Quick and Dirty Guide to C
The single best book on C is The C Programming Language by Kernighan and Richie.
CODE:
Code for execution goes into files with ¡°.c¡± suffix.
Shared decl¡¯s (included using #include ¡°mylib.h¡±) in ¡°header¡± files, end in ¡°.h¡±
COMMENTS:
Characters to the right of // are not interpreted; they¡¯re a comment.
Text between /* and */ (possibly across lines) is commented out.
FUNCTIONS:
A function is a pointer to some code, parameterized by formal parameters, that
may be executed by providing actual parameters. Functions must be declared before
they are used, but code may be provided later. A sqrt function for positive n
might be declared as:
double sqrt(double n) {
double guess;
for (guess = n/2.0; abs(n-guess*guess)>0.001; guess = (n/guess+guess)/2);
return guess;
}
This function has type double (s*sqrt)(double).
printf(¡°%g\n¡±, sqrt(7.0)); //calls sqrt; actuals are always passed by value
Functions parameters are always passed by value. Functions must return a value.
The return value need not be used. Function names with parameters returns the
function pointer. Thus, an alias for sqrt may be declared:
double (*root)(double) = sqrt;
printf(¡°%g\n¡±, root(7.0));
Procedures or valueless functions return ¡®void¡¯.
There must always be a main function that returns an int.
int main(int argc, char **argv) OR int main(int argc, char *argv[])
Program arguments may be accessed as strings through main¡¯s array argv with argc
elements. First is the program name. Function declarations are never nested.
DATA TYPES:
Name! ! char! ! int/long! long long ! float! ! double!!
Size! Description
1 byte! an ASCII value: e.g. ¡®a¡¯ (see: man ascii)
4 bytes !a signed integer: e.g. 97 or hex 0x61, oct 0x141 8 bytes! a longer multi-byte signed integer
4 bytes! a floating-point (possibly fractional) value
8 bytes !a double length float
char, int, and double are most frequently and easily used in small programs
sizeof(double) computes the size of a double in addressable units (bytes)
Zero values represent logical false, nonzero values are logical true.
Math library (#include
CASTING:
Preceding a primitive expression with an alternate parenthesized type converts or
¡°casts¡± value to a new value equivalent in new type:
int a – (int) 3.131; ! //assigns a=3 without complaint
Preceding any other expression with a cast forces new type for unchanged value.
double b = 3.131;
int a = *(int*)&b; //interprets the double b as an integer (not necessarily 3)
STRUCTS and ARRAYS and POINTERS and ADDRESS COMPUTATION:
Structs collect several fields into a single logical type:
struct { int n; double root;} s; //s has two fields, n and root
s.root = sqrt((s.n=7)); //ref fields (N.B. double parens=>assign OK!)
Arrays indicated by right associative brackets ([]) in the type declaration
int a[10]; //a is a 10int array. a[0] is the first element. a[9] is the last
char b[]; //in a function header, b is an array of chars with unknown length
int c[2][3]; //c is an array of 2 arrays of three ints. a[1][0] follows a[0][2]
Array variables (e.g. a,b,c above) cannot be made to point to other arrays
Strings are represented as character arrays terminated by ASCII zero.
Pointers are indicated by left associative asterisk (*) in the type declarations:
int *a; // a is a pointer to an integer
char *b; // b is a pointer to a character
int *c[2]; // c is an array of two pointers
int (*d)[2]; // d is a pointer to an array of 2 integers
Pointers are simply addresses. Pointer variables may be assigned.
Adding 1 computes pointer to the next value by adding sizeof(X) for type X
General int adds to pointer (even 0 or negative values) behave in the same way
Addresses may be computed with the ampersand (&) operator.
An array without an index or a struct without field computes its address:
OPERATIONS:
+, -, *, /, %! ++i –i!!
i++ i–!!
&& || !!!
Arithmetic ops. /truncates on integers, % is remainder.
Add or subtract 1 from i, assign result to i, return new val
Remember i, inc or decrement i, return remembered value
Logical ops. Right side of && and || unless necessary
Bit logical ops: and, or, xor, complement.
Shift right and left: int n=10; n <<2 computes 40.
Assignment is an operator. Result is value assigned.
Perform binary op on left and right, assign result to left
int a[10], b[20];
int *p = a;
p = b;
// two arrays
// p points to first int of array a
// p now points to the first int of array b
switch (
case
//traditional ¡°case statement¡±
// this statement exec¡¯d if val==expr
// quit this when value == expression
//executed if value2 = expression
An array or pointer with an index n in square brackets returns the nth value:
//executed if value3 = expression
// quit
// if matches no other value; may
// optional (but encouraged) quit
// an array
// i is the first element of a
// pointer dereference
//sameasp=&a[0] //sameasp=p+1;sameasp=&a[1];sameasp=a+1
checked; your responsibility not to run off. Don¡¯t assume.
int a[10];
int *p;
int i = a[0];
i = *a;
p = a;
p++;
Bounds are not
An arrow (-> no
struct { int n; double root; } s[1]; //s is pointer to struct or array of 1
s->root = sqrt)s->n = 7); //s->root same as (*s).root or s[0].root
printf(¡°%g\n¡±, s->root);
be first
spaces!) dereferences a pointer to a field:
before primitive type suggests unsigned operations
in global declaration => symbol is for external use
in global declaration => symbol is local to this file
in local decl¡¯n => don¡¯t place on stack; keep value betw¡¯n calls
before declaration defines a new type name, not a new variable
to ints (same as int *(c[2]);
if (
if (
while (
do
for (
&| ^ ~!!
