C Cheat Sheet

C Standard Language Cheat Sheet: Built-In C Methods With Descriptions

Input & Output Functions

int fclose(FILE *stream)

Closes the stream. All buffers are flushed

void clearerr(FILE *stream)

Clears the end-of-file and error indicators for the given stream

int feof(FILE *stream)

Tests the end-of-file indicator for the given stream

int ferror(FILE *stream)

Tests the error indicator for the given stream

int fflush(FILE *stream)

Flushes the output buffer of a stream

int fgetpos(FILE *stream, fpos_t *pos)

Gets the current file position of the stream and writes it to pos

FILE *fopen(const char *filename, const char *mode)

Opens the filename pointed to by filename using the given mode

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)

Reads data from the given stream into the array pointed to by ptr

FILE *freopen(const char *filename, const char *mode, FILE *stream)

Associates a new filename with the given open stream and same time closing the old file in stream

int fseek(FILE *stream, long int offset, int whence)

Sets the file position of the stream to the given offset. The argument offset signifies the number of bytes to seek from the given whence position

int fsetpos(FILE *stream, const fpos_t *pos)

Sets the file position of the given stream to the given position. The argument pos is a position given by the function fgetpos

long int ftell(FILE *stream)

Returns the current file position of the given stream

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

Writes data from the array pointed to by ptr to the given stream

int remove(const char *filename)

Deletes the given filename so that it is no longer accessible

int rename(const char *old_filename, const char *new_filename)

Causes the filename referred to, by old_filename to be changed to new_filename

void rewind(FILE *stream)

Sets the file position to the beginning of the file of the given stream

void setbuf(FILE *stream, char *buffer)

Defines how a stream should be buffered

int setvbuf(FILE *stream, char *buffer, int mode, size_t size)

Another function to define how a stream should be buffered

FILE *tmpfile(void)

Creates a temporary file in binary update mode (wb+)

char *tmpnam(char *str)

Generates and returns a valid temporary filename which does not exist

int fprintf(FILE *stream, const char *format, ...)

Sends formatted output to a stream

int printf(const char *format, ...)

Sends formatted output to stdout

int sprintf(char *str, const char *format, ...)

Sends formatted output to a string

int vfprintf(FILE *stream, const char *format, va_list arg)

Sends formatted output to a stream using an argument list

int vprintf(const char *format, va_list arg)

Sends formatted output to stdout using an argument list

int vsprintf(char *str, const char *format, va_list arg)

Sends formatted output to a string using an argument list

int fscanf(FILE *stream, const char *format, ...)

Reads formatted input from a stream

int scanf(const char *format, ...)

Reads formatted input from stdin

int sscanf(const char *str, const char *format, ...)

Reads formatted input from a string

int fgetc(FILE *stream)

Gets the next character (an unsigned char) from the specified stream and advances the position indicator for the stream

char *fgets(char *str, int n, FILE *stream)

Reads a line from the specified stream and stores it into the string pointed to by str. It stops when either (n-1) characters are read, the newline character is read, or the end-of-file is reached, whichever comes first

int fputc(int char, FILE *stream)

Writes a character (an unsigned char) specified by the argument char to the specified stream and advances the position indicator for the stream

int fputs(const char *str, FILE *stream)

Writes a string to the specified stream up to but not including the null character

int getc(FILE *stream)

Gets the next character (an unsigned char) from the specified stream and advances the position indicator for the stream

int getchar(void)

Gets a character (an unsigned char) from stdin

char *gets(char *str)

Reads a line from stdin and stores it into the string pointed to by, str. It stops when either the newline character is read or when the end-of-file is reached, whichever comes first

int putc(int char, FILE *stream)

Writes a character (an unsigned char) specified by the argument char to the specified stream and advances the position indicator for the stream

int putchar(int char)

Writes a character (an unsigned char) specified by the argument char to stdout

int puts(const char *str)

Writes a string to stdout up to but not including the null character. A newline character is appended to the output

int ungetc(int char, FILE *stream)

Pushes the character char (an unsigned char) onto the specified stream so that the next character is read

void perror(const char *str)

Prints a descriptive error message to stderr. First the string str is printed followed by a colon and then a space

Character Functions

isdigit()

Returns non-0 if the argument is a digit between 0 and 9

isalpha()

Returns a non-0 value if the argument is a letter of the alphabet

isalnum()

Returns a non-0 value if the argument is a letter or digit

islower()

Returns non-0 if the argument is a lowercase letter

isupper()

