C Standard Library

[Last modified : 2003.07.09]

Contents

<assert.h>

void assert(int expression);
Macro used for internal error detection. (Ignored if NDEBUG is defined where <assert.h> is included.) If expression equals zero, message printed on stderr and abort called to terminate execution. Source filename and line number in message are from preprocessor macros __FILE__ and __LINE__.

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<ctype.h>

int isalnum(int c);
isalpha(c) or isdigit(c)
int isalpha(int c);
isupper(c) or islower(c)
int iscntrl(int c);
is control character. In ASCII, control characters are 0x00 (NUL) to 0x1F (US), and 0x7F (DEL)
int isdigit(int c);
is decimal digit
int isgraph(int c);
is printing character other than space
int islower(int c);
is lower-case letter
int isprint(int c);
is printing character (including space). In ASCII, printing characters are 0x20 (' ') to 0x7E ('~')
int ispunct(int c);
is printing character other than space, letter, digit
int isspace(int c);
is space, formfeed, newline, carriage return, tab, vertical tab
int isupper(int c);
is upper-case letter
int isxdigit(int c);
is hexadecimal digit
int tolower(int c);
return lower-case equivalent
int toupper(int c);
return upper-case equivalent

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<errno.h>

errno
object to which certain library functions assign specific positive values on error
EDOM
code used for domain errors
ERANGE
code used for range errors

Notes:

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<float.h>

FLT_RADIX
radix of floating-point representations
FLT_ROUNDS
floating-point rounding mode

Where the prefix "FLT" pertains to type float, "DBL" to type double, and "LDBL" to type long double:

FLT_DIG
DBL_DIG
LDBL_DIG
precision (in decimal digits)
FLT_EPSILON
DBL_EPSILON
LDBL_EPSILON
smallest number x such that 1.0 + x != 1.0
FLT_MANT_DIG
DBL_MANT_DIG
LDBL_MANT_DIG
number of digits, base FLT_RADIX, in mantissa
FLT_MAX
DBL_MAX
LDBL_MAX
maximum number
FLT_MAX_EXP
DBL_MAX_EXP
LDBL_MAX_EXP
largest positive integer exponent to which FLT_RADIX can be raised and remain representable
FLT_MIN
DBL_MIN
LDBL_MIN
minimum normalised number
FLT_MIN_EXP
DBL_MIN_EXP
LDBL_MIN_EXP
smallest negative integer exponent to which FLT_RADIX can be raised and remain representable

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<limits.h>

CHAR_BIT
number of bits in a char
CHAR_MAX
maximum value of type char
CHAR_MIN
minimum value of type char
SCHAR_MAX
maximum value of type signed char
SCHAR_MIN
minimum value of type signed char
UCHAR_MAX
maximum value of type unsigned char
SHRT_MAX
maximum value of type short
SHRT_MIN
minimum value of type short
USHRT_MAX
maximum value of type unsigned short
INT_MAX
maximum value of type int
INT_MIN
minimum value of type int
UINT_MAX
maximum value of type unsigned int
LONG_MAX
maximum value of type long
LONG_MIN
minimum value of type long
ULONG_MAX
maximum value of type unsigned long

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<locale.h>

struct lconv
Describes formatting of monetary and other numeric values:
char* decimal_point;
decimal point for non-monetary values
char* grouping;
sizes of digit groups for non-monetary values
char* thousands_sep;
separator for digit groups for non-monetary values (left of "decimal point")
char* currency_symbol;
currency symbol
char* int_curr_symbol;
international currency symbol
char* mon_decimal_point;
decimal point for monetary values
char* mon_grouping;
sizes of digit groups for monetary values
char* mon_thousands_sep;
separator for digit groups for monetary values (left of "decimal point")
char* negative_sign;
negative sign for monetary values
char* positive_sign;
positive sign for monetary values
char frac_digits;
number of digits to be displayed to right of "decimal point" for monetary values
char int_frac_digits;
number of digits to be displayed to right of "decimal point" for international monetary values
char n_cs_precedes;
whether currency symbol precedes (1) or follows (0) negative monetary values
char n_sep_by_space;
whether currency symbol is (1) or is not (0) separated by space from negative monetary values
char n_sign_posn;
format for negative monetary values:
0
parentheses surround quantity and currency symbol
1
sign precedes quantity and currency symbol
2
sign follows quantity and currency symbol
3
sign immediately precedes currency symbol
4
sign immediately follows currency symbol
char p_cs_precedes;
whether currency symbol precedes (1) or follows (0) positive monetary values
char p_sep_by_space;
whether currency symbol is (1) or is not (0) separated by space from non-negative monetary values
char p_sign_posn;
format for non-negative monetary values, with values as for n_sign_posn
Implementations may change field order and include additional fields. Standard C Library functions use only decimal_point.
struct lconv* localeconv(void);
returns pointer to formatting information for current locale
char* setlocale(int category, const char* locale);
Sets components of locale according to specified category and locale. Returns string describing new locale or null on error. (Implementations are permitted to define values of category additional to those describe here.)
LC_ALL
category argument for all categories
LC_NUMERIC
category for numeric formatting information
LC_MONETARY
category for monetary formatting information
LC_COLLATE
category for information affecting collating functions
LC_CTYPE
category for information affecting character class tests functions
LC_TIME
category for information affecting time conversions functions
NULL
null pointer constant

