Standard Library Functions in C Programming

A function is a block of code that performs particular task.

Functions are used for to prevent writing the same line of code for more than once in a program.

C functions can be classified into two categories

library functions
user-defined functions

Library functions are functions which are already defined in C library. You just need to include appropriate header files to use these functions. They are a;ready declared and defined in C libraries.

Below is the list Standard Library Functions in the C Language

Function	System Include File	Function Prototype	Description
abort	stdlib.h	`void abort(void);`	Stops a program abnormally.
abs	stdlib.h	`int abs(int n);`	Calculates the absolute value of an integer argument n.
acos	math.h	`double acos(double x);`	Calculates the arc cosine of x.
asctime	time.h	`char asctime(const struct tm time);`	Converts the time that is stored as a structure to a character string.
asctime_r	time.h	`char asctime_r (const struct tm tm, char *buf);`	Converts tm that is stored as a structure to a character string. (Restartable version of asctime.)
asin	math.h	`double asin(double x);`	Calculates the arc sine of x.
assert	assert.h	`void assert(int expression);`	Prints a diagnostic message and ends the program if the expression is false.
atan	math.h	`double atan(double x);`	Calculates the arc tangent of x.
atan2	math.h	`double atan2(double y, double x);`	Calculates the arc tangent of y/x.
atexit	stdlib.h	`int atexit(void (*func)(void));`	Registers a function to be called at normal termination.
atof	stdlib.h	`double atof(const char *string);`	Converts string to a double-precision floating-point value.
atoi	stdlib.h	`int atoi(const char *string);`	Converts string to an integer.
atol	stdlib.h	`long int atol(const char *string);`	Converts string to a long integer.
bsearch	stdlib.h	`void bsearch(const void key, const void base, size_t num, size_t size, int (compare) (const void element1, const void element2));`	Performs a binary search on an array of num elements, each of size bytes. The array must be sorted in ascending order by the function pointed to by compare.
btowc	stdio.h wchar.h	`wint_t btowc(int c);`	Determines whether c constitues a valid multibyte character in the initial shift state.
calloc	stdlib.h	`void *calloc(size_t num, size_t size);`	Reserves storage space for an array of num elements, each of size size, and initializes the values of all elements to 0.
catclose⁶	nl_types.h	`int catclose (nl_catd catd);`	Closes a previously opened message catalog.
catgets⁶	nl_types.h	`char catgets(nl_catd catd, int set_id, int msg_id, const char s);`	Retrieves a message from an open message catalog.
catopen⁶	nl_types.h	`nl_catd catopen (const char *name, int oflag);`	Opens a message catalog, which must be done before a message can be retrieved.
ceil	math.h	`double ceil(double x);`	Calculates the double value representing the smallest integer that is greater than or equal to x.
clearerr	stdio.h	`void clearerr(FILE *stream);`	Resets the error indicators and the end-of-file indicator for stream.
clock	time.h	`clock_t clock(void);`	Returns the processor time that has elapsed since the job was started.
cos	math.h	`double cos(double x);`	Calculates the cosine of x.
cosh	math.h	`double cosh(double x);`	Calculates the hyperbolic cosine of x.
ctime	time.h	`char ctime(const time_t time);`	Converts time to a character string.
ctime64	time.h	`char ctime64(const time64_t time);`	Converts time to a character string.
ctime_r	time.h	`char ctime_r(const time_t time, char *buf);`	Converts time to a character string. (Restartable version of ctime.)
ctime64_r	time.h	`char ctime64_r(const time64_t time, char *buf);`	Converts time to a character string. (Restartable version of ctime64.)
difftime	time.h	`double difftime(time_t time2, time_t time1);`	Computes the difference between time2 and time1.
difftime64	time.h	`double difftime64(time64_t time2, time64_t time1);`	Computes the difference between time2 and time1.
div	stdlib.h	`div_t div(int numerator, int denominator);`	Calculates the quotient and remainder of the division of numerator by denominator.
erf	math.h	`double erf(double x);`	Calculates the error function of x.
erfc	math.h	`double erfc(double x);`	Calculates the error function for large values of x.
exit	stdlib.h	`void exit(int status);`	Ends a program normally.
exp	math.h	`double exp(double x);`	Calculates the exponential function of a floating-point argument x.
fabs	math.h	`double fabs(double x);`	Calculates the absolute value of a floating-point argument x.
fclose	stdio.h	`int fclose(FILE *stream);`	Closes the specified stream.
fdopen	stdio.h	`FILE fdopen(int handle, const char type);`	Associates an input or output stream with the file identified by handle.
feof	stdio.h	`int feof(FILE *stream);`	Tests whether the end-of-file flag is set for a given stream.
