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Diffstat (limited to 'c.html.markdown')
| -rw-r--r-- | c.html.markdown | 898 |
1 files changed, 454 insertions, 444 deletions
diff --git a/c.html.markdown b/c.html.markdown index 0190125b..6daabe94 100644 --- a/c.html.markdown +++ b/c.html.markdown @@ -4,6 +4,8 @@ filename: learnc.c contributors: - ["Adam Bard", "http://adambard.com/"] - ["Árpád Goretity", "http://twitter.com/H2CO3_iOS"] + - ["Jakub Trzebiatowski", "http://cbs.stgn.pl"] + - ["Marco Scannadinari", "https://marcoms.github.io"] --- @@ -16,16 +18,20 @@ memory management and C will take you as far as you need to go. ```c // Single-line comments start with // - only available in C99 and later. -/* + /* Multi-line comments look like this. They work in C89 as well. -*/ + */ + +/* +Multi-line comments don't nest /* Be careful */ // comment ends on this line... +*/ // ...not this one! -// Constants: #define <keyword> + // Constants: #define <keyword> #define DAYS_IN_YEAR 365 -//enumeration constants are also ways to declare constants. -enum days {SUN = 1, MON, TUE, WED, THU, FRI, SAT}; -// MON gets 2 automatically, TUE gets 3, etc. + // Enumeration constants are also ways to declare constants. + enum days {SUN = 1, MON, TUE, WED, THU, FRI, SAT}; +// MON gets 2 automatically, TUE gets 3, etc. // Import headers with #include #include <stdlib.h> @@ -34,388 +40,391 @@ enum days {SUN = 1, MON, TUE, WED, THU, FRI, SAT}; // (File names between <angle brackets> are headers from the C standard library.) // For your own headers, use double quotes instead of angle brackets: -#include "my_header.h" +//#include "my_header.h" // Declare function signatures in advance in a .h file, or at the top of // your .c file. -void function_1(char c); +void function_1(); int function_2(void); // Must declare a 'function prototype' before main() when functions occur after // your main() function. -int add_two_ints(int x1, int x2); // function prototype +int add_two_ints(int x1, int x2); // function prototype // Your program's entry point is a function called // main with an integer return type. int main() { - // print output using printf, for "print formatted" - // %d is an integer, \n is a newline - printf("%d\n", 0); // => Prints 0 - // All statements must end with a semicolon - - /////////////////////////////////////// - // Types - /////////////////////////////////////// - - // ints are usually 4 bytes - int x_int = 0; - - // shorts are usually 2 bytes - short x_short = 0; - - // chars are guaranteed to be 1 byte - char x_char = 0; - char y_char = 'y'; // Char literals are quoted with '' - - // longs are often 4 to 8 bytes; long longs are guaranteed to be at least - // 64 bits - long x_long = 0; - long long x_long_long = 0; - - // floats are usually 32-bit floating point numbers - float x_float = 0.0; - - // doubles are usually 64-bit floating-point numbers - double x_double = 0.0; - - // Integral types may be unsigned. - unsigned short ux_short; - unsigned int ux_int; - unsigned long long ux_long_long; - - // chars inside single quotes are integers in machine's character set. - '0' // => 48 in the ASCII character set. - 'A' // => 65 in the ASCII character set. - - // sizeof(T) gives you the size of a variable with type T in bytes - // sizeof(obj) yields the size of the expression (variable, literal, etc.). - printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words) - - - // If the argument of the `sizeof` operator is an expression, then its argument - // is not evaluated (except VLAs (see below)). - // The value it yields in this case is a compile-time constant. - int a = 1; - // size_t is an unsiged integer type of at least 2 bytes used to represent - // the size of an object. - size_t size = sizeof(a++); // a++ is not evaluated - printf("sizeof(a++) = %zu where a = %d\n", size, a); - // prints "sizeof(a++) = 4 where a = 1" (on a 32-bit architecture) - - // Arrays must be initialized with a concrete size. - char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes - int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes - // (assuming 4-byte words) - - - // You can initialize an array to 0 thusly: - char my_array[20] = {0}; - - // Indexing an array is like other languages -- or, - // rather, other languages are like C - my_array[0]; // => 0 - - // Arrays are mutable; it's just memory! - my_array[1] = 2; - printf("%d\n", my_array[1]); // => 2 - - // In C99 (and as an optional feature in C11), variable-length arrays (VLAs) - // can