Returns a non-0 value if the argument is an uppercase letter

Location Functions

char *setlocale(int category, const char *locale) Sets or reads location dependent information

struct lconv *localeconv(void) Sets or reads location dependent information

Mathematics Functions

acos(x) Returns the arc cosine of x in radians

asin(x) Returns the arc sine of x in radians

atan(x) Returns the arc tangent of x in radians

atan2(y, x) Returns the arc tangent in radians of y/x based on the signs of both values to determine the correct quadrant

cos(x) Returns the cosine of a radian angle x

cosh(x) Returns the hyperbolic cosine of x

sin(x) Returns the sine of a radian angle x

sinh(x) Returns the hyperbolic sine of x

tanh(x) Returns the hyperbolic tangent of x

exp(x) Returns the value of e raised to the xth power

frexp(x, int *exponent) The returned value is the mantissa and the integer pointed to by exponent is the exponent. The resultant value is x = mantissa * 2 ^ exponent

ldexp(x, int exponent) Returns x multiplied by 2 raised to the power of exponent

log(x) Returns the natural logarithm (base-e logarithm) of x

log10(x) Returns the common logarithm (base-10 logarithm) of x

modf(x, *integer) The returned value is the fraction component (part after the decimal), and sets integer to the integer component

pow(x, y) Returns x raised to the power of y

sqrt(x) Returns the square root of x

ceil(x) Returns the smallest integer value greater than or equal to x

fabs(x) Returns the absolute value of x

floor(x) Returns the largest integer value less than or equal to x

fmod(x, y) Returns the remainder of x divided by y

Time & Date Functions

char *asctime(const struct tm *timeptr)

Returns a pointer to a string which represents the day and time of the structure timeptr

clock_t clock(void)

Returns the processor clock time used since the beginning of an implementation defined era (normally the beginning of the program)

char *ctime(const time_t *timer)

Returns a string representing the localtime based on the argument timer

double difftime(time_t time1, time_t time2)

Returns the difference of seconds between time1 and time2 (time1-time2)

struct tm *gmtime(const time_t *timer)

The value of timer is broken up into the structure tm and expressed in Coordinated Universal Time (UTC) also known as Greenwich Mean Time (GMT)

struct tm *localtime(const time_t *timer)

The value of timer is broken up into the structure tm and expressed in the local time zone

time_t mktime(struct tm *timeptr)

Converts the structure pointed to by timeptr into a time_t value according to the local time zone

size_t strftime(char *str, size_t maxsize, const char *format, const struct tm *timeptr)

Formats the time represented in the structure timeptr according to the formatting rules defined in format and stored into str

time_t time(time_t *timer)

Calculates the current calender time and encodes it into time_t format

Miscellaneous Functions

void longjmp(jmp_buf environment, int value)

Restores the environment saved by the most recent call to setjmp() macro in the same invocation of the program with the corresponding jmp_buf argument

void (*signal(int sig, void (*func)(int)))(int)

Sets a function to handle signal i.e. a signal handler

int raise(int sig)

Causes signal to be generated. The sig argument is compatible with the SIG macros

malloc()

Provides dynamic memory allocation

rand()

Generates a random number

srand()

Used to set the starting point for rand()

Type Functions

int isalnum(int c) Checks whether the passed character is alphanumeric
int isalpha(int c) Checks whether the passed character is alphabetic
int iscntrl(int c) Checks whether the passed character is control character
int isdigit(int c) Checks whether the passed character is decimal digit
int isgraph(int c) Checks whether the passed character has graphical representation using locale
int islower(int c) Checks whether the passed character is lowercase letter
int isprint(int c) Checks whether the passed character is printable
int ispunct(int c) Checks whether the passed character is a punctuation character
int isspace(int c) Checks whether the passed character is white-space
int isupper(int c) Checks whether the passed character is an uppercase letter
int isxdigit(int c) Checks whether the passed character is a hexadecimal digit
int tolower(int c) Converts uppercase letters to lowercase

General Functions

double atof(const char *str)

Converts the string pointed to, by the argument str to a floating-point number (type double)

int atoi(const char *str)

Converts the string pointed to, by the argument str to an integer (type int)

long int atol(const char *str)

Converts the string pointed to, by the argument str to a long integer (type long int)

double strtod(const char *str, char **endptr)

Converts the string pointed to, by the argument str to a floating-point number (type double)

long int strtol(const char *str, char **endptr, int base)

Converts the string pointed to, by the argument str to a long integer (type long int)

unsigned long int strtoul(const char *str, char **endptr, int base)

Converts the string pointed to, by the argument str to an unsigned long integer (type unsigned long int)

void *calloc(size_t nitems, size_t size)

Allocates the requested memory and returns a pointer to it

void free(void *ptr)