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<math.h>

On domain error, implementation-defined value returned and errno set to EDOM. On range error, errno set to ERANGE and return value is HUGE_VAL with correct sign for overflow, or zero for underflow. Angles are in radians.

HUGE_VAL
magnitude returned (with correct sign) on overflow error
double exp(double x);
exponential of x
double log(double x);
natural logarithm of x
double log10(double x);
base-10 logarithm of x
double pow(double x, double y);
x raised to power y
double sqrt(double x);
square root of x
double ceil(double x);
smallest integer not less than x
double floor(double x);
largest integer not greater than x
double fabs(double x);
absolute value of x
double ldexp(double x, int n);
x times 2 to the power n
double frexp(double x, int* exp);
if x non-zero, returns value, with absolute value in interval [1/2, 1), and assigns to *exp integer such that product of return value and 2 raised to the power *exp equals x; if x zero, both return value and *exp are zero
double modf(double x, double* ip);
returns fractional part and assigns to *ip integral part of x, both with same sign as x
double fmod(double x, double y);
if y non-zero, floating-point remainder of x/y, with same sign as x; if y zero, result is implementation-defined
double sin(double x);
sine of x
double cos(double x);
cosine of x
double tan(double x);
tangent of x
double asin(double x);
arc-sine of x
double acos(double x);
arc-cosine of x
double atan(double x);
arc-tangent of x
double atan2(double y, double x);
arc-tangent of y/x
double sinh(double x);
hyperbolic sine of x
double cosh(double x);
hyperbolic cosine of x
double tanh(double x);
hyperbolic tangent of x

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<setjmp.h>

jmp_buf
type of object holding context information
int setjmp(jmp_buf env);
Saves context information in env and returns zero. Subsequent call to longjmp with same env returns non-zero.
void longjmp(jmp_buf env, int val);
Restores context saved by most recent call to setjmp with specified env. Execution resumes as a second return from setjmp, with returned value val if specified value non-zero, or 1 otherwise.

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<signal.h>

SIGABRT
abnormal termination
SIGFPE
arithmetic error
SIGILL
invalid execution
SIGINT
(asynchronous) interactive attention
SIGSEGV
illegal storage access
SIGTERM
(asynchronous) termination request
SIG_DFL
specifies default signal handling
SIG_ERR
signal return value indicating error
SIG_IGN
specifies that signal should be ignored
void (*signal(int sig, void (*handler)(int)))(int);
Install handler for subsequent signal sig. If handler is SIG_DFL, implementation-defined default behaviour will be used; if SIG_IGN, signal will be ignored; otherwise function pointed to by handler will be invoked with argument sig. In the last case, handling is restored to default behaviour before handler is called. If handler returns, execution resumes where signal occurred. signal returns the previous handler or SIG_ERR on error. Initial state is implementation-defined. Implementations may may define signals additional to those listed here.
int raise(int sig);
Sends signal sig. Returns zero on success.

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<stdarg.h>

va_list
type of object holding context information
void va_start(va_list ap, lastarg);
Initialisation macro which must be called once before any unnamed argument is accessed. Stores context information in ap. lastarg is the last named parameter of the function.
type va_arg(va_list ap, type);
Yields value of the type (type) and value of the next unnamed argument.
void va_end(va_list ap);
Termination macro which must be called once after argument processing and before exit from function.

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<stddef.h>

NULL
Null pointer constant.
offsetof(stype, m)
Offset (in bytes) of member m from start of structure type stype.
ptrdiff_t
Type for objects declared to store result of subtracting pointers.
size_t
Type for objects declared to store result of sizeof operator.