be declared as well. The size of such an array need not be a compile - // time constant: - printf("Enter the array size: "); // ask the user for an array size - char buf[0x100]; - fgets(buf, sizeof buf, stdin); - - // strtoul parses a string to an unsigned integer - size_t size = strtoul(buf, NULL, 10); - int var_length_array[size]; // declare the VLA - printf("sizeof array = %zu\n", sizeof var_length_array); - - // A possible outcome of this program may be: - // > Enter the array size: 10 - // > sizeof array = 40 - - // Strings are just arrays of chars terminated by a NUL (0x00) byte, - // represented in strings as the special character '\0'. - // (We don't have to include the NUL byte in string literals; the compiler - // inserts it at the end of the array for us.) - char a_string[20] = "This is a string"; - printf("%s\n", a_string); // %s formats a string - - printf("%d\n", a_string[16]); // => 0 - // i.e., byte #17 is 0 (as are 18, 19, and 20) - - // If we have characters between single quotes, that's a character literal. - // It's of type `int`, and *not* `char` (for historical reasons). - int cha = 'a'; // fine - char chb = 'a'; // fine too (implicit conversion from int to char) - - //Multi-dimensional arrays: - int multi_array[2][5] = { - {1, 2, 3, 4, 5}, - {6, 7, 8, 9, 0} - } - //access elements: - int array_int = multi_array[0][2]; // => 3 - - /////////////////////////////////////// - // Operators - /////////////////////////////////////// - - // Shorthands for multiple declarations: - int i1 = 1, i2 = 2; - float f1 = 1.0, f2 = 2.0; - - int a, b, c; - a = b = c = 0; - - // Arithmetic is straightforward - i1 + i2; // => 3 - i2 - i1; // => 1 - i2 * i1; // => 2 - i1 / i2; // => 0 (0.5, but truncated towards 0) - - f1 / f2; // => 0.5, plus or minus epsilon - // Floating-point numbers and calculations are not exact - - // Modulo is there as well - 11 % 3; // => 2 - - // Comparison operators are probably familiar, but - // there is no boolean type in c. We use ints instead. - // (Or _Bool or bool in C99.) - // 0 is false, anything else is true. (The comparison - // operators always yield 0 or 1.) - 3 == 2; // => 0 (false) - 3 != 2; // => 1 (true) - 3 > 2; // => 1 - 3 < 2; // => 0 - 2 <= 2; // => 1 - 2 >= 2; // => 1 - - // C is not Python - comparisons don't chain. - int a = 1; - // WRONG: - int between_0_and_2 = 0 < a < 2; - // Correct: - int between_0_and_2 = 0 < a && a < 2; - - // Logic works on ints - !3; // => 0 (Logical not) - !0; // => 1 - 1 && 1; // => 1 (Logical and) - 0 && 1; // => 0 - 0 || 1; // => 1 (Logical or) - 0 || 0; // => 0 - - //Conditional expression ( ? : ) - int a = 5; - int b = 10; - int z; - z = (a > b) ? a : b; // => 10 "if a > b return a, else return b." - - //Increment and decrement operators: - char *s = "iLoveC" - int j = 0; - s[j++]; // => "i". Returns the j-th item of s THEN increments value of j. - j = 0; - s[++j]; // => "L". Increments value of j THEN returns j-th value of s. - // same with j-- and --j - - // Bitwise operators! - ~0x0F; // => 0xF0 (bitwise negation, "1's complement") - 0x0F & 0xF0; // => 0x00 (bitwise AND) - 0x0F | 0xF0; // => 0xFF (bitwise OR) - 0x04 ^ 0x0F; // => 0x0B (bitwise XOR) - 0x01 << 1; // => 0x02 (bitwise left shift (by 1)) - 0x02 >> 1; // => 0x01 (bitwise right shift (by 1)) - - // Be careful when shifting signed integers - the following are undefined: - // - shifting into the sign bit of a signed integer (int a = 1 << 32) - // - left-shifting a negative number (int a = -1 << 2) - // - shifting by an offset which is >= the width of the type of the LHS: - // int a = 1 << 32; // UB if int is 32 bits wide - - /////////////////////////////////////// - // Control Structures - /////////////////////////////////////// - - if (0) { - printf("I am never run\n"); - } else if (0) { - printf("I am also never run\n"); - } else { - printf("I print\n"); - } - - // While loops exist - int ii = 0; - while (ii < 10) { //ANY value not zero is true. - printf("%d, ", ii++); // ii++ increments ii AFTER using it's current value. - } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " - - printf("\n"); - - int kk = 0; - do { - printf("%d, ", kk); - } while (++kk < 10); // ++kk increments kk BEFORE using it's current value. - // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " - - printf("\n"); - - // For loops too - int jj; - for (jj=0; jj < 10; jj++) { - printf("%d, ", jj); - } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " - - printf("\n"); - - // *****NOTES*****: - // Loops and Functions MUST have a body. If no body is needed: - int i; - for (i = 0; i <= 5; i++) { - ; // use semicolon to act as the body (null statement) - } - - // branching with multiple choices: switch() - switch (some_integral_expression) { - case 0: // labels need to be integral *constant* epxressions - do_stuff(); - break; // if you don't break, control flow falls over labels - case 1: - do_something_else(); - break; - default: - // if `some_integral_expression` didn't match any of the labels - fputs("error!