ferror	stdio.h	`int ferror(FILE *stream);`	Tests for an error indicator in reading from or writing to stream.
fflush	stdio.h	`int fflush(FILE *stream);`	Writes the contents of the buffer associated with the output stream.
fgetc	stdio.h	`int fgetc(FILE *stream);`	Reads a single unsigned character from the input stream.
fgetpos	stdio.h	`int fgetpos(FILE stream, fpos_t pos);`	Stores the current position of the file pointer associated with stream into the object pointed to by pos.
fgets	stdio.h	`char fgets(char string, int n, FILE *stream);`	Reads a string from the input stream.
fgetwc	stdio.h wchar.h	`wint_t fgetwc(FILE *stream);`	Reads the next multibyte character from the input stream pointed to by stream.
fgetws	stdio.h wchar.h	`wchar_t fgetws(wchar_t wcs, int n, FILE *stream);`	Reads wide characters from the stream into the array pointed to by wcs.
fileno	stdio.h	`int fileno(FILE *stream);`	Determines the file handle currently associated with stream.
floor	math.h	`double floor(double x);`	Calculates the floating-point value representing the largest integer less than or equal to x.
fmod	math.h	`double fmod(double x, double y);`	Calculates the floating-point remainder of x/y.
fopen	stdio.h	`FILE fopen(const char filename, const char *mode);`	Opens the specified file.
fprintf	stdio.h	`int fprintf(FILE stream, const char format-string, ...);`	Formats and prints characters and values to the output stream.
fputc	stdio.h	`int fputc(int c, FILE *stream);`	Prints a character to the output stream.
fputs	stdio.h	`int fputs(const char string, FILE stream);`	Copies a string to the output stream.
fputwc	stdio.h wchar.h	`wint_t fputwc(wchar_t wc, FILE *stream);`	Converts the wide character wc to a multibyte character and writes it to the output stream pointed to by stream at the current position.
fputws	stdio.h wchar.h	`int fputws(const wchar_t wcs, FILE stream);`	Converts the wide-character string wcs to a multibyte-character string and writes it to stream as a multibyte character string.
fread	stdio.h	`size_t fread(void buffer, size_t size, size_t count, FILE stream);`	Reads up to count items of size length from the input stream, and stores them in buffer.
free	stdlib.h	`void free(void *ptr);`	Frees a block of storage.
freopen	stdio.h	`FILE freopen(const char filename, const char mode, FILE stream);`	Closes stream, and reassigns it to the file specified.
frexp	math.h	`double frexp(double x, int *expptr);`	Separates a floating-point number into its mantissa and exponent.
fscanf	stdio.h	`int fscanf(FILE stream, const char format-string, ...);`	Reads data from stream into locations given by arg-list.
fseek	stdio.h	`int fseek(FILE *stream, long int offset, int origin);`	Changes the current file position associated with stream to a new location.
fsetpos	stdio.h	`int fsetpos(FILE stream, const fpos_t pos);`	Moves the current file position to a new location determined by pos.
ftell	stdio.h	`long int ftell(FILE *stream);`	Gets the current position of the file pointer.
fwide	stdio.h wchar.h	`int fwide(FILE *stream, int mode);`	Determines the orientation of the stream pointed to by stream.
fwprintf	stdio.h wchar.h	`int fwprintf(FILE stream, const wchar_t format, ...);`	Writes output to the stream pointed to by stream.
fwrite	stdio.h	`size_t fwrite(const void buffer, size_t size, size_t count, FILE stream);`	Writes up to count items of size length from buffer to stream.
fwscanf	stdio.h wchar.h	`int fwscanf(FILE stream, const wchar_t format, ...)`	Reads input from the stream pointed to by stream.
gamma	math.h	`double gamma(double x);`	Computes the Gamma Function
getc¹	stdio.h	`int getc(FILE *stream);`	Reads a single character from the input stream.
getchar	stdio.h	`int getchar(void);`	Reads a single character from stdin.
getenv	stdlib.h	`char getenv(const char varname);`	Searches environment variables for varname.
gets	stdio.h	`char gets(char buffer);`	Reads a string from stdin, and stores it in buffer.
getwc	stdio.h wchar.h	`wint_t getwc(FILE *stream);`	Reads the next multibyte character from stream, converts it to a wide character and advances the associated file position indicator for stream.
getwchar	wchar.h	`wint_t getwchar(void);`	Reads the next multibyte character from stdin, converts it to a wide character, and advances the associated file position indicator for stdin.
gmtime	time.h	`struct tm gmtime(const time_t time);`	Converts a time value to a structure of type tm.
gmtime64	time.h	`struct tm gmtime64(const time64_t time);`	Converts a time value to a structure of type tm.