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<stdio.h>

BUFSIZ
Size of buffer used by setbuf.
EOF
Value used to indicate end-of-stream or to report an error.
FILENAME_MAX
Maximum length required for array of characters to hold a filename.
FOPEN_MAX
Maximum number of files which may be open simultaneously.
L_tmpnam
Number of characters required for temporary filename generated by tmpnam.
NULL
Null pointer constant.
SEEK_CUR
Value for origin argument to fseek specifying current file position.
SEEK_END
Value for origin argument to fseek specifying end of file.
SEEK_SET
Value for origin argument to fseek specifying beginning of file.
TMP_MAX
Minimum number of unique filenames generated by calls to tmpnam.
_IOFBF
Value for mode argument to setvbuf specifying full buffering.
_IOFBF
Value for mode argument to setvbuf specifying line buffering.
_IOFBF
Value for mode argument to setvbuf specifying no buffering.
stdin
File pointer for standard input stream. Automatically opened when program execution begins.
stdout
File pointer for standard output stream. Automatically opened when program execution begins.
stderr
File pointer for standard error stream. Automatically opened when program execution begins.
FILE
Type of object holding information necessary to control a stream.
fpos_t
Type for objects declared to store file position information.
size_t
Type for objects declared to store result of sizeof operator.
FILE* fopen(const char* filename, const char* mode);
Opens file named filename and returns a stream, or NULL on failure. mode may be one of the following for text files:
"r"
text reading
"w"
text writing
"a"
text append
"r+"
text update (reading and writing)
"w+"
text update, discarding previous content (if any)
"a+"
text append, reading, and writing at end
or one of those strings with b included (after the first character), for binary files.
FILE* freopen(const char* filename, const char* mode, FILE* stream);
Closes file associated with stream, then opens file filename with specified mode and associates it with stream. Returns stream or NULL on error.
int fflush(FILE* stream);
Flushes stream stream and returns zero on success or EOF on error. Effect undefined for input stream. fflush(NULL) flushes all output streams.
int fclose(FILE* stream);
Closes stream stream (after flushing, if output stream). Returns EOF on error, zero otherwise.
int remove(const char* filename);
Removes specified file. Returns non-zero on failure.
int rename(const char* oldname, const char* newname);
Changes name of file oldname to newname. Returns non-zero on failure.
FILE* tmpfile();
Creates temporary file (mode "wb+") which will be removed when closed or on normal program termination. Returns stream or NULL on failure.
char* tmpname(char s[L_tmpnam]);
Assigns to s (if s non-null) and returns unique name for a temporary file. Unique name is returned for each of the first TMP_MAX invocations.
int setvbuf(FILE* stream, char* buf, int mode, size_t size);
Controls buffering for stream stream. mode is _IOFBF for full buffering, _IOLBF for line buffering, _IONBF for no buffering. Non-null buf specifies buffer of size size to be used; otherwise, a buffer is allocated. Returns non-zero on error. Call must be before any other operation on stream.
void setbuf(FILE* stream, char* buf);
Controls buffering for stream stream. For null buf, turns off buffering, otherwise equivalent to (void)setvbuf(stream,buf,_IOFBF,BUFSIZ).
int fprintf(FILE* stream, const char* format, ...);
Converts (according to format format) and writes output to stream stream. Number of characters written, or negative value on error, is returned. Conversion specifications consist of:
int printf(const char* format, ...);
printf(f, ...) is equivalent to fprintf(stdout, f, ...)
int sprintf(char* s, const char* format, ...);
Like fprintf, but output written into string s, which must be large enough to hold the output, rather than to a stream. Output is NUL-terminated. Returns length (excluding the terminating NUL).
int vfprintf(FILE* stream, const char* format, va_list arg);
Equivalent to fprintf with variable argument list replaced by arg, which must have been initialised by the va_start macro (and may have been used in calls to va_arg).
