| Header file | Description | First released | | {{tag>aio.h|open}} | Asynchronous input and output | Issue 5 |
| {{tag>arpa/inet.h|open}} | Functions for manipulating numeric IP addresses (part of Berkeley sockets) | Issue 6 |
| {{tag>assert.h|open}} | Verify assumptions | Issue 1 |
| {{tag>complex.h|open}} | Complex Arithmetic, see C mathematical functions | Issue 6 |
| {{tag>cpio.h|open}} | Magic numbers for the cpio archive format | Issue 3 |
| {{tag>ctype.h|open}} | Character types | Issue 1 |
| {{tag>dirent.h|open}} | Allows the opening and listing of directories | Issue 2 |
| {{tag>dlfcn.h|open}} | Dynamic linking | Issue 5 |
| {{tag>errno.h|open}} | Retrieving Error Number | Issue 1 |
| {{tag>fcntl.h|open}} | File opening, locking and other operations | Issue 1 |
| {{tag>fenv.h|open}} | Floating-Point Environment (FPE), see C mathematical functions | Issue 6 |
| {{tag>float.h|open}} | Floating-point types, see C data types | Issue 4 |
| {{tag>fmtmsg.h|open}} | Message display structures | Issue 4 |
| {{tag>fnmatch.h|open}} | Filename matching | Issue 4 |
| {{tag>ftw.h|open}} | File tree traversal | Issue 1 |
| {{tag>glob.h|open}} | Pathname "globbing" (pattern-matching) | Issue 4 |
| {{tag>grp.h|open}} | User group information and control | Issue 1 |
| {{tag>iconv.h|open}} | Codeset conversion facility | Issue 4 |
| {{tag>inttypes.h|open}} | Fixed sized integer types, see C data types | Issue 5 |
| {{tag>iso646.h|open}} | Alternative spellings, see C alternative tokens | Issue 5 |
| {{tag>langinfo.h|open}} | Language information constants – builds on C localization functions | Issue 2 |
| {{tag>libgen.h|open}} | Pathname manipulation | Issue 4 |
| {{tag>limits.h|open}} | Implementation-defined constants, see C data types | Issue 1 |
| {{tag>locale.h|open}} | Category macros, see C localization functions | Issue 3 |
| {{tag>math.h|open}} | Mathematical declarations, see C mathematical functions | Issue 1 |
| {{tag>monetary.h|open}} | String formatting of monetary units | Issue 4 |
| {{tag>mqueue.h|open}} | Message queue | Issue 5 |
| {{tag>ndbm.h|open}} | NDBM database operations | Issue 4 |
| {{tag>net/if.h|open}} | Listing of local network interfaces | Issue 6 |
| {{tag>netdb.h|open}} | Translating protocol and host names into numeric addresses (part of Berkeley sockets) | Issue 6 |
| {{tag>netinet/in.h|open}} | Defines Internet protocol and address family (part of Berkeley sockets) | Issue 6 |
| {{tag>netinet/tcp.h|open}} | Additional TCP control options (part of Berkeley sockets) | Issue 6 |
| {{tag>nl_types.h|open}} | Localization message catalog functions | Issue 2 |
| {{tag>poll.h|open}} | Asynchronous file descriptor multiplexing | Issue 4 |
| {{tag>pthread.h|open}} | Defines an API for creating and manipulating POSIX threads | Issue 5 |
| {{tag>pwd.h|open}} | passwd (user information) access and control | Issue 1 |
| regex|regex.h}}{{tag|regex.h|open}} | Regular expression matching | Issue 4 |
| {{tag>sched.h|open}} | Execution scheduling | Issue 5 |
| {{tag>search.h|open}} | Search tables | Issue 1 |
| {{tag>semaphore.h|open}} | POSIX semaphores | Issue 5 |
| {{tag>setjmp.h|open}} | Stack environment declarations | Issue 1 |
