The C POSIX library is a language-independent library (using C calling conventions) that adds functions specific to POSIX systems. POSIX (and the Single Unix Specification) specifies a number of routines that should be available over and above those in the C standard library proper. It was developed at the same time as the ANSI C standard and is closely related to C. Some effort was made to make the C and POSIX libraries compatible, but there are a few POSIX functions that were never introduced into ANSI C.
Facilities are often implemented alongside the C standard library functionality, with varying degrees of closeness. For example, glibc implements functions such as fork within libc.so, but before NPTL was merged into glibc it constituted a separate library with its own linker flag. Often, this POSIX-specified functionality will be regarded as part of the library; the C library proper may be identified as the ANSI or ISO C library.
Header files
| aio.h | Asynchronous input and output. | 
| arpa/inet.h | Definitions for internet operations. | 
| cpio.h | Magic numbers for the cpio archive format. | 
| dirent.h | Allows the opening and listing of directories. | 
| fcntl.h | File opening, locking and other operations. | 
| fmtmsg.h | Message display structures. | 
| fnmatch.h | Filename-matching types. | 
| ftw.h | File tree traversal. | 
| glob.h | Pathname pattern-matching types. | 
| grp.h | User group information and control. | 
| iconv.h | Codeset conversion facility. | 
| langinfo.h | Language information constants. | 
| libgen.h | Definitions for pattern matching functions. | 
| monetary.h | Monetary types. | 
| mqueue.h | Message queues (REALTIME). | 
| ndbm.h | Definitions for ndbm database operations. | 
| net/if.h | Sockets local interfaces. | 
| netdb.h | Definitions for network database operations. | 
| netinet/in.h | Internet address family. | 
| netinet/tcp.h | Definitions for the Internet Transmission Control Protocol (TCP). | 
| nl_types.h | Data types. | 
| poll.h | Definitions for the poll() function. | 
| pthread.h | Defines an API for creating and manipulating POSIX threads. | 
| pwd.h | Passwd (user information) access and control. | 
| regex.h | Regular expression matching types. | 
| sched.h | Execution scheduling. | 
| search.h | Search tables. | 
| semaphore.h | Semaphores. | 
| spawn.h | Create a new process to run an executable program. | 
| strings.h | String operations. | 
| stropts.h | STREAMS interface (STREAMS). | 
| sys/ipc.h | Inter-process communication (IPC). | 
| sys/mman.h | POSIX memory management declarations. | 
| sys/msg.h | POSIX message queues. | 
| sys/resource.h | Definitions for XSI resource operations. | 
| sys/select.h | Select types. | 
| sys/sem.h | POSIX semaphores. | 
| sys/shm.h | XSI shared memory facility. | 
| sys/socket.h | Main sockets header. | 
| sys/stat.h | File information (stat et al.). | 
| sys/statvfs.h | VFS File System information structure. | 
| sys/time.h | Time and date functions and structures. | 
| sys/times.h | File access and modification times structure. | 
| sys/types.h | Various data types used elsewhere. | 
| sys/uio.h | Definitions for vector I/O operations. | 
| sys/un.h | Definitions for UNIX domain sockets. | 
| sys/utsname.h | uname and related structures. | 
| sys/wait.h | Status of terminated child processes. | 
| syslog.h | Definitions for system error logging. | 
| tar.h | Magic numbers for the tar archive format. | 
| termios.h | Allows terminal I/O interfaces. | 
| trace.h | Tracing. | 
| ulimit.h | ulimit commands. | 
| unistd.h | Various essential POSIX functions and constants. | 
| utime.h | File access and modification times. | 
| utmpx.h | User accounting database definitions. | 
| wordexp.h | Word-expansion types. | 
Standard overlap headers
Headers that overlap/extend the C standard.
| assert.h | Verify program assertion. | 
| complex.h | Complex arithmetic. | 
| ctype.h | Character types. | 
| fenv.h | Floating-point environment. | 
| float.h | Floating types. | 
| inttypes.h | Fixed size integer types. | 
| iso646.h | Alternative spellings. | 
| limits.h | Implementation-defined constants. | 
| locale.h | Category macros. | 
| math.h | Mathematical declarations. | 
| setjmp.h | Stack environment declarations. | 
| signal.h | Signals. | 
| stdarg.h | Handle variable argument list. | 
| stdbool.h | Boolean type and values. | 
| stddef.h | Standard type definitions. | 
| stdint.h | Integer types. | 
| stdio.h | Standard buffered input/output. | 
| stdlib.h | Standard library definitions. | 
| string.h | String operations. | 
| tgmath.h | Type-generic macros. | 
| time.h | Time types. | 
| wchar.h | Wide-character handling. | 
| wctype.h | Wide-character classification and mapping utilities. | 
References
Bibliography
- Gallmeister, Bill (January 1, 1995), POSIX.4 Programmers Guide: Programming for the Real World (1st ed.), O'Reilly Media, pp. 564, ISBN 1565920740, http://oreilly.com/catalog/9781565920743/