This document provides a summary of key C programming concepts in 3 sentences or less: The document is a reference card that summarizes C language syntax and standard libraries, including data types, operators, flow control statements, functions, and preprocessor directives. It covers program structure, variables and declarations, input/output functions, mathematical functions, and more. Standard libraries are also summarized, such as string, time, math, and variable argument functions.

1. C Reference Card (ANSI) Constants Flow of Control
long (suffix) L or l statement terminator ;
Program Structure/Functions float (suffix) F or f block delimeters { }
type fnc(type 1 ,. . . ) function declarations exponential form e exit from switch, while, do, for break
type name external variable declarations octal (prefix zero) 0 next iteration of while, do, for continue
main() { main routine hexadecimal (prefix zero-ex) 0x or 0X go to goto label
declarations local variable declarations character constant (char, octal, hex) 'a', 'ooo', 'xhh' label label :
statements newline, cr, tab, backspace n, r, t, b return value from function return expr
} special characters , ?, ', " Flow Constructions
type fnc(arg 1 ,. . . ) { function definition string constant (ends with '0') "abc. . . de" if statement if (expr ) statement
declarations local variable declarations else if (expr ) statement
statements Pointers, Arrays & Structures else statement
return value; declare pointer to type type *name while statement while (expr )
statement
} declare function returning pointer to type type *f()
comments for statement for (expr 1 ; expr 2 ; expr 3 )
/* */ declare pointer to function returning type type (*pf)() statement
main(int argc, char *argv[]) main with args generic pointer type void * do statement do statement
exit(arg ) terminate execution null pointer NULL while(expr );
object pointed to by pointer *pointer
C Preprocessor address of object name &name
switch statement switch (expr ) {
case const 1 : statement 1 break;
include library file #include <filename> array name[dim ] case const 2 : statement 2 break;
include user file #include "filename" multi-dim array name[dim 1 ][dim 2 ]. . . default: statement
replacement text #define name text Structures }
replacement macro #define name(var ) text struct tag { structure template
Example. #define max(A,B) ((A)>(B) ? (A) : (B)) declarations declaration of members ANSI Standard Libraries
undefine #undef name }; <assert.h> <ctype.h> <errno.h> <float.h> <limits.h>
quoted string in replace # create structure struct tag name <locale.h> <math.h> <setjmp.h> <signal.h> <stdarg.h>
concatenate args and rescan ## member of structure from template name.member <stddef.h> <stdio.h> <stdlib.h> <string.h> <time.h>
conditional execution #if, #else, #elif, #endif member of pointed to structure pointer -> member
is name defined, not defined? #ifdef, #ifndef Example. (*p).x and p->x are the same
Character Class Tests <ctype.h>
name defined? defined(name ) single value, multiple type structure union alphanumeric? isalnum(c)
line continuation char bit field with b bits member : b alphabetic? isalpha(c)
control character? iscntrl(c)
Data Types/Declarations Operators (grouped by precedence) decimal digit? isdigit(c)
character (1 byte) char structure member operator name.member printing character (not incl space)? isgraph(c)
integer int structure pointer pointer ->member lower case letter? islower(c)
float (single precision) float printing character (incl space)? isprint(c)
increment, decrement ++, --
float (double precision) double printing char except space, letter, digit? ispunct(c)
plus, minus, logical not, bitwise not +, -, !, ~
short (16 bit integer) short space, formfeed, newline, cr, tab, vtab? isspace(c)
indirection via pointer, address of object *pointer , &name