gmtime_r	time.h	`struct tm gmtime_r (const time_t time, struct tm *result);`	Converts a time value to a structure of type tm. (Restartable version of gmtime.)
gmtime64_r	time.h	`struct tm gmtime64_r (const time64_t time, struct tm *result);`	Converts a time value to a structure of type tm. (Restartable version of gmtime64.)
hypot	math.h	`double hypot(double side1, double side2);`	Calculates the hypotenuse of a right-angled triangle with sides of length side1 and side2.
isalnum	ctype.h	`int isalnum(int c);`	Tests if c is alphanumeric.
isalpha	ctype.h	`int isalpha(int c);`	Tests if c is alphabetic.
isascii	ctype.h	`int isascii(int c);`	Tests if c is within the 7-bit US-ASCII range.
iscntrl	ctype.h	`int iscntrl(int c);`	Tests if c is a control character.
isdigit	ctype.h	`int isdigit(int c);`	Tests if c is a decimal digit.
isgraph	ctype.h	`int isgraph(int c);`	Tests if c is a printable character excluding the space.
islower	ctype.h	`int islower(int c);`	Tests if c is a lowercase letter.
isprint	ctype.h	`int isprint(int c);`	Tests if c is a printable character including the space.
ispunct	ctype.h	`int ispunct(int c);`	Tests if c is a punctuation character.
isspace	ctype.h	`int isspace(int c);`	Tests if c is a whitespace character.
isupper	ctype.h	`int isupper(int c);`	Tests if c is an uppercase letter.
iswalnum	wctype.h	`int iswalnum (wint_t wc);`	Checks for any alphanumeric wide character.
iswalpha	wctype.h	`int iswalpha (wint_t wc);`	Checks for any alphabetic wide character.
iswcntrl	wctype.h	`int iswcntrl (wint_t wc);`	Tests for any control wide character.
iswctype	wctype.h	`int iswctype(wint_t wc, wctype_t wc_prop);`	Determines whether or not the wide character wc has the property wc_prop.
iswdigit	wctype.h	`int iswdigit (wint_t wc);`	Checks for any decimal-digit wide character.
iswgraph	wctype.h	`int iswgraph (wint_t wc);`	Checks for any printing wide character except for the wide-character space.
iswlower	wctype.h	`int iswlower (wint_t wc);`	Checks for any lowercase wide character.
iswprint	wctype.h	`int iswprint (wint_t wc);`	Checks for any printing wide character.
iswpunct	wctype.h	`int iswpunct (wint_t wc);`	Test for a wide non-alphanumeric, non-space character.
iswspace	wctype.h	`int iswspace (wint_t wc);`	Checks for any wide character that corresponds to an implementation-defined set of wide characters for which iswalnum is false.
iswupper	wctype.h	`int iswupper (wint_t wc);`	Checks for any uppercase wide character.
iswxdigit	wctype.h	`int iswxdigit (wint_t wc);`	Checks for any hexadecimal digit character.
isxdigit	wctype.h	`int isxdigit(int c);`	Tests if c is a hexadecimal digit.
j0	math.h	`double j0(double x);`	Calculates the Bessel function value of the first kind of order 0.
j1	math.h	`double j1(double x);`	Calculates the Bessel function value of the first kind of order 1.
jn	math.h	`double jn(int n, double x);`	Calculates the Bessel function value of the first kind of order n.
labs	stdlib.h	`long int labs(long int n);`	Calculates the absolute value of n.
ldexp	math.h	`double ldexp(double x, int exp);`	Returns the value of x multiplied by (2 to the power of exp).
ldiv	stdlib.h	`ldiv_t ldiv(long int numerator, long int denominator);`	Calculates the quotient and remainder of numerator/denominator.
localeconv	locale.h	`struct lconv *localeconv(void);`	Formats numeric quantities in struct lconv according to the current locale.
localtime	time.h	`struct tm localtime(const time_t timeval);`	Converts timeval to a structure of type tm.
localtime64	time.h	`struct tm localtime64(const time64_t timeval);`	Converts timeval to a structure of type tm.
localtime_r	time.h	`struct tm localtime_r (const time_t timeval, struct tm *result);`	Converts a time value to a structure of type tm. (Restartable version of localtime.)
localtime64_r	time.h	`struct tm localtime64_r (const time64_t timeval, struct tm *result);`	Converts a `time` value to a structure of type `tm`. (Restartable version of localtime64.)
log	math.h	`double log(double x);`	Calculates the natural logarithm of x.
log10	math.h	`double log10(double x);`	Calculates the base 10 logarithm of x.
longjmp	setjmp.h	`void longjmp(jmp_buf env, int value);`	Restores a stack environment previously set in env by the setjmp function.