int vprintf(const char* format, va_list arg);
Equivalent to printf with variable argument list replaced by arg, which must have been initialised by the va_start macro (and may have been used in calls to va_arg).
int vsprintf(char* s, const char* format, va_list arg);
Equivalent to sprintf with variable argument list replaced by arg, which must have been initialised by the va_start macro (and may have been used in calls to va_arg).
int fscanf(FILE* stream, const char* format, ...);
Performs formatted input conversion, reading from stream stream according to format format. The function returns when format is fully processed. Returns number of items converted and assigned, or EOF if end-of-file or error occurs before any conversion. Each of the arguments following format must be a pointer. Format string may contain:
int scanf(const char* format, ...);
scanf(f, ...) is equivalent to fscanf(stdin, f, ...)
int sscanf(char* s, const char* format, ...);
Like fscanf, but input read from string s.
int fgetc(FILE* stream);
Returns next character from (input) stream stream, or EOF on end-of-file or error.
char* fgets(char* s, int n, FILE* stream);
Copies characters from (input) stream stream to s, stopping when n-1 characters copied, newline copied, end-of-file reached or error occurs. If no error, s is NUL-terminated. Returns NULL on end-of-file or error, s otherwise.
int fputc(int c, FILE* stream);
Writes c, to stream stream. Returns c, or EOF on error.
char* fputs(const char* s, FILE* stream);
Writes s, to (output) stream stream. Returns non-negative on success or EOF on error.
int getc(FILE* stream);
Equivalent to fgetc except that it may be a macro.
int getchar(void);
Equivalent to getc(stdin).
char* gets(char* s);
Copies characters from stdin into s until newline encountered, end-of-file reached, or error occurs. Does not copy newline. NUL-terminates s. Returns s, or NULL on end-of-file or error. Should not be used because of the potential for buffer overflow.
int putc(int c, FILE* stream);
Equivalent to fputc except that it may be a macro.
int putchar(int c);
putchar(c) is equivalent to putc(c, stdout).
int puts(const char* s);
Writes s (excluding terminating NUL) and a newline to stdout. Returns non-negative on success, EOF on error.
int ungetc(int c, FILE* stream);
Pushes c (which must not be EOF), onto (input) stream stream such that it will be returned by the next read. Only one character of pushback is guaranteed (for each stream). Returns c, or EOF on error.
size_t fread(void* ptr, size_t size, size_t nobj, FILE* stream);
Reads (at most) nobj objects of size size from stream stream into ptr and returns number of objects read. (feof and ferror can be used to check status.)
size_t fwrite(const void* ptr, size_t size, size_t nobj, FILE* stream);
Writes to stream stream, nobj objects of size size from array ptr. Returns number of objects written.
int fseek(FILE* stream, long offset, int origin);
Sets file position for stream stream and clears end-of-file indicator. For a binary stream, file position is set to offset bytes from the position indicated by origin: beginning of file for SEEK_SET, current position for SEEK_CUR, or end of file for SEEK_END. Behaviour is similar for a text stream, but offset must be zero or, for SEEK_SET only, a value returned by ftell. Returns non-zero on error.
long ftell(FILE* stream);
Returns current file position for stream stream, or -1 on error.
void rewind(FILE* stream);
Equivalent to fseek(stream, 0L, SEEK_SET); clearerr(stream).
int fgetpos(FILE* stream, fpos_t* ptr);
Stores current file position for stream stream in *ptr. Returns non-zero on error.
int fsetpos(FILE* stream, const fpos_t* ptr);
Sets current position of stream stream to *ptr. Returns non-zero on error.
void clearerr(FILE* stream);
Clears end-of-file and error indicators for stream stream.
int feof(FILE* stream);
Returns non-zero if end-of-file indicator is set for stream stream.
int ferror(FILE* stream);
Returns non-zero if error indicator is set for stream stream.
void perror(const char* s);
Prints s (if non-null) and strerror(errno) to standard error as would:
fprintf(stderr, "%s: %s\n", (s != NULL ? s : ""), strerror(errno))

