Advantages of Functions:

There are many advantages in using functions:

It makes possible top down modular programming. In this style of programming, the high level logic of the overall problem is solved first while the details of each lower level functions is addressed later.
The length of the source program can be reduced by using functions at appropriate places.
It becomes uncomplicated to locate and separate a faulty function for further study.
A function may be used later by many other programs this means that a c programmer can use function written by others.
A function can be used to keep away from rewriting the same block of codes which we are going use two or more locations in a program. This is especially useful if the code involved is long or complicated.

All C Functions

# and ##	manipulate strings
#define	define variables
#error	display an error message
#if, #ifdef, #ifndef, #else, #elif, #endif	conditional operators
#include	insert the contents of another file
#line	set line and file information
#pragma	implementation specific command
#undef	used to undefine variables
Predefined preprocessor variables	miscellaneous preprocessor variables
abort	stops the program
abs	absolute value
acos	arc cosine
asctime	a textual version of the time
asin	arc sine
assert	stops the program if an expression isn't true
atan2	arc tangent, using signs to determine quadrants
atexit	sets a function to be called when the program exits
atof	converts a string to a double
atoi	converts a string to an integer
atol	converts a string to a long
bsearch	perform a binary search
calloc	allocates and clears a chunk of memory
ceil	the smallest integer not less than a certain value
clearerr	clears errors
clock	returns the amount of time that the program has been running
cos	cosine
cosh	hyperbolic cosine
ctime	returns a specifically formatted version of the time
difftime	the difference between two times
div	returns the quotient and remainder of a division
exit	stop the program
exp	returns “e” raised to a given power
fabs	absolute value for floating-point numbers
fclose	close a file
feof	true if at the end-of-file
ferror	checks for a file error
fflush	writes the contents of the output buffer
fgetc	get a character from a stream
fgetpos	get the file position indicator
fgets	get a string of characters from a stream
floor	returns the largest integer not greater than a given value
fmod	returns the remainder of a division
fopen	open a file
fprintf	print formatted output to a file
fputc	write a character to a file
fputs	write a string to a file
fread	read from a file
free	returns previously allocated memory to the operating system
freopen	open an existing stream with a different name
frexp	decomposes a number into scientific notation
fscanf	read formatted input from a file
fseek	move to a specific location in a file
fsetpos	move to a specific location in a file
ftell	returns the current file position indicator
fwrite	write to a file
getc	read a character from a file
getchar	read a character from STDIN
getenv	get enviornment information about a variable
gets	read a string from STDIN
gmtime	returns a pointer to the current Greenwich Mean Time
isalnum	true if a character is alphanumeric
isalpha	true if a character is alphabetic
iscntrl	true if a character is a control character
isdigit	true if a character is a digit
isgraph	true if a character is a graphical character
islower	true if a character is lowercase
isprint	true if a character is a printing character
ispunct	true if a character is punctuation
isspace	true if a character is a space character
isupper	true if a character is an uppercase character
isxdigit	true if a character is a hexidecimal character
labs	absolute value for long integers
ldexp	computes a number in scientific notation
ldiv	returns the quotient and remainder of a division, in long integer form
localtime	returns a pointer to the current time
log	natural logarithm
log10	natural logarithm, in base 10
longjmp	start execution at a certain point in the program
malloc	allocates memory
memchr	searches an array for the first occurance of a character
memcmp	compares two buffers
memcpy	copies one buffer to another
memmove	moves one buffer to another
memset	fills a buffer with a character
mktime	returns the calendar version of a given time
modf	decomposes a number into integer and fractional parts
perror	displays a string version of the current error to STDERR
pow	returns a given number raised to another number
printf	write formatted output to STDOUT
putc	write a character to a stream
putchar	write a character to STDOUT
putenv	add/modify the environmental settings
puts	write a string to STDOUT
qsort	perform a quicksort
raise	send a signal to the program
rand	returns a pseudorandom number
realloc	changes the size of previously allocated memory
remove	erase a file
rename	rename a file
rewind	move the file position indicator to the beginning of a file
scanf	read formatted input from STDIN
setbuf	set the buffer for a specific stream
setjmp	set execution to start at a certain point
setlocale	sets the current locale
setvbuf	set the buffer and size for a specific stream
signal	register a function as a signal handler
sin	sine
sinh	hyperbolic sine
sprintf	write formatted output to a buffer
sqrt	square root
srand	initialize the random number generator
sscanf	read formatted input from a buffer
strcat	concatenates two strings
strchr	finds the first occurance of a character in a string
strcmp	compares two strings
strcoll	compares two strings in accordance to the current locale
strcpy	copies one string to another
strcspn	searches one string for any characters in another
strerror	returns a text version of a given error code
strftime	returns individual elements of the date and time
strlen	returns the length of a given string
strncat	concatenates a certain amount of characters of two strings
strncmp	compares a certain amount of characters of two strings
strncpy	copies a certain amount of characters from one string to another
strpbrk	finds the first location of any character in one string, in another string
strrchr	finds the last occurance of a character in a string
strspn	returns the length of a substring of characters of a string
strstr	finds the first occurance of a substring of characters
strtod	converts a string to a double
strtok	finds the next token in a string
strtol	converts a string to a long
strtoul	converts a string to an unsigned long
strxfrm	converts a substring so that it can be used by string comparison functions
system	perform a system call
tan	tangent
tanh	hyperbolic tangent
time	returns the current calendar time of the system
tmpfile	return a pointer to a temporary file
tmpnam	return a unique filename
tolower	converts a character to lowercase
toupper	converts a character to uppercase
ungetc	puts a character back into a stream
va_arg	use variable length parameter lists
vprintf, vfprintf, and vsprintf	write formatted output with variable argument lists
vscanf, vfscanf, vsscanf	gets formatted input from stdin with variable argument lists

The reference site for this content is http://www.cppreference.com/wiki/c/all

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Advantages of Functions:

All C Functions

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