malloc	stdlib.h	`void *malloc(size_t size);`	Reserves a block of storage.
mblen	stdlib.h	`int mblen(const char *string, size_t n);`	Determines the length of a multibyte character string.
mbrlen	wchar.h	`int mbrlen (const char s, size_t n, mbstate_t ps);`	Determines the length of a multibyte character. (Restartable version of mblen.)
mbrtowc	wchar.h	`int mbrtowc (wchar_t pwc, const char s, size_t n, mbstate_t *ps);`	Convert a multibyte character to a wide character (Restartable version of mbtowc.)
mbsinit	wchar.h	`int mbsinit (const mbstate_t *ps);`	Test state object `*ps` for initial state.
mbsrtowcs	wchar.h	`size_t mbsrtowc (wchar_t dst, const char src, size_t len, mbstate_t ps);`	Convert multibyte string to a wide character string. (Restartable version of mbstowcs.)
mbstowcs	stdlib.h	`size_t mbstowcs(wchar_t pwc, const char string, size_t n);`	Converts the multibyte characters in string to their corresponding wchar_t codes, and stores not more than n codes in pwc.
mbtowc	stdlib.h	`int mbtowc(wchar_t pwc, const char string, size_t n);`	Stores the wchar_t code corresponding to the first n bytes of multibyte character string into the wchar_t character pwc.
memchr	string.h	`void memchr(const void buf, int c, size_t count);`	Searches the first count bytes of buf for the first occurrence of c converted to an unsigned character.
memcmp	string.h	`int memcmp(const void buf1, const void buf2, size_t count);`	Compares up to count bytes of buf1 and buf2.
memcpy	string.h	`void memcpy(void dest, const void *src, size_t count);`	Copies count bytes of src to dest.
memmove	string.h	`void memmove(void dest, const void *src, size_t count);`	Copies count bytes of src to dest. Allows copying between objects that overlap.
memset	string.h	`void memset(void dest, int c, size_t count);`	Sets count bytes of dest to a value c.
mktime	time.h	`time_t mktime(struct tm *time);`	Converts local time into calendar time.
mktime64	time.h	`time64_t mktime64(struct tm *time);`	Converts local time into calendar time.
modf	math.h	`double modf(double x, double *intptr);`	Breaks down the floating-point value x into fractional and integral parts.
nextafter	math.h	`double nextafter(double x, double y);`	Calculates the next representable value after x in the direction of y.
nextafterl	math.h	`long double nextafterl(long double x, long double y);`	Calculates the next representable value after x in the direction of y.
nexttoward	math.h	`double nexttoward(double x, long double y);`	Calculates the next representable value after x in the direction of y.
nexttowardl	math.h	`long double nexttowardl(long double x, long double y);`	Calculates the next representable value after x in the direction of y.
nl_langinfo	langinfo.h	`char *nl_langinfo(nl_item item);`	Retrieve from the current locale the string that describes the requested information specified by item .
perror	stdio.h	`void perror(const char *string);`	Prints an error message to stderr.
pow	math.h	`double pow(double x, double y);`	Calculates the value x to the power y.
printf	stdio.h	`int printf(const char *format-string, ...);`	Formats and prints characters and values to stdout.
putc	stdio.h	`int putc(int c, FILE *stream);`	Prints c to the output stream.
putchar	stdio.h	`int putchar(int c);`	Prints c to stdout.
putenv	stdlib.h	`int putenv(const char varname);`	Sets the value of an environment variable by altering an existing variable or creating a new one.
puts	stdio.h	`int puts(const char *string);`	Prints a string to stdout.
putwc	stdio.h wchar.h	`wint_t putwchar(wchar_t wc, FILE *stream);`	Converts the wide character wc to a multibyte character, and writes it to the stream at the current position.
putwchar	wchar.h	`wint_t putwchar(wchar_t wc);`	Converts the wide character wc to a multibyte character and writes it to stdout.
qsort	stdlib.h	`void qsort(void base, size_t num, size_t width, int(compare)(const void element1, const void element2));`	Performs a quick sort of an array of num elements, each of width bytes in size.
raise	signal.h	`int raise(int sig);`	Sends the signal sig to the running program.
rand	stdlib.h	`int rand(void);`	Returns a pseudo-random integer.
rand_r	stdlib.h	`int rand_r(void);`	Returns a pseudo-random integer. (Restartable version)
realloc	stdlib.h	`void realloc(void ptr, size_t size);`	Changes the size of a previously reserved storage block.
regcomp	regex.h	`int regcomp(regex_t preg, const char pattern, int cflags);`	Compiles the source regular expression pointed to by pattern into an executable version and stores it in the location pointed to by preg.