\n", stderr); - exit(-1); - break; - } - - - /////////////////////////////////////// - // Typecasting - /////////////////////////////////////// - - // Every value in C has a type, but you can cast one value into another type - // if you want (with some constraints). - - int x_hex = 0x01; // You can assign vars with hex literals - - // Casting between types will attempt to preserve their numeric values - printf("%d\n", x_hex); // => Prints 1 - printf("%d\n", (short) x_hex); // => Prints 1 - printf("%d\n", (char) x_hex); // => Prints 1 - - // Types will overflow without warning - printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long) - - // For determining the max value of a `char`, a `signed char` and an `unisigned char`, - // respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h> - - // Integral types can be cast to floating-point types, and vice-versa. - printf("%f\n", (float)100); // %f formats a float - printf("%lf\n", (double)100); // %lf formats a double - printf("%d\n", (char)100.0); - - /////////////////////////////////////// - // Pointers - /////////////////////////////////////// - - // A pointer is a variable declared to store a memory address. Its declaration will - // also tell you the type of data it points to. You can retrieve the memory address - // of your variables, then mess with them. - - int x = 0; - printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable - // (%p formats an object pointer of type void *) - // => Prints some address in memory; - - - // Pointers start with * in their declaration - int *px, not_a_pointer; // px is a pointer to an int - px = &x; // Stores the address of x in px - printf("%p\n", (void *)px); // => Prints some address in memory - printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer)); - // => Prints "8, 4" on a typical 64-bit system - - // To retrieve the value at the address a pointer is pointing to, - // put * in front to de-reference it. - // Note: yes, it may be confusing that '*' is used for _both_ declaring a - // pointer and dereferencing it. - printf("%d\n", *px); // => Prints 0, the value of x - - // You can also change the value the pointer is pointing to. - // We'll have to wrap the de-reference in parenthesis because - // ++ has a higher precedence than *. - (*px)++; // Increment the value px is pointing to by 1 - printf("%d\n", *px); // => Prints 1 - printf("%d\n", x); // => Prints 1 - - // Arrays are a good way to allocate a contiguous block of memory - int x_array[20]; //declares array of size 20 (cannot change size) - int xx; - for (xx = 0; xx < 20; xx++) { - x_array[xx] = 20 - xx; - } // Initialize x_array to 20, 19, 18,... 2, 1 + // print output using printf, for "print formatted" + // %d is an integer, \n is a newline + printf("%d\n", 0); // => Prints 0 + // All statements must end with a semicolon + + /////////////////////////////////////// + // Types + /////////////////////////////////////// + + // ints are usually 4 bytes + int x_int = 0; + + // shorts are usually 2 bytes + short x_short = 0; + + // chars are guaranteed to be 1 byte + char x_char = 0; + char y_char = 'y'; // Char literals are quoted with '' + + // longs are often 4 to 8 bytes; long longs are guaranteed to be at least + // 64 bits + long x_long = 0; + long long x_long_long = 0; + + // floats are usually 32-bit floating point numbers + float x_float = 0.0f; // 'f' suffix here denotes floating point literal + + // doubles are usually 64-bit floating-point numbers + double x_double = 0.0; // real numbers without any suffix are doubles + + // Integral types may be unsigned. + unsigned short ux_short; + unsigned int ux_int; + unsigned long long ux_long_long; + + // chars inside single quotes are integers in machine's character set. + '0'; // => 48 in the ASCII character set. + 'A'; // => 65 in the ASCII character set. + + // sizeof(T) gives you the size of a variable with type T in bytes + // sizeof(obj) yields the size of the expression (variable, literal, etc.). + printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words) + + + // If the argument of the `sizeof` operator is an expression, then its argument + // is not evaluated (except VLAs (see below)). + // The value it yields in this case is a compile-time constant. + int a = 1; + // size_t is an unsigned integer type of at least 2 bytes used to represent + // the size of an object. + size_t size = sizeof(a++); // a++ is not evaluated + printf("sizeof(a++) = %zu where a = %d\n", size, a); + // prints "sizeof(a++) = 4 where a = 1" (on a 32-bit architecture) + + // Arrays must be initialized with a concrete size. + char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes + int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes + // (assuming 4-byte words) + + + // You can initialize an array to 0 thusly: + char my_array[20] = {0}; + + // Indexing an array is like other languages -- or, + // rather, other languages are like C + my_array[0]; // => 0 + + // Arrays are mutable; it's just memory! + my_array[1] = 2; + printf("%d\n", my_array[1]); // => 2 + + // In C99 (and as an optional feature in C11), variable-length arrays (VLAs) + // can be declared as well. The size of such an array need not be a compile + // time constant: + printf("Enter the array size: "); // ask the user for an array size + char buf[0x100]; + fgets(buf, sizeof buf, stdin); + + // strtoul parses a string to an unsigned integer + size_t size2 = strtoul(buf, NULL, 10); + int var_length_array[size2]; // declare the VLA + printf("sizeof array = %zu\n", sizeof var_length_array); + + // A possible outcome of this program may be: + // > Enter the array size: 10 + // > sizeof array = 40 + + // Strings are just arrays of chars terminated by a NULL (0x00) byte, + // represented in strings as the special character '\0'. + // (We don't have to include the NULL byte in string literals; the compiler + // inserts it at the end of the array for us.) + char a_string[20] = "This is a string"; + printf("%s\n", a_string); // %s formats a string + + printf("%d\n", a_string[16]); // => 0 + // i.e., byte #17 is 0 (as are 18, 19, and 20) + + // If we have characters between single quotes, that's a character literal. + // It's of type `int`, and *not* `char` (for historical reasons). + int cha = 'a'; // fine + char chb = 'a'; // fine too (implicit conversion from int to char) + + //Multi-dimensional arrays: + int multi_array[2][5] = { + {1, 2, 3, 4, 5}, + {6, 7, 8, 9, 0} + }; + //access elements: + int array_int = multi_array[0][2]; // => 3 + + /////////////////////////////////////// + // Operators + /////////////////////////////////////// + + // Shorthands for multiple declarations: + int i1 = 1, i2 = 2; + float f1 = 1.0, f2 = 2.0; + + int b, c; + b = c = 0; + + // Arithmetic is straightforward + i1 + i2; // => 3 + i2 - i1; // => 1 + i2 * i1; // => 2 + i1 / i2; // => 0 (0.5, but truncated towards 0) + + // You need to cast at least one integer to float to get a floating-point result + (float)i1 / i2 // => 0.5f + i1 / (double)i2 // => 0.5 // Same with double + f1 / f2; // => 0.5, plus or minus epsilon + // Floating-point numbers and calculations are not exact + + // Modulo is there as well + 11 % 3; // => 2 + + // Comparison operators are probably familiar, but + // there is no Boolean type in c. We use ints instead. + // (Or _Bool or bool in C99.) + // 0 is false, anything else is true. (The comparison + // operators always yield 0 or 1.) + 3 == 2; // => 0 (false) + 3 != 2; // => 1 (true) + 3 > 2; // => 1 + 3 < 2; // => 0 + 2 <= 2; // => 1 + 2 >= 2; // => 1 + + // C is not Python - comparisons don't chain. + // Warning: The line below will compile, but it means `(0 < a) < 2`. + // This expression is always true, because (0 < a) could be either 1 or 0. + // In this case it's 1, because (0 < 1). + int between_0_and_2 = 0 < a < 2; + // Instead use: + int between_0_and_2 = 0 < a && a < 2; + + // Logic works on ints + !3; // => 0 (Logical not) + !0; // => 1 + 1 && 1; // => 1 (Logical and) + 0 && 1; // => 0 + 0 || 1; // => 1 (Logical or) + 0 || 0; // => 0 + + //Conditional expression ( ? : ) + int e = 5; + int f = 10; + int z; + z = (e > f) ? e : f; // => 10 "if e > f return e, else return f." + + //Increment and decrement operators: + char *s = "iLoveC"; + int j = 0; + s[j++]; // => "i". Returns the j-th item of s THEN increments value of j. + j = 0; + s[++j]; // => "L". Increments value of j THEN returns j-th value of s. + // same with j-- and --j + + // Bitwise