succinct reference for the most used parts of C11 Standard Library, everything except time.h fits on one printed page. NOT meant to be comprehensive, but a reference for learning.

clib_short_notes.c

/* use $ man 3 <function> */
#include <stdlib.h>
void *calloc(size_t nitems, size_t size);
void *malloc(size_t size);
void *realloc(void *ptr, size_t size);
void free(void *ptr);
int rand(void); /* [0, RAND_MAX), use random() or arc4random() instead */
void srand(unsigned int seed);

#include <math.h>
double exp(double x);
double log(double x);
double pow(double x, double y);
double sqrt(double x);
double ceil(double x);
double fabs(double x);
double floor(double x);
double fmod(double x, double y);

#include <stdarg.h>
typedef va_list;
void va_start(va_list ap, last_arg); /* must call to start */
type va_arg(va_list ap, type);       /* get next arg */
void va_end(va_list ap);             /* must call to end */

#include <stddef.h>
#define NULL 0
typedef size_t

#include <stdio.h>
int printf(const char *format, ...); /* prefix: f=file, s=string, v=var arg */
int scanf(const char *format, ...); /* prefix: f=file, s=string */
FILE *fopen(const char *filename, const char *mode);
char *fgets(char *s, int n, FILE *stream);
int fputs(const char *s, FILE *stream);

#include <string.h>
size_t strlen(const char *s);

/* copies & moves are to dst from src: dst<-src */
void *memchr(const void *s, int c, size_t n); /* find c in s within n */
void *memcpy(void *dst, const void *src, size_t n);  /* faster */
void *memmove(void *dst, const void *src, size_t n); /* overlap safe */
void *memset(void *s, int c, size_t n); /* put c in s n times */
char *strncat(char *dst, const char *src, size_t n); /* append dst += src */
char *strcpy(char *dst, const char *src);
char *strncpy(char *dst, const char *src, size_t n);

/* compare. result 0 means ==; <0 means s1 < s2; >0 means s1 > s2 */
int memcmp(const void *s1, const void *s2, size_t n);
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);

/* searching (t = text, p = pattern) */
void *memchr(const void *s, int c, size_t n);
char *strchr(const char *s, int c);  /* find c in s */
char *strrchr(const char *s, int c); /* reverse search */
char *strpbrk(const char *t, const char *p); /* find s2 in s1 */
size_t strcspn(const char *t, const char *p); /* "complementary span" */
size_t strspn(const char *t, const char *p);
char *strtok(char *t, const char *p); /* tokenized search */




#include <time.h>
typedef clock_t; /* relative time, like a stopwatch "stop clock" */
typedef time_t;  /* absolute time, like a calendar, "real time" */
struct tm {      /* time struct, used to make human-readable */
  int tm_sec;    /* seconds after the minute (0 to 61, for leap secs) */
  int tm_min;    /* minutes after the hour (0 to 59) */
  int tm_hour;   /* hours since midnight (0 to 23) */
  int tm_mday;   /* day of the month (1 to 31) */
  int tm_mon;    /* months since January (0 to 11) NOTICE! 0-indexed months! */
  int tm_year;   /* years since 1900 */
  int tm_wday;   /* days since Sunday (0 to 6 Sunday=0) */
  int tm_yday;   /* days since January 1 (0 to 365) */
  int tm_isdst;  /* Daylight Savings Time (0 if not DST) */
};
/* create */
clock_t clock(void);
time_t time(time_t *timer); /* either call w/ 0 & use return OR with &time */
double difftime(time_t time1, time_t time2); /* t1 - t2 in seconds */
/* convert types */
struct tm *gmtime(const time_t *timer);    /* UTC, time_t to struct tm */
struct tm *localtime(const time_t *timer); /* local, time_t to struct tm */
time_t mktime(struct tm *timeptr); /* struct tm to time_t */
/* to/from strings */
char *asctime(const struct tm *timeptr);
char *ctime(const time_t *timer);
size_t strftime(char *str, size_t max, const char *fmt, const struct tm *tptr);