**lldiv**(3)] _off_t_ _Include_: __. Alternatively, __, __, __, __, __, or __. Used for file sizes. According to POSIX, this shall be a signed integer type. _Versions_: __ and __ define _off_t_ since POSIX.1-2008. _Conforming_ _to_: POSIX.1-2001 and later. _Notes_: On some architectures, the width of this type can be con- trolled with the feature test macro ___**FILE**___**OFFSET**___**BITS**. _See_ _also_: [**lseek**(2)], [**mmap**(2)], **posix**___**[fadvise**(2)], [**pread**(2)], **trun-** [**cate**(2)], [**fseeko**(3)], [**lockf**(3)], **posix**___**[fallocate**(3)], **fea-** **ture**___**test**___**[macros**(7)] _pid_t_ _Include_: __. Alternatively, __, __, __, __, __, __, __, __, __, __, __, or __. This type is used for storing process IDs, process group IDs, and session IDs. According to POSIX, it shall be a signed inte- ger type, and the implementation shall support one or more pro- gramming environments where the width of _pid_t_ is no greater than the width of the type _long_. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**fork**(2)], [**getpid**(2)], [**getppid**(2)], [**getsid**(2)], [**gettid**(2)], [**getpgid**(2)], [**kill**(2)], **pidfd**___**[open**(2)], **sched**___**[setscheduler**(2)], **wait-** [**pid**(2)], [**sigqueue**(3)], [**credentials**(7)], _ptrdiff_t_ _Include_: __. Used for a count of elements, and array indices. It is the re- sult of subtracting two pointers. According to the C language standard, it shall be a signed integer type capable of storing values in the range [**PTRDIFF**___**MIN**, **PTRDIFF**___**MAX**]. The length modifier for _ptrdiff_t_ for the [**printf**(3)] and the [**scanf**(3)] families of functions is **t**; resulting commonly in **%td** or **%ti** for printing _ptrdiff_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _size_t_ and _ssize_t_ types in this page. _regex_t_ _Include_: __. typedef struct { size_t re_nsub; /* Number of parenthesized subexpressions. */ } regex_t; This is a structure type used in regular expression matching. It holds a compiled regular expression, compiled with **reg-** [**comp**(3)]. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**regex**(3)] _regmatch_t_ _Include_: __. typedef struct { regoff_t rm_so; /* Byte offset from start of string to start of substring */ regoff_t rm_eo; /* Byte offset from start of string of the first character after the end of substring */ } regmatch_t; This is a structure type used in regular expression matching. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**regexec**(3)] _regoff_t_ _Include_: __. According to POSIX, it shall be a signed integer type capable of storing the largest value that can be stored in either a _ptrdiff_t_ type or a _ssize_t_ type. _Versions_: Prior to POSIX.1-2008, the type was capable of storing the largest value that can be stored in either an _off_t_ type or a _ssize_t_ type. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: the _regmatch_t_ structure and the _ptrdiff_t_ and _ssize_t_ types in this page. _sigevent_ _Include_: __. Alternatively, __, __, or __. struct sigevent { int sigev_notify; /* Notification type */ int sigev_signo; /* Signal number */ union sigval sigev_value; /* Signal value */ void (*sigev_notify_function)(union sigval); /* Notification function */ pthread_attr_t *sigev_notify_attributes; /* Notification attributes */ }; For further details about this type, see [**sigevent**(7)]. _Versions_: __ and __ define _sigevent_ since POSIX.1-2008. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **timer**___**[create**(2)], **getaddrinfo**___**[a**(3)], **lio**___**[listio**(3)], **mq**___**[notify**(3)] See also the _aiocb_ structure in this page. _siginfo_t_ _Include_: __. Alternatively, __. typedef struct { int si_signo; /* Signal number */ int si_code; /* Signal code */ pid_t si_pid; /* Sending process ID */ uid_t si_uid; /* Real user ID of sending process */ void *si_addr; /* Address of faulting instruction */ int si_status; /* Exit value or signal */ union sigval si_value; /* Signal value */ } siginfo_t; Information associated with a signal. For further details on this structure (including additional, Linux-specific fields), see [**sigaction**(2)]. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **pidfd**___**send**___**[signal**(2)], **rt**___**[sigqueueinfo**(2)], **sigac-** [**tion**(2)], [**sigwaitinfo**(2)], [**psiginfo**(3)] _sigset_t_ _Include_: __. Alternatively, __, or __. This is a type that represents a set of signals. According to POSIX, this shall be an integer or structure type. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **epoll**___**[pwait**(2)], [**ppoll**(2)], [**pselect**(2)], [**sigaction**(2)], [**signalfd**(2)], [**sigpending**(2)], [**sigprocmask**(2)], [**sigsuspend**(2)], **sig-** [**waitinfo**(2)], [**signal**(7)] _sigval_ _Include_: __. union sigval { int sigval_int; /* Integer value */ void *sigval_ptr; /* Pointer value */ }; Data passed with a signal. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **pthread**___**[sigqueue**(3)], [**sigqueue**(3)], [**sigevent**(7)] See also the _sigevent_ structure and the _siginfo_t_ type in this page. _size_t_ _Include_: __ or __. Alternatively, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, or __. Used for a count of bytes. It is the result of the _sizeof_ oper- ator. According to the C language standard, it shall be an un- signed integer type capable of storing values in the range [0, **SIZE**___**MAX**]. According to POSIX, the implementation shall support one or more programming environments where the width of _size_t_ is no greater than the width of the type _long_. The length modifier for _size_t_ for the [**printf**(3)] and the [**scanf**(3)] families of functions is **z**; resulting commonly in **%zu** or **%zx** for printing _size_t_ values. _Versions_: __, __, __, __, __, __, __, and __ define _size_t_ since POSIX.1-2008. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**read**(2)], [**write**(2)], [**fread**(3)], [**fwrite**(3)], [**memcmp**(3)], [**memcpy**(3)], [**memset**(3)], [**offsetof**(3)] See also the _ptrdiff_t_ and _ssize_t_ types in this page. _ssize_t_ _Include_: __. Alternatively, __, __, __, __, __, __, __, or __. Used for a count of bytes or an error indication. According to POSIX, it shall be a signed integer type capable of storing val- ues at least in the range [-1, **SSIZE**___**MAX**], and the implementa- tion shall support one or more programming environments where the width of _ssize_t_ is no greater than the width of the type _long_. Glibc and most other implementations provide a length modifier for _ssize_t_ for the [**printf**(3)] and the [**scanf**(3)] families of func- tions, which is **z**; resulting commonly in **%zd** or **%zi** for printing _ssize_t_ values. Although **z** works for _ssize_t_ on most implemen- tations, portable POSIX programs should avoid using it--for ex- ample, by converting the value to _intmax_t_ and using its length modifier (**j**). _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**read**(2)], [**readlink**(2)], [**readv**(2)], [**recv**(2)], [**send**(2)], [**write**(2)] See also the _ptrdiff_t_ and _size_t_ types in this page. _suseconds_t_ _Include_: __. Alternatively, __, or __. Used for time in microseconds. According to POSIX, it shall be a signed integer type capable of storing values at least in the range [-1, 1000000], and the implementation shall support one or more programming environments where the width of _suseconds_t_ is no greater than the width of the type _long_. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: the _timeval_ structure in this page. _time_t_ _Include_: __ or __. Alternatively, __, __, __, __, __, __, __, or __. Used for time in seconds. According to POSIX, it shall be an integer type. _Versions_: __ defines _time_t_ since POSIX.1-2008. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**stime**(2)], [**time**(2)], [**ctime**(3)], [**difftime**(3)] _timer_t_ _Include_: __. Alternatively, __. Used for timer ID returned by **timer**___**[create**(2)]. According to POSIX, there are no defined comparison or assignment operators for this type. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: **timer**___**[create**(2)], **timer**___**[delete**(2)], **timer**___**[getoverrun**(2)], **timer**___**[settime**(2)] _timespec_ _Include_: __. Alternatively, __, __, __, __, __, or __. struct timespec { time_t tv_sec; /* Seconds */ long tv_nsec; /* Nanoseconds */ }; Describes times in seconds and nanoseconds. _Conforming_ _to_: C11 and later; POSIX.1-2001 and later. _See_ _also_: **clock**___**[gettime**(2)], **clock**___**[nanosleep**(2)], [**nanosleep**(2)], **timerfd**___**[gettime**(2)], **timer**___**[gettime**(2)] _timeval_ _Include_: __. Alternatively, __, __, or __. struct timeval { time_t tv_sec; /* Seconds */ suseconds_t tv_usec; /* Microseconds */ }; Describes times in seconds and microseconds. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**gettimeofday**(2)], [**select**(2)], [**utimes**(2)], [**adjtime**(3)], **fu-** [**times**(3)], [**timeradd**(3)] _uid_t_ _Include_: __. Alternatively, __, __, __, __, __, or __. A type used to hold user IDs. According to POSIX, this shall be an integer type. _Conforming_ _to_: POSIX.1-2001 and later. _See_ _also_: [**chown**(2)], [**getuid**(2)], [**geteuid**(2)], [**getresuid**(2)], **getpw-** [**nam**(2)], [**credentials**(7)] _uintmax_t_ _Include_: __. Alternatively, __. An unsigned integer type capable of representing any value of any unsigned integer type supported by the implementation. Ac- cording to the C language standard, it shall be capable of stor- ing values in the range [0, **UINTMAX**___**MAX**]. The macro **UINTMAX**___**C**() expands its argument to an integer con- stant of type _uintmax_t_. The length modifier for _uintmax_t_ for the [**printf**(3)] and the [**scanf**(3)] families of functions is **j**; resulting commonly in **%ju** or **%jx** for printing _uintmax_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _Bugs_: _uintmax_t_ is not large enough to represent values of type _unsigned_ ___int128_ in implementations where _unsigned_ ___int128_ is defined and _unsigned_ _long_ _long_ is less than 128 bits wide. _See_ _also_: the _intmax_t_ type in this page. _uint_N__t_ _Include_: __. Alternatively, __. _uint8_t_, _uint16_t_, _uint32_t_, _uint64_t_ An unsigned integer type of a fixed width of exactly N bits, N being the value specified in its type name. According to the C language standard, they shall be capable of storing values in the range [0, **UINT**N___**MAX**], substituting N by the appropriate num- ber. According to POSIX, _uint8_t_, _uint16_t_, and _uint32_t_ are re- quired; _uint64_t_ is only required in implementations that pro- vide integer types with width 64; and all other types of this form are optional. The length modifiers for the _uint_N__t_ types for the [**printf**(3)] family of functions are expanded by macros of the forms **PRIu**N, **PRIo**N, **PRIx**N, and **PRIX**N (defined in __); resulting for example in **%"PRIu32"** or **%"PRIx32"** for printing _uint32_t_ values. The length modifiers for the _uint_N__t_ types for the [**scanf**(3)] fam- ily of functions are expanded by macros of the forms **SCNu**N, **SC-** **No**N, **SCNx**N, and **SCNX**N (defined in __); resulting for example in **%"SCNu16"** or **%"SCNx16"** for scanning _uint16_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _intmax_t_, _int_N__t_, and _uintmax_t_ types in this page. _uintptr_t_ _Include_: __. Alternatively, __. An unsigned integer type such that any valid (_void_ _*_) value can be converted to this type and back. According to the C language standard, it shall be capable of storing values in the range [0, **UINTPTR**___**MAX**]. The length modifier for _uintptr_t_ for the [**printf**(3)] family of functions is expanded by the macros **PRIuPTR**, **PRIoPTR**, **PRIxPTR**, and **PRIXPTR** (defined in __); resulting commonly in **%"PRIuPTR"** or **%"PRIxPTR"** for printing _uintptr_t_ values. The length modifier for _uintptr_t_ for the [**scanf**(3)] family of func- tions is expanded by the macros **SCNuPTR,** **SCNoPTR,** **SCNxPTR**, and **SCNXPTR** (defined in __); resulting commonly in **%"SC-** **NuPTR"** or **%"SCNxPTR"** for scanning _uintptr_t_ values. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: the _intptr_t_ and _void_ _*_ types in this page. _va_list_ _Include_: __. Alternatively, __, or __. Used by functions with a varying number of arguments of varying types. The function must declare an object of type _va_list_ which is used by the macros **va**___**[start**(3)], **va**___**[arg**(3)], **va**___**[copy**(3)], and **va**___**[end**(3)] to traverse the list of arguments. _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: **va**___**[start**(3)], **va**___**[arg**(3)], **va**___**[copy**(3)], **va**___**[end**(3)] _void_ _*_ According to the C language standard, a pointer to any object type may be converted to a pointer to _void_ and back. POSIX fur- ther requires that any pointer, including pointers to functions, may be converted to a pointer to _void_ and back. Conversions from and to any other pointer type are done implic- itly, not requiring casts at all. Note that this feature pre- vents any kind of type checking: the programmer should be care- ful not to convert a _void_ _*_ value to a type incompatible to that of the underlying data, because that would result in undefined behavior. This type is useful in function parameters and return value to allow passing values of any type. The function will typically use some mechanism to know the real type of the data being passed via a pointer to _void_. A value of this type can't be dereferenced, as it would give a value of type _void_, which is not possible. Likewise, pointer arithmetic is not possible with this type. However, in GNU C, pointer arithmetic is allowed as an extension to the standard; this is done by treating the size of a _void_ or of a function as 1. A consequence of this is that _sizeof_ is also allowed on _void_ and on function types, and returns 1. The conversion specifier for _void_ _*_ for the [**printf**(3)] and the [**scanf**(3)] families of functions is **p**. _Versions_: The POSIX requirement about compatibility between _void_ _*_ and function pointers was added in POSIX.1-2008 Technical Cor- rigendum 1 (2013). _Conforming_ _to_: C99 and later; POSIX.1-2001 and later. _See_ _also_: [**malloc**(3)], [**memcmp**(3)], [**memcpy**(3)], [**memset**(3)] See also the _intptr_t_ and _uintptr_t_ types in this page. **NOTES** The structures described in this manual page shall contain, at least, the members shown in their definition, in no particular order. Most of the integer types described in this page don't have a corre- sponding length modifier for the [**printf**(3)] and the [**scanf**(3)] families of functions. To print a value of an integer type that doesn't have a length modifier, it should be converted to _intmax_t_ or _uintmax_t_ by an explicit cast. To scan into a variable of an integer type that doesn't have a length modifier, an intermediate temporary variable of type _int-_ _max_t_ or _uintmax_t_ should be used. When copying from the temporary variable to the destination variable, the value could overflow. If the type has upper and lower limits, the user should check that the value is within those limits, before actually copying the value. The example below shows how these conversions should be done. **Conventions** **used** **in** **this** **page** In "Conforming to" we only concern ourselves with C99 and later and POSIX.1-2001 and later. Some types may be specified in earlier ver- sions of one of these standards, but in the interests of simplicity we omit details from earlier standards. In "Include", we first note the "primary" header(s) that define the type according to either the C or POSIX.1 standards. Under "Alterna- tively", we note additional headers that the standards specify shall define the type. **EXAMPLES** The program shown below scans from a string and prints a value stored in a variable of an integer type that doesn't have a length modifier. The appropriate conversions from and to _intmax_t_, and the appropriate range checks, are used as explained in the notes section above. #include #include #include #include int main (void) { static const char *const str = "500000 us in half a second"; suseconds_t us; intmax_t tmp; /* Scan the number from the string into the temporary variable */ sscanf(str, "%jd", &tmp); /* Check that the value is within the valid range of suseconds_t */ if (tmp < -1 || tmp > 1000000) { fprintf(stderr, "Scanned value outside valid range!\n"); exit(EXIT_FAILURE); } /* Copy the value to the suseconds_t variable 'us' */ us = tmp; /* Even though suseconds_t can hold the value -1, this isn't a sensible number of microseconds */ if (us < 0) { fprintf(stderr, "Scanned value shouldn't be negative!\n"); exit(EXIT_FAILURE); } /* Print the value */ printf("There are %jd microseconds in half a second.\n", (intmax_t) us); exit(EXIT_SUCCESS); } **SEE** **ALSO** **feature**___**test**___**[macros**(7)], [**standards**(7)] **COLOPHON** This page is part of release 5.10 of the Linux _man-pages_ project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at . Linux 2020-12-21 [SYSTEM_DATA_TYPES(7)]