diff options
Diffstat (limited to 'c.html.markdown')
| -rw-r--r-- | c.html.markdown | 104 |
1 files changed, 79 insertions, 25 deletions
diff --git a/c.html.markdown b/c.html.markdown index 57ce72d2..a57be1dc 100644 --- a/c.html.markdown +++ b/c.html.markdown @@ -8,6 +8,9 @@ contributors: - ["Marco Scannadinari", "https://marcoms.github.io"] - ["Zachary Ferguson", "https://github.io/zfergus2"] - ["himanshu", "https://github.com/himanshu81494"] + - ["Joshua Li", "https://github.com/JoshuaRLi"] + - ["Dragos B. Chirila", "https://github.com/dchirila"] + - ["Heitor P. de Bittencourt", "https://github.com/heitorPB/"] --- Ah, C. Still **the** language of modern high-performance computing. @@ -16,6 +19,16 @@ C is the lowest-level language most programmers will ever use, but it more than makes up for it with raw speed. Just be aware of its manual memory management and C will take you as far as you need to go. +> **About compiler flags** +> +> By default, gcc and clang are pretty quiet about compilation warnings and +> errors, which can be very useful information. Explicitly using stricter +> compiler flags is recommended. Here are some recommended defaults: +> +> `-Wall -Wextra -Werror -O2 -std=c99 -pedantic` +> +> For information on what these flags do as well as other flags, consult the man page for your C compiler (e.g. `man 1 gcc`) or just search online. + ```c // Single-line comments start with // - only available in C99 and later. @@ -76,8 +89,12 @@ int main (int argc, char** argv) // Types /////////////////////////////////////// - // All variables MUST be declared at the top of the current block scope - // we declare them dynamically along the code for the sake of the tutorial + // Compilers that are not C99-compliant require that variables MUST be + // declared at the top of the current block scope. + // Compilers that ARE C99-compliant allow declarations near the point where + // the value is used. + // For the sake of the tutorial, variables are declared dynamically under + // C99-compliant standards. // ints are usually 4 bytes int x_int = 0; @@ -90,7 +107,7 @@ int main (int argc, char** argv) 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 + // 8 bytes long x_long = 0; long long x_long_long = 0; @@ -130,6 +147,17 @@ int main (int argc, char** argv) // You can initialize an array to 0 thusly: char my_array[20] = {0}; + // where the "{0}" part is called an "array initializer". + // NOTE that you get away without explicitly declaring the size of the array, + // IF you initialize the array on the same line. So, the following declaration + // is equivalent: + char my_array[] = {0}; + // BUT, then you have to evaluate the size of the array at run-time, like this: + size_t my_array_size = sizeof(my_array) / sizeof(my_array[0]); + // WARNING If you adopt this approach, you should evaluate the size *before* + // you begin passing the array to function (see later discussion), because + // arrays get "downgraded" to raw pointers when they are passed to functions + // (so the statement above will produce the wrong result inside the function). // Indexing an array is like other languages -- or, // rather, other languages are like C @@ -269,7 +297,7 @@ int main (int argc, char** argv) // While loops exist int ii = 0; - while (ii < 10) { //ANY value not zero is true. + while (ii < 10) { //ANY value less than ten is true. printf("%d, ", ii++); // ii++ increments ii AFTER using its current value. } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, " @@ -302,7 +330,7 @@ int main (int argc, char** argv) // branching with multiple choices: switch() switch (a) { - case 0: // labels need to be integral *constant* expressions + case 0: // labels need to be integral *constant* expressions (such as enums) printf("Hey, 'a' equals 0!\n"); break; // if you don't break, control flow falls over labels case 1: @@ -324,7 +352,7 @@ int main (int argc, char** argv) using "goto" in C */ typedef enum { false, true } bool; - // for C don't have bool as data type :( + // for C don't have bool as data type before C99 :( bool disaster = false; int i, j; for(i=0;i<100;++i) @@ -336,10 +364,10 @@ int main (int argc, char** argv) goto error; } error : - printf("Error occured at i = %d & j = %d.\n", i, j); + printf("Error occurred at i = %d & j = %d.\n", i, j); /* https://ideone.com/GuPhd6 - this will print out "Error occured at i = 52 & j = 99." + this will print out "Error occurred at i = 51 & j = 99." */ /////////////////////////////////////// @@ -363,8 +391,8 @@ int main (int argc, char** argv) // 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("%f\n", (double) 100); // %f always formats a double... + printf("%f\n", (float) 100); // ...even with a float. printf("%d\n", (char)100.0); /////////////////////////////////////// @@ -422,7 +450,7 @@ int main (int argc, char** argv) // 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); + printf("%zu, %zu\n", sizeof(arraythethird), sizeof(ptr)); // probably prints "40, 4" or "40, 8" // Pointers are incremented and decremented based on their type @@ -439,16 +467,24 @@ int main (int argc, char** argv) *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints) + // Be careful passing user-provided