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<stdlib.h>

EXIT_FAILURE
Value for status argument to exit indicating failure.
EXIT_SUCCESS
Value for status argument to exit indicating success.
RAND_MAX
Maximum value returned by rand().
NULL
Null pointer constant.
div_t
Return type of div(). Structure having members:
int quot;
quotient
int rem;
remainder
ldiv_t
Return type of ldiv(). Structure having members:
long quot;
quotient
long rem;
remainder
size_t
Type for objects declared to store result of sizeof operator.
int abs(int n);
long labs(long n);
Returns absolute value of n.
div_t div(int num, int denom);
ldiv_t ldiv(long num, long denom);
Returns quotient and remainder of num/denom.
double atof(const char* s);
Equivalent to strtod(s, (char**)NULL) except that errno is not necessarily set on conversion error.
int atoi(const char* s);
Equivalent to (int)strtol(s, (char**)NULL, 10) except that errno is not necessarily set on conversion error.
long atol(const char* s);
Equivalent to strtol(s, (char**)NULL, 10) except that errno is not necessarily set on conversion error.
double strtod(const char* s, char** endp);
Converts initial characters (ignoring leading white space) of s to type double. If endp non-null, stores pointer to unconverted suffix in *endp. On overflow, sets errno to ERANGE and returns HUGE_VAL with the appropriate sign; on underflow, sets errno to ERANGE and returns zero; otherwise returns converted value.
long strtol(const char* s, char** endp, int base);
Converts initial characters (ignoring leading white space) of s to type long. If endp non-nu ll, stores pointer to unconverted suffix in *endp. If base between 2 and 36, that base used for conversion; if zero, leading (after any sign) 0X or 0x implies hexadecimal, leading 0 (after any sign) implies octal, otherwise decimal assumed. Leading 0X or 0x permitted for base hexadecimal. On overflow, sets errno to ERANGE and returns LONG_MAX or LONG_MIN (as appropriate for sign); otherwise returns converted value.
unsigned long strtoul(const char* s, char** endp, int base);
As for strtol except result is unsigned long and value on overflow is ULONG_MAX.
void* calloc(size_t nobj, size_t size);
Returns pointer to zero-initialised newly-allocated space for an array of nobj objects each of size size, or NULL on error.
void* malloc(size_t size);
Returns pointer to uninitialised newly-allocated space for an object of size size, or NULL on error.
void* realloc(void* p, size_t size);
Returns pointer to newly-allocated space for an object of size size, initialised, to minimum of old and new sizes, to existing contents of p (if non-null), or NULL on error. On success, old object deallocated, otherwise unchanged.
void free(void* p);
If p non-null, deallocates space to which it points.
void abort();
Terminates program abnormally, by calling raise(SIGABRT).
void exit(int status);
Terminates program normally. Functions installed using atexit are called (in reverse order to that in which installed), open files are flushed, open streams are closed and control is returned to environment. status is returned to environment in implementation-dependent manner. Zero or EXIT_SUCCESS indicates successful termination and EXIT_FAILURE indicates unsuccessful termination. Implementations may define other values.
int atexit(void (*fcm)(void));
Registers fcn to be called when program terminates normally (or when main returns). Returns non-zero on failure.
int system(const char* s);
If s is not NULL, passes s to environment for execution, and returns status reported by command processor; if s is NULL, non-zero returned if environment has a command processor.
char* getenv(const char* name);
Returns string associated with name name from implementation's environment, or NULL if no such string exists.
void* bsearch(const void* key, const void* base, size_t n, size_t size, int (*cmp)(const void* keyval, const void* datum));
Searches ordered array base (of n objects each of size size) for item matching key according to comparison function cmp. cmp must return negative value if first argument is less than second, zero if equal and positive if greater. Items of base are assumed to be in ascending order (according to cmp). Returns a pointer to an item matching key, or NULL if none found.
void qsort(void* base, size_t n, size_t size, int (*cmp)(const void*, const void*));
Arranges into ascending order array base (of n objects each of size size) according to comparison function cmp. cmp must return negative value if first argument is less than second, zero if equal and positive if greater.
int rand(void);
Returns pseudo-random number in range 0 to RAND_MAX.
void srand(unsigned int seed);
Uses seed as seed for new sequence of pseudo-random numbers. Initial seed is 1.

