Header Files In C

List of Header File available in c is given below(few only)

<aio.h>               <libgen.h>          <spawn.h>                <sys/time.h>
<arpa/inet.h>      <limits.h>            <stdarg.h>                <sys/times.h>
<assert.h>          <locale.h>           <stdbool.h>              <sys/types.h>
<complex.h>      <math.h>             <stddef.h>               <sys/uio.h>
<conio.h>          <monetary.h>       <stdint.h>                <sys/un.h>
<ctype.h>          <mqueue.h>         <stdio.h>                 <sys/utsname.h>
<dirent.h>          <ndbm.h>            <stdlib.h>                <sys/wait.h>
<dlfcn.h>           <net/if.h>              <string.h>                <syslog.h>
<errno.h>          <netdb.h>             <strings.h>              <tar.h>
<fcntl.h>            <netinet/in.h>        <stropts.h>             <termios.h>
<fenv.h>            <netinet/tcp.h>     <sys/ipc.h>              <tgmath.h>
<float.h>            <nl_types.h>        <sys/mman.h>         <time.h>
<fmtmsg.h>        <poll.h>               <sys/msg.h>            <trace.h>
<fnmatch.h>       <pthread.h>         <sys/resource.h>     <ulimit.h>
<ftw.h>              <pwd.h>              <sys/select.h>         <unistd.h>
<glob.h>            <regex.h>             <sys/sem.h>           <utime.h>
<grp.h>             <sched.h>             <sys/shm.h>           <utmpx.h>
<iconv.h>          <search.h>            <sys/socket.h>       <wchar.h>
<inttypes.h>       <semaphore.h>     <sys/stat.h>           <wctype.h>
<iso646.h>        <setjmp.h>           <sys/statvfs.h>        <wordexp.h>
<langinfo.h>       <signal.h>

out of the above mention Header File mostly used Header File at beginning level is
<stdio.h>
<conio.h>
<strings.h>
<math.h>
Hear file <stdio.h>
This header file is  used  in every program because it include the definition of printf, scanf getc,getchar,putc,putchar etc.
see the fig below

we can see that without stdio.h header file , the compiler cannot identify the definition of printf .

Hear file <conio.h>
This header file is  used  in every program because it include the definition of clrscr, getch ,ungetch,putch,gotoxy, etc.
see the image below

we can see that without conio.h header file , the compiler cannot identify the definition of clrscr and getch .

Header file <math.h>
To include the inbuile mathematical function like cos,sine, log, sqrt(square root), we need to
 include the header file <math.h>

header file <string.h>
this header file used for string (sequence of character) hadling,and manipulation like
strlen,strcpy,strcat etc