>>< <= >= Comparison operators (useful only on primitive types) ?:! ! If-like expression: (x%2==0)?¡±even¡±:¡±odd¡±
,! ! computing value is last: a, = b,c,d; exec¡¯s b,c,d then a=d
STATEMENTS:
Angle brackets identify syntactic elements and don¡¯t appear in real statements
//semicolon indicates end of a simple statement
//quits the tightest loop or switch immediately
//jumps to next loop test, skipping rest of loop body
//quits this function, returns x as value
//curly-brace groups statements into 1 compound (no 😉
! ! }
KEY WORDS
unsigned! extern!! static!! ! ! typedef!
break;! !
Quick and Dirty Guide to C
I/O (#include
Default input comes from ¡°stdin¡±; output goes to ¡°stdout¡±; errors to ¡°stderr¡±.
Standard input and output routines are declared in stdio.h: #include
Function! Description
fopen(name, ¡°r¡±) opens file name for read, returns FILE *f; ¡°w¡± allows write
A GOOD FIRST PROGRAM:
#include
#include
int main(int argc, char** argv){
printf(¡°Hello, world.\n¡±);
return 0; }
A WORD COUNT (WC)
#include
#include
int main(int argc, char **argv){
int charCount=0, wordCount=0, lineCount=0;
int doChar=0, doWord=0, doLine=0, inWord = 0;
int c;
char *fileName = 0;
FILE *f = stdin;
while (argv++, –argc) {
fclose(f)!
getchar()!
ungetch(c)!
putchar(c)!
fgetc(f)!
ungetc(c,f)!
fputc(c,f)!
fgets(s,n, f)!
fputs(s,f)!
scanf(p,…) !
printf(p, …)
fprintf(f,p,…) same, but print to file f fscanf(f,p,…) same, but read from file f sscanf(s,p,…) same, but read from string s sprintf(s,p,…) same, as printf, but to string s feof(f)! return true iff at end of file f
malloc(n)! free(p)! calloc(n,s)!
alloc n bytes of memory; for type T: p = (T*)malloc(sizeof(t));
free memory pointed at p; must have been alloc¡¯d; don¡¯t re-free
alloc n-array size s & clear; typ: a = (T*)calloc(n, sizeof(T));
closes file f
read 1 char from stdin or pushback; is EOF (int -1) if none
pushback char c into stdin for re-reading; don¡¯t change c
write 1 char, c, to stdout
same as getchar(), but reads from file f
same as ungetchar() but onto file f
same as putchar(c), but onto file f
read string of n-1 chars to a s from f or til eof or \n
writes string s to f: e.g. fputs(¡°Hello world\n¡±, stdout);
reads … args using format p (below); put &w/non-pointers
write … args using format p (below); pass args as is
Formats use format characters preceded by escape %; other chars written as is>
!
!
!
!
! !}
char!
%c!
%d!
%s!
%g !
meaning! ! ! ! character! ! ! decimal integer! ! ! string! ! ! ! general floating point! !
char! meaning
\n! newline (control-j) \t! tab (control-i)
\\! slash
%%! perent
MEMORY (%include
! ! ! ! !
MATH (#include
All functions take and return double unless otherwise noted:
sin(a), cos(a), tan(a)! sine, cosine, tangent of double (in radians) asine(y),acos(x),atan(r) principle inverse of above
atan2(y,x)! ! sqrt(x)! ! log(x)!! ! exp(p)!! ! pow(x,y)! ! ceil(x)! ! floor(x)! !
principal inverse of tan(y/x) in same quadrant as (x,y)
root of x
natural logarithm of x; others: log2(x) and log10(x)
e to the power of p; others: exp2(x) and exp10(x)
x to the power of y; like (expy*log(x))
smallest integer (returned as double) no less than x
largest integer (returned as double) no greater than y
#include
abs(x)!! ! random()! ! srandom(seed)! !
absolute value of x
returns a random long
seeds the random generator with a new random seed
STRINGS (#include
strlen(s)! return length of string; number of characters before ASCII 0 strcpy(d,s)! copy string s to d and return d; N.B. parameter order like = strncpy(d,s,n)!copy at most n characters of s to d and terminate; returns d stpcpy(d,s)! like strcpy, but returns pointer to ASCII 0 terminarot in d strcmp(s,t)! compare strings s and t and return first difference; 0=> equal strncmp(s,t,n)!stop after at most n characters; needn¡¯t be null terminated memcpy(d,s,n) copy exactly n bytes from s to d; may fail if s overlaps d memmove(d,s,n) (slow) copy n bytes from s to d; won¡¯t fail if s overlaps d
COMPILING:
gcc prog.c! # compiles prog.c into a.out run result with ./a.out gcc -o prog prog.c # compiles prog.c into prog; run result with ./prog gcc -g -o prog prog.c # as above, but allows for debugging
Content borrowed and updated (with permission)
from Duane A. Bailey¡¯s guidelines from 2007.
if (!strcmp(*argv,¡±-c¡±)) doChar=1;
else if (!strcmp(*argv,¡±-w¡±)) doWord=1;
else if (!strcmp(*argv,¡±-l¡±)) doLine=1;
else if (!(f = fopen((fileName = *argv), ¡°r¡±))){
printf(¡°Usage: wc [-l] [-w] [-c]\n¡±); return 1;
}
if (!(doChar || doWord || doLine)) doChar = doWord = doLine = 1;
while (EOF != (c= fgetc(f))){
charCount++;
if (c == ¡®\n¡¯) lineCount++;
if (!iswpace(c)) {
if (!inWord) { inWord = 1; wordcount++; }
} else { inWord = 0; }
}
if (doLine) printf(¡°%8d¡±, lineCount);
if (doWord) printf(¡°%8d¡±, wordCount);
if (doChar) printf(¡°%8d¡±, charCount);
if (fileName) printf(¡° %s¡±, fileName);
printf(¡°\n¡±);
}
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