operators! + ~0x0F; // => 0xF0 (bitwise negation, "1's complement") + 0x0F & 0xF0; // => 0x00 (bitwise AND) + 0x0F | 0xF0; // => 0xFF (bitwise OR) + 0x04 ^ 0x0F; // => 0x0B (bitwise XOR) + 0x01 << 1; // => 0x02 (bitwise left shift (by 1)) + 0x02 >> 1; // => 0x01 (bitwise right shift (by 1)) + + // Be careful when shifting signed integers - the following are undefined: + // - shifting into the sign bit of a signed integer (int a = 1 << 32) + // - left-shifting a negative number (int a = -1 << 2) + // - shifting by an offset which is >= the width of the type of the LHS: + // int a = 1 << 32; // UB if int is 32 bits wide + + /////////////////////////////////////// + // Control Structures + /////////////////////////////////////// + + if (0) { + printf("I am never run\n"); + } else if (0) { + printf("I am also never run\n"); + } else { + printf("I print\n"); + } + + // While loops exist + int ii = 0; + while (ii < 10) { //ANY value not zero is true. + printf("%d, ", ii++); // ii++ increments ii AFTER using its current value. + } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " + + printf("\n"); + + int kk = 0; + do { + printf("%d, ", kk); + } while (++kk < 10); // ++kk increments kk BEFORE using its current value. + // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " + + printf("\n"); + + // For loops too + int jj; + for (jj=0; jj < 10; jj++) { + printf("%d, ", jj); + } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " + + printf("\n"); + + // *****NOTES*****: + // Loops and Functions MUST have a body. If no body is needed: + int i; + for (i = 0; i <= 5; i++) { + ; // use semicolon to act as the body (null statement) + } + + // branching with multiple choices: switch() + switch (a) { + case 0: // labels need to be integral *constant* expressions + printf("Hey, 'a' equals 0!\n"); + break; // if you don't break, control flow falls over labels + case 1: + printf("Huh, 'a' equals 1!\n"); + break; + default: + // if `some_integral_expression` didn't match any of the labels + fputs("error!\n", stderr); + exit(-1); + break; + } + + /////////////////////////////////////// + // Typecasting + /////////////////////////////////////// + + // Every value in C has a type, but you can cast one value into another type + // if you want (with some constraints). + + int x_hex = 0x01; // You can assign vars with hex literals + + // Casting between types will attempt to preserve their numeric values + printf("%d\n", x_hex); // => Prints 1 + printf("%d\n", (short) x_hex); // => Prints 1 + printf("%d\n", (char) x_hex); // => Prints 1 + + // Types will overflow without warning + printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long) + + // For determining the max value of a `char`, a `signed char` and an `unsigned char`, + // respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h> + + // Integral types can be cast to floating-point types, and vice-versa. + printf("%f\n", (float)100); // %f formats a float + printf("%lf\n", (double)100); // %lf formats a double + printf("%d\n", (char)100.0); + + /////////////////////////////////////// + // Pointers + /////////////////////////////////////// + + // A pointer is a variable declared to store a memory address. Its declaration will + // also tell you the type of data it points to. You can retrieve the memory address + // of your variables, then mess with them. + + int x = 0; + printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable + // (%p formats an object pointer of type void *) + // => Prints some address in memory; + + + // Pointers start with * in their declaration + int *px, not_a_pointer; // px is a pointer to an int + px = &x; // Stores the address of x in px + printf("%p\n", (void *)px); // => Prints some address in memory + printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer)); + // => Prints "8, 4" on a typical 64-bit system + + // To retrieve the value at the address a pointer is pointing to, + // put * in front to dereference it. + // Note: yes, it may be confusing that '*' is used for _both_ declaring a + // pointer and dereferencing it. + printf("%d\n", *px); // => Prints 0, the value of x + + // You can also change the value the pointer is pointing to. + // We'll have to wrap the dereference in parenthesis because + // ++ has a higher precedence than *. + (*px)++; // Increment the value px is pointing to by 1 + printf("%d\n", *px); // => Prints 1 + printf("%d\n", x); // => Prints 1 + + // Arrays are a good way to allocate a contiguous block of memory + int x_array[20]; //declares array of size 20 (cannot change size) + int xx; + for (xx = 0; xx < 20; xx++) { + x_array[xx] = 20 - xx; + } // Initialize x_array to 20, 19, 18,... 