values to malloc! If you want + // to be safe, you can use calloc instead (which, unlike malloc, also zeros out the memory) + int* my_other_ptr = calloc(20, sizeof(int)); + // Note that there is no standard way to get the length of a // dynamically allocated array in C. Because of this, if your arrays are // going to be passed around your program a lot, you need another variable // to keep track of the number of elements (size) of an array. See the // functions section for more info. - int size = 10; - int *my_arr = malloc(sizeof(int) * size); + size_t size = 10; + int *my_arr = calloc(size, sizeof(int)); // Add an element to the array size++; my_arr = realloc(my_arr, sizeof(int) * size); + if (my_arr == NULL) { + //Remember to check for realloc failure! + return + } my_arr[10] = 5; // Dereferencing memory that you haven't allocated gives @@ -503,9 +539,11 @@ Example: in-place string reversal void str_reverse(char *str_in) { char tmp; - int ii = 0; + size_t ii = 0; size_t len = strlen(str_in); // `strlen()` is part of the c standard library - for (ii = 0; ii < len / 2; ii++) { + // NOTE: length returned by `strlen` DOESN'T include the + // terminating NULL byte ('\0') + for (ii = 0; ii < len / 2; ii++) { // in C99 you can directly declare type of `ii` here tmp = str_in[ii]; str_in[ii] = str_in[len - ii - 1]; // ii-th char from end str_in[len - ii - 1] = tmp; @@ -546,7 +584,7 @@ array in C. */ // Size must be passed! // Otherwise, this function has no way of knowing how big the array is. -void printIntArray(int *arr, int size) { +void printIntArray(int *arr, size_t size) { int i; for (i = 0; i < size; i++) { printf("arr[%d] is: %d\n", i, arr[i]); @@ -559,7 +597,7 @@ printIntArray(my_arr, size); // will print "arr[0] is: 1" etc */ -// if referring to external variables outside function, must use extern keyword. +// if referring to external variables outside function, you should use the extern keyword. int i = 0; void testFunc() { extern int i; //i here is now using external variable i @@ -570,6 +608,14 @@ static int j = 0; //other files using testFunc2() cannot access variable j void testFunc2() { extern int j; } +// The static keyword makes a variable inaccessible to code outside the +// compilation unit. (On almost all systems, a "compilation unit" is a .c +// file.) static can apply both to global (to the compilation unit) variables, +// functions, and function-local variables. When using static with +// function-local variables, the variable is effectively global and retains its +// value across function calls, but is only accessible within the function it +// is declared in. Additionally, static variables are initialized to 0 if not +// declared with some other starting value. //**You may also declare functions as static to make them private** /////////////////////////////////////// @@ -656,6 +702,7 @@ typedef void (*my_fnp_type)(char *); // ... // my_fnp_type f; + //Special characters: /* '\a'; // alert (bell) character @@ -683,7 +730,8 @@ typedef void (*my_fnp_type)(char *); "%3.2f"; // minimum 3 digits left and 2 digits right decimal float "%7.4s"; // (can do with strings too) "%c"; // char -"%p"; // pointer +"%p"; // pointer. NOTE: need to (void *)-cast the pointer, before passing + // it as an argument to `printf`. "%x"; // hexadecimal "%o"; // octal "%%"; // prints % @@ -738,14 +786,20 @@ as the C file. /* Like c source files macros can be defined in headers and used in files */ /* that include this header file. */ #define EXAMPLE_NAME "Dennis Ritchie" -/* Function macros can also be defined. */ -#define ADD(a, b) (a + b) + +/* Function macros can also be defined. */ +#define ADD(a, b) ((a) + (b)) + +/* Notice the parenthesis surrounding the arguments -- this is important to */ +/* ensure that a and b don't get expanded in an unexpected way (e.g. consider */ +/* MUL(x, y) (x * y); MUL(1 + 2, 3) would expand to (1 + 2 * 3), yielding an */ +/* incorrect result) */ /* Structs and typedefs can be used for consistency between files. */ -typedef struct node +typedef struct Node { int val; - struct node *next; + struct Node *next; } Node; /* So can enumerations. */ @@ -769,14 +823,14 @@ Best to find yourself a copy of [K&R, aka "The C Programming Language"](https:// It is *the* book about C, written by Dennis Ritchie, the creator of C, and Brian Kernighan. Be careful, though - it's ancient and it contains some inaccuracies (well, ideas that are not considered good anymore) or now-changed practices. -Another good resource is [Learn C The Hard Way](http://c.learncodethehardway.org/book/). +Another good resource is [Learn C The Hard Way](http://learncodethehardway.org/c/). If you have a question, read the [compl.lang.c Frequently Asked Questions](http://c-faq.com). 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 style](https://www.kernel.org/doc/Documentation/CodingStyle). +[Linux kernel coding style](https://www.kernel.org/doc/Documentation/process/coding-style.rst). Other than that, Google is your friend. -[1] http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member +[1] [Why isn't sizeof for a struct equal to the sum of sizeof of each member?](http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member) |