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The C POSIX library is a specification of a C standard library for POSIX systems. It was developed at the same time as the ANSI C standard. Some effort was made to make POSIX compatible with standard C; POSIX includes additional functions to those introduced in standard C.
一言以蔽之：POSIX库函数是C标准库函数的超集。
1 标准C库
<assert.h> Contains the assert macro, used to assist with detecting logical errors and other types of bug in debugging versions of a program.
<complex.h> C99 A set of functions for manipulating complex numbers.
<ctype.h> Defines set of functions used to classify characters by their types or to convert between upper and lower case in a way that is independent of the used character set (typically ASCII or one of its extensions, although implementations utilizing EBCDIC are also known).
<errno.h> For testing error codes reported by library functions.
<fenv.h> C99 Defines a set of functions for controlling floating-point environment.
<float.h> Defines macro constants specifying the implementation-specific properties of the floating-point library.
<inttypes.h> C99 Defines exact width integer types.
<iso646.h> NA1 Defines several macros that implement alternative ways to express several standard tokens. For programming in ISO 646 variant character sets.
<limits.h> Defines macro constants specifying the implementation-specific properties of the integer types.
<locale.h> Defines localization functions.
<math.h> Defines common mathematical functions.
<setjmp.h> Declares the macros setjmp and longjmp, which are used for non-local exits.
<signal.h> Defines signal handling functions.
<stdalign.h> C11 For querying and specifying the alignment of objects.
<stdarg.h> For accessing a varying number of arguments passed to functions.
<stdatomic.h> C11 For atomic operations on data shared between threads.
<stdbool.h> C99 Defines a boolean data type.
<stddef.h> Defines several useful types and macros.
<stdint.h> C99 Defines exact width integer types.
<stdio.h> Defines core input and output functions
<stdlib.h> Defines numeric conversion functions, pseudo-random numbers generation functions, memory allocation, process control functions
<stdnoreturn.h> C11 For specifying non-returning functions.
<string.h> Defines string handling functions.
<tgmath.h> C99 Defines type-generic mathematical functions.
<threads.h> C11 Defines functions for managing multiple Threads as well as mutexes and condition variables.
<time.h> Defines date and time handling functions
<uchar.h> C11 Types and functions for manipulating Unicode characters.
<wchar.h> NA1 Defines wide string handling functions.
<wctype.h> NA1 Defines set of functions used to classify wide characters by their types or to convert between upper and lower case- 1
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2 POSIX库（标准C库除外）
<aio.h> Asynchronous input and output Issue 5
<arpa/inet.h> Functions for manipulating numeric IP addresses (part of Berkeley sockets) Issue 6
<assert.h> Verify assumptions ??
<complex.h> Complex Arithmetic, see C mathematical functions ??
<cpio.h> Magic numbers for the cpio archive format Issue 3
<dirent.h> Allows the opening and listing of directories Issue 2
<dlfcn.h> Dynamic linking Issue 5
<errno.h> Retrieving Error Number ??
<fcntl.h> File opening, locking and other operations Issue 1
<fenv.h> Floating-Point Environment (FPE), see C mathematical functions ??
<float.h> Floating-point types, see C data types ??
<fmtmsg.h> Message display structures Issue 4
<fnmatch.h> Filename matching Issue 4
<ftw.h> File tree traversal Issue 1
<glob.h> Pathname "globbing" (pattern-matching) Issue 4
<grp.h> User group information and control Issue 1
<iconv.h> Codeset conversion facility Issue 4
<inttypes.h> Fixed sized integer types, see C data types ??
<iso646.h> Alternative spellings, see C alternative tokens ??
<langinfo.h> Language information constants – builds on C localization functions Issue 2
<libgen.h> Pathname manipulation Issue 4
<limits.h> Implementation-defined constants, see C data types ??
<locale.h> Category macros, see C localization functions ??
<math.h> Mathematical declarations, see C mathematical functions ??
<monetary.h> String formatting of monetary units Issue 4
<mqueue.h> Message queue Issue 5
<ndbm.h> NDBM database operations Issue 4
<net/if.h> Listing of local network interfaces Issue 6
<netdb.h> Translating protocol and host names into numeric addresses (part of Berkeley sockets) Issue 6
<netinet/in.h> Defines Internet protocol and address family (part of Berkeley sockets) Issue 6
<netinet/tcp.h> Additional TCP control options (part of Berkeley sockets) Issue 6
<nl_types.h> Localization message catalog functions Issue 2
<poll.h> Asynchronous file descriptor multiplexing Issue 4
<pthread.h> Defines an API for creating and manipulating POSIX threads Issue 5
<pwd.h> passwd (user information) access and control Issue 1
<regex.h> Regular expression matching Issue 4
<sched.h> Execution scheduling Issue 5
<search.h> Search tables Issue 1
<semaphore.h> POSIX semaphores Issue 5
<setjmp.h> Stack environment declarations ??
<signal.h> Signals, see C signal handling ??
<spawn.h> Process spawning Issue 6
<stdarg.h> Handle Variable Argument List ??
<stdbool.h> Boolean type and values, see C data types ??
<stddef.h> Standard type definitions, see C data types ??
<stdint.h> Integer types, see C data types ??
<stdio.h> Standard buffered input/output, see C file input/output ??
<stdlib.h> Standard library definitions, see C standard library ??
<string.h> Several String Operations, see C string handling ??
<strings.h> Case-insensitive string comparisons Issue 4
<stropts.h> Stream manipulation, including ioctl Issue 4
<sys/ipc.h> Inter-process communication (IPC) Issue 2
<sys/mman.h> Memory management, including POSIX shared memory and memory mapped files Issue 4
<sys/msg.h> POSIX message queues Issue 2
<sys/resource.h> Resource usage, priorities, and limiting Issue 4
<sys/select.h> Synchronous I/O multiplexing Issue 6
<sys/sem.h> XSI (SysV style) semaphores Issue 2
<sys/shm.h> XSI (SysV style) shared memory Issue 2
<sys/socket.h> Main Berkley sockets header Issue 6
<sys/stat.h> File information (stat et al.) Issue 1
<sys/statvfs.h> File System information Issue 4
<sys/time.h> Time and date functions and structures Issue 4
<sys/times.h> File access and modification times Issue 1
<sys/types.h> Various data types used elsewhere Issue 1
<sys/uio.h> Vectored I/O operations Issue 4
<sys/un.h> Unix domain sockets Issue 6
<sys/utsname.h> Operating system information, including uname Issue 1
<sys/wait.h> Status of terminated child processes (see wait) Issue 3
<syslog.h> System error logging Issue 4
<tar.h> Magic numbers for the tar archive format Issue 3
<termios.h> Allows terminal I/O interfaces Issue 3
<tgmath.h> Type-Generic Macros, see C mathematical functions ??
<time.h> Type-Generic Macros, see C date and time functions ??
<trace.h> Tracing of runtime behavior (DEPRECATED) Issue 6
<ulimit.h> Resource limiting (DEPRECATED in favor of <sys/resource.h>) Issue 1
<unistd.h> Various essential POSIX functions and constants Issue 1
<utime.h> inode access and modification times Issue 3
<utmpx.h> User accounting database functions Issue 4
<wchar.h> Wide-Character Handling, see C string handling ??
<wctype.h> Wide-Character Classification and Mapping Utilities, see C character classification ??
<wordexp.h> Word-expansion like the shell would perform