long (32 bit integer) long upper case letter? isupper(c)
cast expression to type (type) expr
positive and negative signed hexadecimal digit? isxdigit(c)
size of an object sizeof
only positive unsigned convert to lower case? tolower(c)
pointer to int, float,. . . *int, *float,. . . multiply, divide, modulus (remainder) *, /, % convert to upper case? toupper(c)
enumeration constant enum add, subtract +, -
constant (unchanging) value const String Operations <string.h>
declare external variable extern left, right shift [bit ops] <<, >> s,t are strings, cs,ct are constant strings
register variable register comparisons >, >=, <, <= length of s strlen(s)
local to source file static comparisons ==, != copy ct to s strcpy(s,ct)
no value void up to n chars strncpy(s,ct,n)
bitwise and &
structure struct concatenate ct after s strcat(s,ct)
create name by data type typedef typename bitwise exclusive or ^
up to n chars strncat(s,ct,n)
size of an object (type is size_t) sizeof object bitwise or (incl) | compare cs to ct strcmp(cs,ct)
size of a data type (type is size_t) sizeof(type name) logical and && only first n chars strncmp(cs,ct,n)
pointer to first c in cs strchr(cs,c)
Initialization logical or ||
pointer to last c in cs strrchr(cs,c)
initialize variable type name=value conditional expression expr 1 ? expr 2 : expr 3
copy n chars from ct to s memcpy(s,ct,n)
initialize array type name[]={value 1 ,. . . } assignment operators +=, -=, *=, . . . copy n chars from ct to s (may overlap) memmove(s,ct,n)
initialize char string char name[]="string " expression evaluation separator , compare n chars of cs with ct memcmp(cs,ct,n)
pointer to first c in first n chars of cs memchr(cs,c,n)
Unary operators, conditional expression and assignment oper-
put c into first n chars of cs memset(s,c,n)
ators group right to left; all others group left to right.
c 1999 Joseph H. Silverman Permissions on back. v1.3
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2. C Reference Card (ANSI) Standard Utility Functions <stdlib.h> Integer Type Limits <limits.h>
absolute value of int n abs(n) The numbers given in parentheses are typical values for the
Input/Output <stdio.h> absolute value of long n labs(n) constants on a 32-bit Unix system.
quotient and remainder of ints n,d div(n,d) CHAR_BIT bits in char (8)
Standard I/O
retursn structure with div_t.quot and div_t.rem CHAR_MAX max value of char (127 or 255)
standard input stream stdin
quotient and remainder of longs n,d ldiv(n,d) CHAR_MIN min value of char (−128 or 0)
standard output stream stdout
returns structure with ldiv_t.quot and ldiv_t.rem INT_MAX max value of int (+32,767)
standard error stream stderr
pseudo-random integer [0,RAND_MAX] rand() INT_MIN min value of int (−32,768)
end of file EOF
set random seed to n srand(n) LONG_MAX max value of long (+2,147,483,647)
get a character getchar()
terminate program execution exit(status) LONG_MIN min value of long (−2,147,483,648)
print a character putchar(chr )
pass string s to system for execution system(s) SCHAR_MAX max value of signed char (+127)
print formatted data printf("format ",arg 1 ,. . . )
Conversions SCHAR_MIN min value of signed char (−128)
print to string s sprintf(s,"format ",arg 1 ,. . . )
convert string s to double atof(s) SHRT_MAX max value of short (+32,767)
read formatted data scanf("format ",&name 1 ,. . . )
convert string s to integer atoi(s) SHRT_MIN min value of short (−32,768)
read from string s sscanf(s,"format ",&name 1 ,. . . )
convert string s to long atol(s) UCHAR_MAX max value of unsigned char (255)
read line to string s (< max chars) gets(s,max)
convert prefix of s to double strtod(s,endp) UINT_MAX max value of unsigned int (65,535)
print string s puts(s)
convert prefix of s (base b) to long strtol(s,endp,b) ULONG_MAX max value of unsigned long (4,294,967,295)
File I/O