2, 1 // Declare a pointer of type int and initialize it to point to x_array - int* x_ptr = x_array; - // x_ptr now points to the first element in the array (the integer 20). - // This works because arrays often decay into pointers to their first element. - // For example, when an array is passed to a function or is assigned to a pointer, - // it decays into (implicitly converted to) a pointer. - // Exceptions: when the array is the argument of the `&` (address-od) operator: - int arr[10]; - int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`! - // It's of type "pointer to array" (of ten `int`s). - // or when the array is a string literal used for initializing a char array: - char arr[] = "foobarbazquirk"; - // or when it's the argument of the `sizeof` or `alignof` operator: - int arr[10]; - int *ptr = arr; // equivalent with int *ptr = &arr[0]; - printf("%zu, %zu\n", sizeof arr, sizeof ptr); // probably prints "40, 4" or "40, 8" - - - // Pointers are incremented and decremented based on their type - // (this is called pointer arithmetic) - printf("%d\n", *(x_ptr + 1)); // => Prints 19 - printf("%d\n", x_array[1]); // => Prints 19 - - // You can also dynamically allocate contiguous blocks of memory with the - // standard library function malloc, which takes one argument of type size_t - // representing the number of bytes to allocate (usually from the heap, although this - // may not be true on e. g. embedded systems - the C standard says nothing about it). - int *my_ptr = malloc(sizeof(*my_ptr) * 20); - for (xx = 0; xx < 20; xx++) { - *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx - } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints) + int* x_ptr = x_array; + // x_ptr now points to the first element in the array (the integer 20). + // This works because arrays often decay into pointers to their first element. + // For example, when an array is passed to a function or is assigned to a pointer, + // it decays into (implicitly converted to) a pointer. + // Exceptions: when the array is the argument of the `&` (address-of) operator: + int arr[10]; + int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`! + // It's of type "pointer to array" (of ten `int`s). + // or when the array is a string literal used for initializing a char array: + char otherarr[] = "foobarbazquirk"; + // or when it's the argument of the `sizeof` or `alignof` operator: + int arraythethird[10]; + int *ptr = arraythethird; // equivalent with int *ptr = &arr[0]; + printf("%zu, %zu\n", sizeof arraythethird, sizeof ptr); // probably prints "40, 4" or "40, 8" + + + // Pointers are incremented and decremented based on their type + // (this is called pointer arithmetic) + printf("%d\n", *(x_ptr + 1)); // => Prints 19 + printf("%d\n", x_array[1]); // => Prints 19 + + // You can also dynamically allocate contiguous blocks of memory with the + // standard library function malloc, which takes one argument of type size_t + // representing the number of bytes to allocate (usually from the heap, although this + // may not be true on e.g. embedded systems - the C standard says nothing about it). + int *my_ptr = malloc(sizeof(*my_ptr) * 20); + for (xx = 0; xx < 20; xx++) { + *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx + } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints) // Dereferencing memory that you haven't allocated gives // "unpredictable results" - the program is said to invoke "undefined behavior" - printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash. - - // When you're done with a malloc'd block of memory, you need to free it, - // or else no one else can use it until your program terminates - // (this is called a "memory leak"): - free(my_ptr); - - // Strings are arrays of char, but they are usually represented as a - // pointer-to-char (which is a pointer to the first element of the array). - // It's good practice to use `const char *' when referring to a string literal, - // since string literals shall not be modified (i. e. "foo"[0] = 'a' is ILLEGAL.) - const char *my_str = "This is my very own string literal"; - printf("%c\n", *my_str); // => 'T' - - // This is not the case if the string is an array - // (potentially initialized with a string literal) - // that resides in writable memory, as in: - char foo[] = "foo"; - foo[0] = 'a'; // this