C Library Functions

ctype.h
int isalnum(int n) Returns a nonzero value if n is a letter of a decimal digit.
int isalpha(int n) Returns a nonzero value if n is a letter.
int iscntrl(int n) Returns a nonzero value if n is a delete character or an ordinary control character.
int isdigit(int n) Returns a nonzero value if n is a decimal digit.
int isgraph(int n) Returns a nonzero value if n is a printing character other than a space.
int islower(int n) Returns a nonzero value if n is a lower case letter.
int isprint(int n) Returns a nonzero value if n is a printing character.
int ispunct(int n) Returns a nonzero value if n is a punctuation character.
int isspace(int n) Returns a nonzero value if n is a space, tab, carriage return, new line, vertical tab, or form feed character.
int isupper(int n) Returns a nonzero value if n is an upper case letter.
int isxdigit(int n) Returns a nonzero value if n is a hexadecimal digit.
int tolower(int n) Returns the coresponding lower-case letter if n is an upper-case letter. Otherwise, n is returned unchanged.
int toupper(int n) Returns the coresponding upper-case letter if n is a lower-case letter. Otherwise, n is returned unchanged.

math.h
double acos(double x) Returns the arc cosine in radians of x (x must be in the range -1 to 1).
double asin(double x) Returns the arc sine in radians of x (x must be in the range -1 to 1).
double atan(double x) Returns the arc tangent in radians of x.
double atan2(double x1, double x2) Returns the arc tangent in radians of x1 / x2.
double atof(const char *s) Returns the double value represented in the character string s.
double ceil(double x) Returns the minimum integer (as a double) greater than or equal to x.
double cos(double x) Returns the cosine of x (x in radians).
double cosh(double x) Returns the hyperbolic cosine of x.
double exp(double x) Returns e to the power of x.
double fabs(double x) Returns the absolute value of x.
double floor(double x) Returns the maximum integer (as a double) less than or equal to x.
double fmod(double x1, double x2) Returns x1 modulo x2.
double frexp(double x, int *n) Returns the base-2 mantissa of x, and stores the exponent in n.
long labs(long n) Returns the absolute value of n.
double ldexp(double x, int n) Returns x times 2 to the power of n.
double log(double x) Returns the natural logarithm of x.
double log10(double x) Returns the base-10 logarithm of x.
double frexp(double x, double *n) Returns the fractional part of x, and stores the integer part in n.
double pow(double x1, double x2) Returns x1 to the power of x2.
double sin(double x) Returns the sine of x (x in radians).
double sinh(double x) Returns the hyperbolic sine of x.
double sqrt(double x) Returns the square root x.
double tan(double x) Returns the tangent of x (x in radians).
double tanh(double x) Returns the hyperbolic tangent of x.

stdlib.h
void abort() Terminates the program and displays an error message.
int abs(int x) Returns the absolute value of x.
int atexit(void (*func)(void)) Sets the function, pointed to by func, to be called upon normal program termination. Returns zero if successful, or a nonzero value if unsuccessful.
int atoi(const char *s) Returns the integer represented in the string array s.
long atol(const char *s) Returns the long integer represented in the string array s.
void *calloc(size_t n1, size_t n2) Allocates in memory an array of n1 blocks of size n2 bytes. If successful, a pointer to the beginning of the array is returned. Otherwise, NULL is returned.
div_t div(int n1, int n2) Divides n1 by n2, and returns the quotient and remainder in a div_t structure (components are quot and rem).
void exit(int s) Terminates a program with the status value s.
void free(void *p) Deallocates the memory area pointed to by p that was previously allocated by a call to calloc, malloc or realloc.
char *getenv(const char *s) Returns the string array associated with the environment variable named in string array s. If unsuccessful, NULL is returned
ldiv_t ldiv(long n1, long n2) Divides n1 by n2, and returns the quotient and remainder in an ldiv_t structure (components are quot and rem).
void *malloc(size_t n) Allocates in memory an array of size n bytes. Returns a pointer to the beginning of the array if successful, or NULL if unsuccessful.
void qsort(void *p, size_t n, size_t s, int (*func)(const void*, const void*)) Sorts the array pointed to by pointer p, with n blocks of size s, using the "quicksort" algorithm based on the function pointed to by func.
int rand(void) Returns the next pseudorandom number.
void *realloc(void *p, size_t n) Allocates in memory an array of size n bytes and copies up to n bytes from the array pointed to by p to the new array. Returns a pointer to the beginning of the new array, or NULL if unsuccessful.
void srand(unsigned int s) Initializes the pseudorandom number generator using the seed s.
double strtod(const char *s, char **p) Returns the double value represented in the string array s. If p is not NULL, a pointer to the first character in s that terminated the evaluation is returned in *p.
long strtol(const char *s, char **p, int b) Returns the long integer value represented in the string array s in the numerical base b. If p is not NULL, a pointer to the first character in s that terminated the evaluation is returned in *p.
long strtoul(const char *s, char **p, int b) Returns the unsigned long integer value represented in the string array s in the numerical base b. If p is not NULL, a pointer to the first character in s that terminated the evaluation is returned in *p.
int system(const char *s) Executes the operating system command in the string array s.