Deallocates the memory previously allocated by a call to calloc, malloc, or realloc

void *malloc(size_t size)

Allocates the requested memory and returns a pointer to it

void *realloc(void *ptr, size_t size)

Attempts to resize the memory block pointed to by ptr that was previously allocated with a call to malloc or calloc

void abort(void)

Causes an abnormal program termination

int atexit(void (*func)(void))

Causes the specified function func to be called when the program terminates normally

void exit(int status)

Causes the program to terminate normally

char *getenv(const char *name)

Searches for the environment string pointed to by name and returns the associated value to the string

int system(const char *string)

The command specified by string is passed to the host environment to be executed by the command processor

void *bsearch(const void *key, const void *base, size_t nitems, size_t size, int (*compar)(const void *, const void *))

Performs a binary search

void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void*))

Sorts an array

int abs(int x)

Returns the absolute value of x

div_t div(int numer, int denom)

Divides numer (numerator) by denom (denominator)

long int labs(long int x)

Returns the absolute value of x

ldiv_t ldiv(long int numer, long int denom)

Divides numer (numerator) by denom (denominator)

int rand(void)

Returns a pseudo-random number in the range of 0 to RAND_MAX

void srand(unsigned int seed)

This function seeds the random number generator used by the function rand

int mblen(const char *str, size_t n)

Returns the length of a multibyte character pointed to by the argument str

size_t mbstowcs(schar_t *pwcs, const char *str, size_t n)

Converts the string of multibyte characters pointed to by the argument str to the array pointed to by pwcs

int mbtowc(whcar_t *pwc, const char *str, size_t n)

Examines the multibyte character pointed to by the argument str

size_t wcstombs(char *str, const wchar_t *pwcs, size_t n)

Converts the codes stored in the array pwcs to multibyte characters and stores them in the string str

int wctomb(char *str, wchar_t wchar)

Examines the code which corresponds to a multibyte character given by the argument wchar

String Functions

strcat()

Concatenates a copy of string2 to string1

strcmp()

Compares two strings

strcpy()

Copies contents of string2 to string1

void *memchr(const void *str, int c, size_t n)

Searches for the first occurrence of the character c (an unsigned char) in the first n bytes of the string pointed to, by the argument str

int memcmp(const void *str1, const void *str2, size_t n)

Compares the first n bytes of string1 and string2

void *memcpy(void *dest, const void *src, size_t n)

Copies n characters from src to dest

void *memmove(void *dest, const void *src, size_t n)

Another function to copy n characters from string2 to string1

void *memset(void *str, int c, size_t n)

Copies the character c (an unsigned char) to the first n characters of the string pointed to, by the argument str

char *strcat(char *dest, const char *src)

Appends the string pointed to, by src to the end of the string pointed to by dest

char *strncat(char *dest, const char *src, size_t n)

Appends the string pointed to, by src to the end of the string pointed to, by dest up to n characters long

char *strchr(const char *str, int c)

Searches for the first occurrence of the character c (an unsigned char) in the string pointed to, by the argument str

int strcmp(const char *str1, const char *str2)

Compares the string pointed to, by string1 to the string pointed to by string2

int strncmp(const char *str1, const char *str2, size_t n)

Compares at most the first n bytes of string1 and string2

int strcoll(const char *str1, const char *str2)

Compares string str1 to str2. The result is dependent on the LC_COLLATE setting of the location

char *strcpy(char *dest, const char *src)

Copies the string pointed to, by src to dest

char *strncpy(char *dest, const char *src, size_t n)

Copies up to n characters from the string pointed to, by src to dest

size_t strcspn(const char *str1, const char *str2)

Calculates the length of the initial segment of string1 which consists entirely of characters not in string2

char *strerror(int errnum)

Searches an internal array for the error number errnum and returns a pointer to an error message string

size_t strlen(const char *str)

Computes the length of the string str up to but not including the terminating null character

char *strpbrk(const char *str1, const char *str2)

Finds the first character in the string string1 that matches any character specified in string2

char *strrchr(const char *str, int c)

Searches for the last occurrence of the character c (an unsigned char) in the string pointed to by the argument str

size_t strspn(const char *str1, const char *str2)

Calculates the length of the initial segment of string1 which consists entirely of characters in string2

char *strstr(const char *haystack, const char *needle)

Finds the first occurrence of the entire string needle (not including the terminating null character) which appears in the string haystack

char *strtok(char *str, const char *delim)

Breaks string str into a series of tokens separated by delim

size_t strxfrm(char *dest, const char *src, size_t n)

Transforms the first n characters of the string src into current locale and places them in the string dest