same, but unsigned long strtoul(s,endp,b) USHRT_MAX max value of unsigned short (65,536)
declare file pointer FILE *fp
Storage Allocation
pointer to named file fopen("name ","mode ")
allocate storage malloc(size), calloc(nobj,size) Float Type Limits <float.h>
modes: r (read), w (write), a (append) FLT_RADIX radix of exponent rep (2)
change size of object realloc(pts,size)
get a character getc(fp) FLT_ROUNDS floating point rounding mode
deallocate space free(ptr)
write a character putc(chr ,fp) FLT_DIG decimal digits of precision (6)
Array Functions
write to file fprintf(fp,"format ",arg 1 ,. . . )
search array for key bsearch(key,array,n,size,cmp()) FLT_EPSILON smallest x so 1.0 + x = 1.0 (10−5 )
read from file fscanf(fp,"format ",arg 1 ,. . . )
sort array ascending order qsort(array,n,size,cmp()) FLT_MANT_DIG number of digits in mantissa
close file fclose(fp)
FLT_MAX maximum floating point number (1037 )
non-zero if error ferror(fp) Time and Date Functions <time.h> FLT_MAX_EXP maximum exponent
non-zero if EOF feof(fp)
processor time used by program clock() FLT_MIN minimum floating point number (10−37 )
read line to string s (< max chars) fgets(s,max,fp )
Example. clock()/CLOCKS_PER_SEC is time in seconds FLT_MIN_EXP minimum exponent
write string s fputs(s,fp )
current calendar time time() DBL_DIG decimal digits of precision (10)
Codes for Formatted I/O: "%-+ 0w.pmc"
- left justify time2 -time1 in seconds (double) difftime(time2 ,time1 ) DBL_EPSILON smallest x so 1.0 + x = 1.0 (10−9 )
+ print with sign arithmetic types representing times clock_t,time_t DBL_MANT_DIG number of digits in mantissa
space print space if no sign structure type for calendar time comps tm DBL_MAX max double floating point number (1037 )
0 pad with leading zeros tm_sec seconds after minute DBL_MAX_EXP maximum exponent
w min field width tm_min minutes after hour DBL_MIN min double floating point number (10−37 )
p precision tm_hour hours since midnight DBL_MIN_EXP minimum exponent
m conversion character: tm_mday day of month
h short, l long, L long double tm_mon months since January
c conversion character: tm_year years since 1900
d,i integer u unsigned tm_wday days since Sunday
c single char s char string tm_yday days since January 1
f double e,E exponential tm_isdst Daylight Savings Time flag
o octal x,X hexadecimal convert local time to calendar time mktime(tp)
p pointer n number of chars written convert time in tp to string asctime(tp)
g,G same as f or e,E depending on exponent convert calendar time in tp to local time ctime(tp)
convert calendar time to GMT gmtime(tp)
Variable Argument Lists <stdarg.h> convert calendar time to local time localtime(tp)
declaration of pointer to arguments va_list name; format date and time info strftime(s,smax,"format ",tp)
initialization of argument pointer va_start(name ,lastarg ) tp is a pointer to a structure of type tm
lastarg is last named parameter of the function
access next unamed arg, update pointer va_arg(name ,type) Mathematical Functions <math.h>
call before exiting function va_end(name ) Arguments and returned values are double
trig functions sin(x), cos(x), tan(x)
inverse trig functions asin(x), acos(x), atan(x)
arctan(y/x) atan2(y,x)
hyperbolic trig functions sinh(x), cosh(x), tanh(x) May 1999 v1.3. Copyright c 1999 Joseph H. Silverman
exponentials & logs exp(x), log(x), log10(x)
Permission is granted to make and distribute copies of this card pro-
exponentials & logs (2 power) ldexp(x,n), frexp(x,*e) vided the copyright notice and this permission notice are preserved on
division & remainder modf(x,*ip), fmod(x,y) all copies.
powers pow(x,y), sqrt(x) Send comments and corrections to J.H. Silverman, Math. Dept., Brown
rounding ceil(x), floor(x), fabs(x) Univ., Providence, RI 02912 USA. jhs@math.brown.edu
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