| {{tag>signal.h|open}} | Signals, see C signal handling | Issue 1 |
| {{tag>spawn.h|open}} | Process spawning | Issue 6 |
| {{tag>stdarg.h|open}} | Handle Variable Argument List | Issue 4 |
| {{tag>stdbool.h|open}} | Boolean type and values, see C data types | Issue 6 |
| {{tag>stddef.h|open}} | Standard type definitions, see C data types | Issue 4 |
| {{tag>stdint.h|open}} | Integer types, see C data types | Issue 6 |
| {{tag>stdio.h|open}} | Standard buffered input/output, see C file input/output | Issue 1 |
| {{tag>stdlib.h|open}} | Standard library definitions, see C standard library | Issue 3 |
| {{tag>string.h|open}} | Several String Operations, see C string handling | Issue 1 |
| {{tag>strings.h|open}} | Case-insensitive string comparisons | Issue 4 |
| {{tag>stropts.h|open}} | Stream manipulation, including ioctl | Issue 4 |
| {{tag>sys/ipc.h|open}} | Inter-process communication (IPC) | Issue 2 |
| {{tag>sys/mman.h|open}} | Memory management, including POSIX shared memory and memory mapped files | Issue 4 |
| {{tag>sys/msg.h|open}} | POSIX message queues | Issue 2 |
| {{tag>sys/resource.h|open}} | Resource usage, priorities, and limiting | Issue 4 |
| {{tag>sys/select.h|open}} | Synchronous I/O multiplexing | Issue 6 |
| {{tag>sys/sem.h|open}} | XSI (SysV style) semaphores | Issue 2 |
| {{tag>sys/shm.h|open}} | XSI (SysV style) shared memory | Issue 2 |
| {{tag>sys/socket.h|open}} | Main Berkeley sockets header | Issue 6 |
| {{tag>sys/stat.h|open}} | File information (stat et al.) | Issue 1 |
| {{tag>sys/statvfs.h|open}} | File System information | Issue 4 |
| {{tag>sys/time.h|open}} | Time and date functions and structures | Issue 4 |
| {{tag>sys/times.h|open}} | File access and modification times | Issue 1 |
| {{tag>sys/types.h|open}} | Various data types used elsewhere | Issue 1 |
| {{tag>sys/uio.h|open}} | Vectored I/O operations | Issue 4 |
| {{tag>sys/un.h|open}} | Unix domain sockets | Issue 6 |
| {{tag>sys/utsname.h|open}} | Operating system information, including uname | Issue 1 |
| {{tag>sys/wait.h|open}} | Status of terminated child processes (see wait) | Issue 3 |
| {{tag>syslog.h|open}} | System error logging | Issue 4 |
| {{tag>tar.h|open}} | Magic numbers for the tar archive format | Issue 3 |
| {{tag>termios.h|open}} | Allows terminal I/O interfaces | Issue 3 |
| {{tag>tgmath.h|open}} | Type-Generic Macros, see C mathematical functions | Issue 1 |
| {{tag>time.h|open}} | Type-Generic Macros, see C date and time functions | Issue 1 |
| {{tag>trace.h|open}} | Tracing of runtime behavior (DEPRECATED) | Issue 6 |
| {{tag>ulimit.h|open}} | Resource limiting (DEPRECATED in favor of ) | Issue 1 | |
| {{tag>unistd.h|open}} | Various essential POSIX functions and constants | Issue 1 |
| {{tag>utime.h|open}} | inode access and modification times | Issue 3 |
| {{tag>utmpx.h|open}} | User accounting database functions | Issue 4 |
| {{tag>wchar.h|open}} | Wide-Character Handling, see C string handling | Issue 4 |
| {{tag>wctype.h|open}} | Wide-Character Classification and Mapping Utilities, see C character classification | Issue 5 |
| {{tag>wordexp.h|open}} | Word-expansion like the shell would perform | Issue 4 |