regerror	regex.h	`size_t regerror(int errcode, const regex_t preg, char errbuf, size_t errbuf_size);`	Finds the description for the error code errcode for the regular expression preg.
regexec	regex.h	`int regexec(const regex_t preg, const char string, size_t nmatch, regmatch_t *pmatch, int eflags);`	Compares the null-ended string string against the compiled regular expression preg to find a match between the two.
regfree	regex.h	`void regfree(regex_t *preg);`	Frees any memory that was allocated by regcomp to implement the regular expression preg.
remove	stdio.h	`int remove(const char *filename);`	Deletes the file specified by filename.
rename	stdio.h	`int rename(const char oldname, const char newname);`	Renames the specified file.
rewind	stdio.h	`void rewind(FILE *stream);`	Repositions the file pointer associated with stream to the beginning of the file.
scanf	stdio.h	`int scanf(const char *format-string, ...);`	Reads data from stdin into locations given by arg-list.
setbuf	stdio.h	`void setbuf(FILE stream, char buffer);`	Controls buffering for stream.
setjmp	setjmp.h	`int setjmp(jmp_buf env);`	Saves a stack environment that can be subsequently restored by longjmp.
setlocale	locale.h	`char setlocale(int category, const char locale);`	Changes or queries variables defined in the locale.
setvbuf	stdio.h	`int setvbuf(FILE stream, char buf, int type, size_t size);`	Controls buffering and buffer size for stream.
signal	signal.h	`void(signal (int sig, void(func)(int))) (int);`	Registers func as a signal handler for the signal sig.
sin	math.h	`double sin(double x);`	Calculates the sine of x.
sinh	math.h	`double sinh(double x);`	Calculates the hyperbolic sine of x.
snprintf	stdio.h	`int snprintf(char outbuf, size_t n, const char, ...)`	Same as sprintf except that the function will stop after n characters have been written to outbuf.
sprintf	stdio.h	`int sprintf(char buffer, const char format-string, ...);`	Formats and stores characters and values in buffer.
sqrt	math.h	`double sqrt(double x);`	Calculates the square root of x.
srand	stdlib.h	`void srand(unsigned int seed);`	Sets the seed for the pseudo-random number generator.
sscanf	stdio.h	`int sscanf(const char buffer, const char format, ...);`	Reads data from buffer into the locations given by arg-list.
strcasecmp	strings.h	`int srtcasecmp(const char string1, const char string2);`	Compares strings without case sensitivity.
strcat	string.h	`char strcat(char string1, const char *string2);`	Concatenates string2 to string1.
strchr	string.h	`char strchr(const char string, int c);`	Locates the first occurrence of c in string.
strcmp	string.h	`int strcmp(const char string1, const char string2);`	Compares the value of string1 to string2.
strcoll	string.h	`int strcoll(const char string1, const char string2);`	Compares two strings using the collating sequence in the current locale.
strcpy	string.h	`char strcpy(char string1, const char *string2);`	Copies string2 into string1.
strcspn	string.h	`size_t strcspn(const char string1, const char string2)`;	Returns the length of the initial substring of string1 consisting of characters not contained in string2.
strerror	string.h	`char *strerror(int errnum);`	Maps the error number in errnum to an error message string.
strfmon	wchar.h	`int strfmon (char s, size_t maxsize, const char format, ...);`	Converts monetary value to string.
strftime	time.h	`size_t strftime (char dest, size_t maxsize, const char format, const struct tm *timeptr);`	Stores characters in an array pointed to by dest, according to the string determined by format .
strlen	string.h	`size_t strlen(const char *string);`	Calculates the length of string.
strncasecmp	strings.h	`int strncasecmp(const char string1, const char string2, size_t count);`	Compares strings without case sensitivity.
strncat	string.h	`char strncat(char string1, const char *string2, size_t count);`	Concatenates up to count characters of string2 to string1.
strncmp	string.h	`int strncmp(const char string1, const char string2, size_t count);`	Compares up to count characters of string1 and string2.
strncpy	string.h	`char strncpy(char string1, const char *string2, size_t count);`	Copies up to count characters of string2 to string1.
strpbrk	string.h	`char strpbrk(const char string1, const char *string2);`	Locates the first occurrence in string1 of any character in string2.
strptime⁴	time.h	`char strptime (const char buf, const char format, struct tm tm);`	Date and time conversion
strrchr	string.h	`char strrchr(const char string, int c);`	Locates the last occurrence of c in string.
strspn	string.h	`size_t strspn(const char string1, const char string2);`	Returns the length of the initial substring of string1 consisting of characters contained in string2.
strstr	string.h	`char strstr(const char string1, const char *string2);`	Returns a pointer to the first occurrence of string2 in string1.