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<string.h>

NULL
Null pointer constant.
size_t
Type for objects declared to store result of sizeof operator.
char* strcpy(char* s, const char* ct);
Copies ct to s including terminating NUL and returns s.
char* strncpy(char* s, const char* ct, size_t n);
Copies at most n characters of ct to s. Pads with NUL characters if ct is of length less than n. Note that this may leave s without NUL-termination. Return s.
char* strcat(char* s, const char* ct);
Concatenate ct to s and return s.
char* strncat(char* s, const char* ct, size_t n);
Concatenate at most n characters of ct to s. NUL-terminates s and return it.
int strcmp(const char* cs, const char* ct);
Compares cs with ct, returning negative value if cs<ct, zero if cs==ct, positive value if cs>ct.
int strncmp(const char* cs, const char* ct, size_t n);
Compares at most (the first) n characters of cs and ct, returning negative value if cs<ct, zero if cs==ct, positive value if cs>ct.
int strcoll(const char* cs, const char* ct);
Compares cs with ct according to locale, returning negative value if cs<ct, zero if cs==ct, positive value if cs>ct.
char* strchr(const char* cs, int c);
Returns pointer to first occurrence of c in cs, or NULL if not found.
char* strrchr(const char* cs, int c);
Returns pointer to last occurrence of c in cs, or NULL if not found.
size_t strspn(const char* cs, const char* ct);
Returns length of prefix of cs which consists of characters which are in ct.
size_t strcspn(const char* cs, const char* ct);
Returns length of prefix of cs which consists of characters which are not in ct.
char* strpbrk(const char* cs, const char* ct);
Returns pointer to first occurrence in cs of any character of ct, or NULL if none is found.
char* strstr(const char* cs, const char* ct);
Returns pointer to first occurrence of ct within cs, or NULL if none is found.
size_t strlen(const char* cs);
Returns length of cs.
char* strerror(int n);
Returns pointer to implementation-defined message string corresponding with error n.
char* strtok(char* s, const char* t);
Searches s for next token delimited by any character from ct. Non-NULL s indicates the first call of a sequence. If a token is found, it is NUL-terminated and returned, otherwise NULL is returned. ct need not be identical for each call in a sequence.
size_t strxfrm(char* s, const char* ct, size_t n);
Stores in s no more than n characters (including terminating NUL) of a string produced from ct according to a locale-specific transformation. Returns length of entire transformed string.
void* memcpy(void* s, const void* ct, size_t n);
Copies n characters from ct to s and returns s. s may be corrupted if objects overlap.
void* memmove(void* s, const void* ct, size_t n);
Copies n characters from ct to s and returns s. s will not be corrupted if objects overlap.
int memcmp(const void* cs, const void* ct, size_t n);
Compares at most (the first) n characters of cs and ct, returning negative value if cs<ct, zero if cs==ct, positive value if cs>ct.
void* memchr(const void* cs, int c, size_t n);
Returns pointer to first occurrence of c in first n characters of cs, or NULL if not found.
void* memset(void* s, int c, size_t n);
Replaces each of the first n characters of s by c and returns s.

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<time.h>

CLOCKS_PER_SEC
The number of clock_t units per second.
NULL
Null pointer constant.
clock_t
An arithmetic type elapsed processor representing time.
time_t
An arithmetic type representing calendar time.
struct tm
Represents the components of calendar time:
int tm_sec;
seconds after the minute
int tm_min;
minutes after the hour
int tm_hour;
hours since midnight
int tm_mday;
day of the month
int tm_mon;
months since January
int tm_year;
years since 1900
int tm_wday;
days since Sunday
int tm_yday;
days since January 1
int tm_isdst;
Daylight Saving Time flag : is positive if DST is in effect, zero if not in effect, negative if information not known.
Implementations may change field order and include additional fields.
clock_t clock(void);
Returns elapsed processor time used by program or -1 if not available.
time_t time(time_t* tp);
Returns current calendar time or -1 if not available. If tp is non-NULL, return value is also assigned to *tp.
double difftime(time_t time2, time_t time1);
Returns the difference in seconds between time2 and time1.
time_t mktime(struct tm* tp);
If necessary, adjusts fields of *tp to fall withing normal ranges. Returns the corresponding calendar time, or -1 if it cannot be represented.
char* asctime(const struct tm* tp);
Returns the given time as a string of the form:
Sun Jan 3 13:08:42 1988\n\0
char* ctime(const time_t* tp);
Returns string equivalent to calendar time tp converted to local time. Equivalent to:
asctime(localtime(tp))
struct tm* gmtime(const time_t* tp);
Returns calendar time *tp converted to Coordinated Universal Time, or NULL if not available.
struct tm* localtime(const time_t* tp);
Returns calendar time *tp converted into local time.
size_t strftime(char* s, size_t smax, const char* fmt, const struct tm* tp);
Formats *tp into s according to fmt. Places no more than smax characters into s, and returns number of characters produced (excluding terminating NUL), or 0 if greater than smax. Formatting conversions (%c) are:
A
name of weekday
a
abbreviated name of weekday
B
name of month
b
abbreviated name of month
c
local date and time representation
d
day of month [01-31]
H
hour (24-hour clock) [00-23]
I
hour (12-hour clock) [01-12]
j
day of year [001-366]
M
minute [00-59]
m
month [01-12]
p
local equivalent of "AM" or "PM"
S
second [00-61]
U
week number of year (Sunday as 1st day of week) [00-53]
W
week number of year (Monday as 1st day of week) [00-53]
w
weekday (Sunday as 0) [0-6]
X
local time representation
x
local date representation
Y
year with century
y
year without century [00-99]
Z
name (if any) of time zone
%
%

Local time may differ from calendar time because of time zone.

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