is legal, foo now contains "aoo" - - function_1(); + printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash. + + // When you're done with a malloc'd block of memory, you need to free it, + // or else no one else can use it until your program terminates + // (this is called a "memory leak"): + free(my_ptr); + + // Strings are arrays of char, but they are usually represented as a + // pointer-to-char (which is a pointer to the first element of the array). + // It's good practice to use `const char *' when referring to a string literal, + // since string literals shall not be modified (i.e. "foo"[0] = 'a' is ILLEGAL.) + const char *my_str = "This is my very own string literal"; + printf("%c\n", *my_str); // => 'T' + + // This is not the case if the string is an array + // (potentially initialized with a string literal) + // that resides in writable memory, as in: + char foo[] = "foo"; + foo[0] = 'a'; // this is legal, foo now contains "aoo" + + function_1(); } // end main function /////////////////////////////////////// @@ -427,16 +436,16 @@ int main() { int add_two_ints(int x1, int x2) { - return x1 + x2; // Use return to return a value + return x1 + x2; // Use return to return a value } /* -Functions are call by value. When a function is called, the arguments passed to -the function are copies of the original arguments (except arrays). Anything you -do to the arguments in the function do not change the value of the original -argument where the function was called. +Functions are call by value. When a function is called, the arguments passed to +≈the function are copies of the original arguments (except arrays). Anything you +do to the arguments in the function do not change the value of the original +argument where the function was called. -Use pointers if you need to edit the original argument values. +Use pointers if you need to edit the original argument values. Example: in-place string reversal */ @@ -444,14 +453,14 @@ Example: in-place string reversal // A void function returns no value void str_reverse(char *str_in) { - char tmp; - int ii = 0; - size_t len = strlen(str_in); // `strlen()` is part of the c standard library - for (ii = 0; ii < len / 2; ii++) { - tmp = str_in[ii]; - str_in[ii] = str_in[len - ii - 1]; // ii-th char from end - str_in[len - ii - 1] = tmp; - } + char tmp; + int ii = 0; + size_t len = strlen(str_in); // `strlen()` is part of the c standard library + for (ii = 0; ii < len / 2; ii++) { + tmp = str_in[ii]; + str_in[ii] = str_in[len - ii - 1]; // ii-th char from end + str_in[len - ii - 1] = tmp; + } } /* @@ -463,13 +472,13 @@ printf("%s\n", c); // => ".tset a si sihT" //if referring to external variables outside function, must use extern keyword. int i = 0; void testFunc() { - extern int i; //i here is now using external variable i + extern int i; //i here is now using external variable i } //make external variables private to source file with static: -static int i = 0; //other files using testFunc() cannot access variable i -void testFunc() { - extern int i; +static int j = 0; //other files using testFunc() cannot access variable i +void testFunc2() { + extern int j; } //**You may also declare functions as static to make them private** @@ -486,8 +495,8 @@ my_type my_type_var = 0; // Structs are just collections of data, the members are allocated sequentially, // in the order they are written: struct rectangle { - int width; - int height; + int width; + int height; }; // It's not generally true that @@ -497,20 +506,20 @@ struct rectangle { void function_1() { - struct rectangle my_rec; + struct rectangle my_rec; - // Access struct members with . - my_rec.width = 10; - my_rec.height = 20; + // Access struct members with . + my_rec.width = 10; + my_rec.height = 20; - // You can declare pointers to structs - struct rectangle *my_rec_ptr = &my_rec; + // You can declare pointers to structs + struct rectangle *my_rec_ptr = &my_rec; - // Use dereferencing to set struct pointer members... - (*my_rec_ptr).width = 30; + // Use dereferencing to set struct pointer members... + (*my_rec_ptr).width = 30; - // ... or even better: prefer the -> shorthand for the sake of readability - my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10; + // ... or even better: prefer the -> shorthand for the sake of readability + my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10; } // You can apply a typedef to a struct for convenience @@ -518,34 +527,34 @@ typedef struct rectangle rect; int area(rect r) { - return