strtod	stdlib.h	`double strtod(const char nptr, char *endptr);`	Converts nptr to a double precision value.
strtod32	stdlib.h	`_Decimal32 strtod32(const char nptr, char *endptr);`	Converts nptr to a single-precision decimal floating-point value.
strtod64	stdlib.h	`_Decimal64 strtod64(const char nptr, char *endptr);`	Converts nptr to a double-precision decimal floating-point value.
strtod128	stdlib.h	`_Decimal128 strtod128(const char nptr, char *endptr);`	Converts nptr to a quad-precision decimal floating-point value.
strtof	stdlib.h	`float strtof(const char nptr, char *endptr);`	Converts nptr to a float value.
strtok	string.h	`char strtok(char string1, const char *string2);`	Locates the next token in string1 delimited by the next character in string2.
strtok_r	string.h	`char strtok_r(char string, const char seps, char *lasts);`	Locates the next token in string delimited by the next character in seps. (Restartable version of strtok.)
strtol	stdlib.h	`long int strtol(const char nptr, char *endptr, int base);`	Converts nptr to a signed long integer.
strtold	stdlib.h	`long double strtold(const char nptr, char *endptr);`	Converts nptr to a long double value.
strtoul	stdlib.h	`unsigned long int strtoul(const char string1, char *string2, int base);`	Converts string1 to an unsigned long integer.
strxfrm	string.h	`size_t strxfrm(char string1, const char string2, size_t count);`	Converts string2 and places the result in string1. The conversion is determined by the program's current locale.
swprintf	wchar.h	`int swprintf(wchar_t wcsbuffer, size_t n, const wchar_t format, ...);`	Formats and stores a series of wide characters and values into the wide-character buffer wcsbuffer.
swscanf	wchar.h	`int swscanf (const wchar_t buffer, const wchar_t format, ...)`	Reads data from buffer into the locations given by arg-list.
system	stdlib.h	`int system(const char *string);`	Passes string to the system command analyzer.
tan	math.h	`double tan(double x);`	Calculates the tangent of x.
tanh	math.h	`double tanh(double x);`	Calculates the hyperbolic tangent of x.
time	time.h	`time_t time(time_t *timeptr);`	Returns the current calendar time.
time64	time.h	`time64_t time64(time64_t *timeptr);`	Returns the current calendar time.
tmpfile	stdio.h	`FILE *tmpfile(void);`	Creates a temporary binary file and opens it.
tmpnam	stdio.h	`char tmpnam(char string);`	Generates a temporary file name.
toascii	ctype.h	`int toascii(int c);`	Converts c to a character in the 7-bit US-ASCII character set.
tolower	ctype.h	`int tolower(int c);`	Converts c to lowercase.
toupper	ctype.h	`int toupper(int c);`	Converts c to uppercase.
towctrans	wctype.h	`wint_t towctrans(wint_t wc, wctrans_t desc);`	Translates the wide character wc based on the mapping described by desc.
towlower	wctype.h	`wint_t towlower (wint_t wc);`	Converts uppercase letter to lowercase letter.
towupper	wctype.h	`wint_t towupper (wint_t wc);`	Converts lowercase letter to uppercase letter.
ungetc	stdio.h	`int ungetc(int c, FILE *stream);`	Pushes c back onto the input stream.
ungetwc	stdio.h wchar.h	`wint_t ungetwc(wint_t wc, FILE *stream);`	Pushes the wide character wc back onto the input stream.
va_arg	stdarg.h	`var_type va_arg(va_list arg_ptr, var_type);`	Returns the value of one argument and modifies arg_ptr to point to the next argument.
va_end	stdarg.h	`void va_end(va_list arg_ptr);`	Facilitates normal return from variable argument list processing.
va_start	stdarg.h	`void va_start(va_list arg_ptr, variable_name);`	Initializes arg_ptr for subsequent use by va_arg and va_end.
vfprintf	stdio.h stdarg.h	`int vfprintf(FILE stream, const char format, va_list arg_ptr);`	Formats and prints characters to the output stream using a variable number of arguments.
vfscanf	stdio.h stdarg.h	`int vfscanf(FILE stream, const char format, va_list arg_ptr);`	Reads data from a specified stream into locations given by a variable number of arguments.
vfwprintf	stdarg.h stdio.h wchar.h	`int vfwprintf(FILE stream, const wchar_t format, va_list arg);`	Equivalent to fwprintf, except that the variable argument list is replaced by arg.
vfwscanf	stdio.h stdarg.h	`int vfwscanf(FILE stream, const wchar_t format, va_list arg_ptr);`	Reads wide data from a specified stream into locations given by a variable number of arguments.