stdio.h
void clearerr(FILE *f) Sets the error and end-of-file indecators of the file f to zero.
int fclose(FILE *f) Closes the file f. Returns a nonzero value if successful, or zero if unsuccessful.
int feof(FILE *f) Returns a nonzero value if end-of-file is detected in file f. Otherwise zero is returned
int ferror(FILE *f) Returns a nonzero value if an error is detected in file f. Otherwise zero is returned.
int fflush(FILE *f) Flushes file f and writes any remaining characters to the associated file. Returns a nonzero value if successful, or zero if unsuccessful.
int fget(FILE *f) Returns the next available character in file f, or EOF if unsuccessful.
int fgetpos(FILE *f, fpos_t *p) Stores in p the file pointer in file f. Returns a nonzero value if successful, or zero if unsuccessful.
char *fgets(char *s, int n, FILE *f) Stores in the character array s the next n-1 characters in file f, or to a new line character if one is encountered. Returns the pointer s, or NULL of unsuccessful.
FILE *fopen(const char *s, const char *m) Opens the file named in the character string s, using the access mode specified in the character string m. Returns a pointer to the FILE struct, or NULL if the open was unsuccessful.
int fprintf(FILE *f, const char *s [, args]) Writes to file f using the format string s along with any arguments (args) listed. Returns the number of characters that were written.
int fputc(int c, FILE *f) Writes to file f the character c. Returns the character c if successful, or EOF if unsuccessful.
int fputs(const char *s, FILE *f) Writes to file f the character string s. Returns the zero if successful, or a nonzero value if unsuccessful.
size_t fread(void *p, size_t n1, size_t n2, FILE *f) Reads from file f up to n2 blocks of size n1 bytes, and stores them in the array beginning at pointer p. Returns the number of if successful, or EOF if unsuccessful.
FILE *freopen(char *s, char *m, FILE *f) Closes the file associated with f, if it exists and is open, and reassociates f with the file named in the character string s. The file is opened using the access mode specified in the character string m. Returns a pointer to the FILE struct, or NULL if the open was unsuccessful.
int fscanf(FILE *f, const char *s [, ptrs]) Reads from file f using the format string s along with any variable pointers (ptrs) listed. Returns the number of characters that were read.
int fseek(FILE *f, long p, int m) Moves the position of the file pointer in file f a distance of p bytes relative to the starting point indicated by mode m (SEEK_SET = from beginning of file, SEEK_CUR = from current pointer position, SEEK_END = from end of file). Returns a nonzero value if successful, or zero if unsuccessful.
int fsetpos(FILE *f, long p) Moves the position of the file pointer in file f to byte position p. Returns a nonzero value if successful, or zero if unsuccessful.
long ftell(FILE *f) Returns the current byte position of the file pointer in file f, or -1 if unsuccessful.
int fwrite(void *p, size_t n1, size_t n2, FILE *f) Reads from file f up to n2 blocks of size n1 bytes, and stores them in the array beginning at pointer p. Returns the character c if successful, or EOF if unsuccessful.
int getc(FILE *f) Returns as an integer the ASCII code of the next character in file f.
int getchar() Returns as an integer the ASCII code of the next available character entered at the keyboard (stdin).
char *gets(char *s) Reads a string of characters from the keyboard (stdin), up to, but not including a newline character, and stores them in the string array s. Returns a pointer to s, or NULL if unsuccessful.
int printf(const char *s [, args]) Writes to standard output using the format string s along with any arguments (args) listed. Returns the number of characters that were written.
int putc(char c, FILE *f) Writes to file f the character c. Returns c if successful, or EOF if unsuccessful.
int putchar(char c) Writes to standard output the character c. Returns c if successful, or EOF if unsuccessful.
int puts(const char *s) Writes to standard output the string array s. Returns zero if successful, or a nonzero value if unsuccessful.
int remove(const char *s) Deletes the file specified in the string array s. Returns zero if successful, or a nonzero value if unsuccessful.
int rename(const char *s1, const char *s2) Renames the file specified in the string array s1 to the name specified in the string array s2. Returns zero if successful, or a nonzero value if unsuccessful.
int rewind(FILE *f) Sets the file pointer in file f to the beginning of the file. Returns zero if successful, or a nonzero value if unsuccessful.
int scanf(const char *s [, ptrs]) Reads from standard input using the format string s along with any variable pointers (ptrs) listed. Returns the number of characters that were read.

fopen access modes
stringreadwritecreatereplaceappend
ryesnononono
wnoyesyesyesno
anonoyesnoyes
r+yesyesnonono
w+yesyesyesyesno
a+yesnoyesnoyes