r.width * r.height; + return r.width * r.height; } // if you have large structs, you can pass them "by pointer" to avoid copying // the whole struct: -int area(const rect *r) +int areaptr(const rect *r) { - return r->width * r->height; + return r->width * r->height; } /////////////////////////////////////// -// Function pointers +// Function pointers /////////////////////////////////////// /* -At runtime, functions are located at known memory addresses. Function pointers are -much like any other pointer (they just store a memory address), but can be used +At run time, functions are located at known memory addresses. Function pointers are +much like any other pointer (they just store a memory address), but can be used to invoke functions directly, and to pass handlers (or callback functions) around. However, definition syntax may be initially confusing. Example: use str_reverse from a pointer */ void str_reverse_through_pointer(char *str_in) { - // Define a function pointer variable, named f. - void (*f)(char *); // Signature should exactly match the target function. - f = &str_reverse; // Assign the address for the actual function (determined at runtime) - // f = str_reverse; would work as well - functions decay into pointers, similar to arrays - (*f)(str_in); // Just calling the function through the pointer - // f(str_in); // That's an alternative but equally valid syntax for calling it. + // Define a function pointer variable, named f. + void (*f)(char *); // Signature should exactly match the target function. + f = &str_reverse; // Assign the address for the actual function (determined at run time) + // f = str_reverse; would work as well - functions decay into pointers, similar to arrays + (*f)(str_in); // Just calling the function through the pointer + // f(str_in); // That's an alternative but equally valid syntax for calling it. } /* @@ -557,39 +566,40 @@ typedef void (*my_fnp_type)(char *); // Then used when declaring the actual pointer variable: // ... -// my_fnp_type f; +// my_fnp_type f; //Special characters: -'\a' // alert (bell) character -'\n' // newline character -'\t' // tab character (left justifies text) -'\v' // vertical tab -'\f' // new page (formfeed) -'\r' // carriage return -'\b' // backspace character -'\0' // null character. Usually put at end of strings in C lang. - // hello\n\0. \0 used by convention to mark end of string. -'\\' // backslash -'\?' // question mark -'\'' // single quote -'\"' // double quote -'\xhh' // hexadecimal number. Example: '\xb' = vertical tab character -'\ooo' // octal number. Example: '\013' = vertical tab character +/* +'\a'; // alert (bell) character +'\n'; // newline character +'\t'; // tab character (left justifies text) +'\v'; // vertical tab +'\f'; // new page (form feed) +'\r'; // carriage return +'\b'; // backspace character +'\0'; // NULL character. Usually put at end of strings in C. +// hello\n\0. \0 used by convention to mark end of string. +'\\'; // backslash +'\?'; // question mark +'\''; // single quote +'\"'; // double quote +'\xhh'; // hexadecimal number. Example: '\xb' = vertical tab character +'\0oo'; // octal number. Example: '\013' = vertical tab character //print formatting: -"%d" // integer -"%3d" // integer with minimum of length 3 digits (right justifies text) -"%s" // string -"%f" // float -"%ld" // long -"%3.2f" // minimum 3 digits left and 2 digits right decimal float -"%7.4s" // (can do with strings too) -"%c" // char -"%p" // pointer -"%x" // hexidecimal -"%o" // octal -"%%" // prints % - +"%d"; // integer +"%3d"; // integer with minimum of length 3 digits (right justifies text) +"%s"; // string +"%f"; // float +"%ld"; // long +"%3.2f"; // minimum 3 digits left and 2 digits right decimal float +"%7.4s"; // (can do with strings too) +"%c"; // char +"%p"; // pointer +"%x"; // hexadecimal +"%o"; // octal +"%%"; // prints % +*/ /////////////////////////////////////// // Order of Evaluation /////////////////////////////////////// @@ -628,7 +638,7 @@ If you have a question, read the [compl.lang.c Frequently Asked Questions](http: It's very important to use proper spacing, indentation and to be consistent with your coding style in general. Readable code is better than clever code and fast code. For a good, sane coding style to adopt, see the -[Linux kernel coding stlye](https://www.kernel.org/doc/Documentation/CodingStyle). +[Linux kernel coding style](https://www.kernel.org/doc/Documentation/CodingStyle). Other than that, Google is your friend. |