vprintf	stdio.h stdarg.h	`int vprintf(const char *format, va_list arg_ptr);`	Formats and prints characters to stdout using a variable number of arguments.
vscanf	stdio.h stdarg.h	`int vscanf(const char *format, va_list arg_ptr);`	Reads data from stdin into locations given by a variable number of arguments.
vsprintf	stdio.h stdarg.h	`int vsprintf(char target-string, const char format, va_list arg_ptr);`	Formats and stores characters in a buffer using a variable number of arguments.
vsnprintf	stdio.h	`int vsnprintf(char outbuf, size_t n, const char, va_list);`	Same as vsprintf except that the function will stop after n characters have been written to outbuf.
vsscanf	stdio.h stdarg.h	`int vsscanf(const char buffer, const char format, va_list arg_ptr);`	Reads data from a buffer into locations given by a variable number of arguments.
vswprintf	stdarg.h wchar.h	`int vswprintf(wchar_t wcsbuffer, size_t n, const wchar_t format, va_list arg);`	Formats and stores a series of wide characters and values in the buffer wcsbuffer.
vswscanf	stdio.h wchar.h	`int vswscanf(const wchar_t buffer, const wchar_t format, va_list arg_ptr);`	Reads wide data from a buffer into locations given by a variable number of arguments.
vwprintf	stdarg.h wchar.h	`int vwprintf(const wchar_t *format, va_list arg);`	Equivalent to wprintf, except that the variable argument list is replaced by arg.
vwscanf	stdio.h wchar.h	`int vwscanf(const wchar_t *format, va_list arg_ptr);`	Reads wide data from stdin into locations given by a variable number of arguments.
wcrtomb	wchar.h	`int wcrtomb (char s, wchar_t wchar, mbstate_t pss);`	Converts a wide character to a multibyte character. (Restartable version of wctomb.)
wcscat	wchar.h	`wchar_t wcscat(wchar_t string1, const wchar_t *string2);`	Appends a copy of the string pointed to by string2 to the end of the string pointed to by string1.
wcschr	wchar.h	`wchar_t wcschr(const wchar_t string, wchar_t character);`	Searches the wide-character string pointed to by string for the occurrence of character.
wcscmp	wchar.h	`int wcscmp(const wchar_t string1, const wchar_t string2);`	Compares two wide-character strings, string1 and string2.
wcscoll	wchar.h	`int wcscoll (const wchar_t wcs1, const wchar_t wcs2);`	Compares two wide-character strings using the collating sequence in the current locale.
wcscpy	wchar.h	`wchar_t wcscpy(wchar_t string1, const wchar_t *string2);`	Copies the contents of string2 (including the ending wchar_t null character) into string1.
wcscspn	wchar.h	`size_t wcscspn(const wchar_t string1, const wchar_t string2);`	Determines the number of wchar_t characters in the initial segment of the string pointed to by string1 that do not appear in the string pointed to by string2.
wcsftime	wchar.h	`size_t wcsftime(wchar_t wdest, size_t maxsize, const wchar_t format, const struct tm *timeptr);`	Converts the time and date specification in the timeptr structure into a wide-character string.
wcslen	wchar.h	`size_t wcslen(const wchar_t *string);`	Computes the number of wide-characters in the string pointed to by string.
wcslocaleconv	locale.h	`struct wcslconv *wcslocaleconv(void);`	Formats numeric quantities in struct wcslconv according to the current locale.
wcsncat	wchar.h	`wchar_t wcsncat(wchar_t string1, const wchar_t *string2, size_t count);`	Appends up to count wide characters from string2 to the end of string1, and appends a wchar_t null character to the result.
wcsncmp	wchar.h	`int wcsncmp(const wchar_t string1, const wchar_t string2, size_t count);`	Compares up to count wide characters in string1 to string2.
wcsncpy	wchar.h	`wchar_t wcsncpy(wchar_t string1, const wchar_t *string2, size_t count);`	Copies up to count wide characters from string2 to string1.
wcspbrk	wchar.h	`wchar_t wcspbrk(const wchar_t string1, const wchar_t *string2);`	Locates the first occurrence in the string pointed to by string1 of any wide characters from the string pointed to by string2.
wcsptime	wchar.h	`wchar_t wcsptime (const wchar_t buf, const wchar_t format, struct tm tm );`	Date and time conversion. Equivalent to strptime(), except that it uses wide characters.
wcsrchr	wchar.h	`wchar_t wcsrchr(const wchar_t string, wchar_t character);`	Locates the last occurrence of character in the string pointed to by string.
wcsrtombs⁴	wchar.h	`size_t wcsrtombs (char dst, const wchar_t src, size_t len, mbstate_t ps);`	Converts wide character string to multibyte string. (Restartable version of wcstombs.)
wcsspn	wchar.h	`size_t wcsspn(const wchar_t string1, const wchar_t string2);`	Computes the number of wide characters in the initial segment of the string pointed to by string1, which consists entirely of wide characters from the string pointed to by string2.
wcsstr	wchar.h	`wchar_t wcsstr(const wchar_t wcs1, const wchar_t *wcs2);`	Locates the first occurrence of wcs2 in wcs1.
wcstod	wchar.h	`double wcstod(const wchar_t nptr, wchar_t *endptr);`	Converts the initial portion of the wide-character string pointed to by nptr to a double value.
wcstod32	wchar.h	`_Decimal32 wcstod32(const wchar_t nptr, wchar_t *endptr);`	Converts the initial portion of the wide-character string pointed to by nptr to a single-precision decimal floating-point value.
wcstod64	wchar.h	`_Decimal64 wcstod64(const wchar_t nptr, wchar_t *endptr);`	Converts the initial portion of the wide-character string pointed to by nptr to a double-precision decimal floating-point value.
wcstod128	wchar.h	`_Decimal128 wcstod128(const wchar_t nptr, wchar_t *endptr);`	Converts the initial portion of the wide-character string pointed to by nptr to a quad-precision decimal floating-point value.
wcstok	wchar.h	`wchar_t wcstok(wchar_t wcs1, const wchar_t wcs2, wchar_t *ptr)`	Breaks wcs1 into a sequence of tokens, each of which is delimited by a wide character from the wide string pointed to by wcs2.
wcstol	wchar.h	`long int wcstol(const wchar_t nptr, wchar_t *endptr, int base);`	Converts the initial portion of the wide-character string pointed to by nptr to a long integer value.
wcstombs	stdlib.h	`size_t wcstombs(char dest, const wchar_t string, size_t count);`	Converts the wchar_t string into a multibyte string dest.
wcstoul	wchar.h	`unsigned long int wcstoul(const wchar_t nptr, wchar_t *endptr, int base);`	Converts the initial portion of the wide-character string pointed to by nptr to an unsigned long integer value.
wcsxfrm	wchar.h	`size_t wcsxfrm (wchar_t wcs1, const wchar_t wcs2, size_t n);`	Transforms a wide-character string to values which represent character collating weights and places the resulting wide-character string into an array.
wctob	stdarg.h wchar.h	`int wctob(wint_t wc);`	Determines whether wc corresponds to a member of the extended character set whose multibyte character representation is a single byte when in the initial shift state.
wctomb	stdlib.h	`int wctomb(char *string, wchar_t character);`	Converts the wchar_t value of character into a multibyte string.
wctrans	wctype.h	`wctrans_t wctrans(const char *property);`	Constructs a value with type wctrans_t that describes a mapping between wide characters identified by the string argument property.
wctype	wchar.h	`wctype_t wctype (const char *property);`	Obtains handle for character property classification.
wcwidth	wchar.h	`int wcswidth(const wchar_t *pwcs>, size_t n);`	Determine the display width of a wide character string.
wmemchr	wchar.h	`wchar_t wmemchr(const wchar_t s, wchar_t c, size_t n);`	Locates the first occurrence of c in the initial n wide characters of the object pointed to by s.
wmemcmp	wchar.h	`int wmemcmp(const wchar_t s1, const wchar_t s2, size_t n);`	Compares the first n wide characters of the object pointed to by s1 to the first n characters of the object pointed to by s2.
wmemcpy	wchar.h	`wchar_t wmemcpy(wchar_t s1, const wchar_t *s2, size_t n);`	Copies n wide characters from the object pointed to by s2 to the object pointed to by s1 .
wmemmove	wchar.h	`wchar_t wmemmove(wchar_t s1, const wchar_t *s2, size_t n);`	Copies n wide characters from the object pointed to by s2 to the object pointed to by s1 .
wmemset	wchar.h	`wchar_t wmemset(wchar_t s, wchar_t c, size_t n);`	Copies the value of c into each of the first n wide characters of the object pointed to by s.
wprintf	wchar.h	`int wprintf(const wchar_t *format, ...);`	Equivalent to fwprintf with the argument stdout interposed before the arguments to wprintf.
wscanf	wchar.h	`int wscanf(const wchar_t *format, ...);`	Equivalent to fwscanf with the argument stdin interposed before the arguments of wscanf.
y0	math.h	`double y0(double x);`	Calculates the Bessel function value of the second kind of order 0.
y1	math.h	`double y1(double x);`	Calculates the Bessel function value of the second kind of order 1.
yn	math.h	`double yn(int n, double x);`	Calculates the Bessel function value of the second kind of order n.

Standard Library Functions in C Programming

Samuel Yang