Hidden features of C

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I know there is a standard behind all C compiler implementations, so there should be no hidden features. Despite that, I am sure all C developers have hidden/secret tricks they use all the time.

Answer 1

More of a trick of the GCC compiler, but you can give branch indication hints to the compiler (common in the Linux kernel)

#define likely(x)       __builtin_expect((x),1)
#define unlikely(x)     __builtin_expect((x),0)

see: http://kerneltrap.org/node/4705

What I like about this is that it also adds some expressiveness to some functions.

void foo(int arg)
{
     if (unlikely(arg == 0)) {
           do_this();
           return;
     }
     do_that();
     ...
}

Answer 2

int8_t
int16_t
int32_t
uint8_t
uint16_t
uint32_t

These are an optional item in the standard, but it must be a hidden feature, because people are constantly redefining them. One code base I've worked on (and still do, for now) has multiple redefinitions, all with different identifiers. Most of the time it's with preprocessor macros:

#define INT16 short
#define INT32  long

And so on. It makes me want to pull my hair out. Just use the freaking standard integer typedefs!

Answer 3

The comma operator isn't widely used. It can certainly be abused, but it can also be very useful. This use is the most common one:

for (int i=0; i<10; i++, doSomethingElse())
{
  /* whatever */
}

But you can use this operator anywhere. Observe:

int j = (printf("Assigning variable j\n"), getValueFromSomewhere());

Each statement is evaluated, but the value of the expression will be that of the last statement evaluated.

Answer 4

initializing structure to zero

struct mystruct a = {0};

this will zero all stucture elements.

Answer 5

Function pointers. You can use a table of function pointers to implement, e.g., fast indirect-threaded code interpreters (FORTH) or byte-code dispatchers, or to simulate OO-like virtual methods.

Then there are hidden gems in the standard library, such as qsort(),bsearch(), strpbrk(), strcspn() [the latter two being useful for implementing a strtok() replacement].

A misfeature of C is that signed arithmetic overflow is undefined behavior (UB). So whenever you see an expression such as x+y, both being signed ints, it might potentially overflow and cause UB.

Answer 6

Multi-character constants:

int x = 'ABCD';

This sets x to 0x41424344 (or 0x44434241, depending on architecture).

EDIT: This technique is not portable, especially if you serialize the int. However, it can be extremely useful to create self-documenting enums. e.g.

enum state {
    stopped = 'STOP',
    running = 'RUN!',
    waiting = 'WAIT',
};

This makes it much simpler if you're looking at a raw memory dump and need to determine the value of an enum without having to look it up.

Answer 7

I never used bit fields but they sound cool for ultra-low-level stuff.

struct cat {
    unsigned int legs:3;  // 3 bits for legs (0-4 fit in 3 bits)
    unsigned int lives:4; // 4 bits for lives (0-9 fit in 4 bits)
    // ...
};

cat make_cat()
{
    cat kitty;
    kitty.legs = 4;
    kitty.lives = 9;
    return kitty;
}

This means that sizeof(cat) can be as small as sizeof(char).

---

Incorporated comments by Aaron and leppie, thanks guys.

Answer 8

C has a standard but not all C compilers are fully compliant (I've not seen any fully compliant C99 compiler yet!).

That said, the tricks I prefer are those that are non-obvious and portable across platforms as they rely on the C semantic. They usually are about macros or bit arithmetic.

For example: swapping two unsigned integer without using a temporary variable:

...
a ^= b ; b ^= a; a ^=b;
...

or "extending C" to represent finite state machines like:

FSM {
  STATE(x) {
    ...
    NEXTSTATE(y);
  }

  STATE(y) {
    ...
    if (x == 0) 
      NEXTSTATE(y);
    else 
      NEXTSTATE(x);
  }
}

that can be achieved with the following macros:

#define FSM
#define STATE(x)      s_##x :
#define NEXTSTATE(x)  goto s_##x

In general, though, I don't like the tricks that are clever but make the code unnecessarily complicated to read (as the swap example) and I love the ones that make the code clearer and directly conveying the intention (like the FSM example).

Answer 9

Interlacing structures like Duff's Device:

strncpy(to, from, count)
char *to, *from;
int count;
{
    int n = (count + 7) / 8;
    switch (count % 8) {
    case 0: do { *to = *from++;
    case 7:      *to = *from++;
    case 6:      *to = *from++;
    case 5:      *to = *from++;
    case 4:      *to = *from++;
    case 3:      *to = *from++;
    case 2:      *to = *from++;
    case 1:      *to = *from++;
               } while (--n > 0);
    }
}

Answer 10

I'm very fond of designated initializers, added in C99 (and supported in gcc for a long time):

#define FOO 16
#define BAR 3

myStructType_t myStuff[] = {
    [FOO] = { foo1, foo2, foo3 },
    [BAR] = { bar1, bar2, bar3 },
    ...

The array initialization is no longer position dependent. If you change the values of FOO or BAR, the array initialization will automatically correspond to their new value.

Answer 11

C99 has some awesome any-order structure initialization.

struct foo{
  int x;
  int y;
  char* name;
};

void main(){
  struct foo f = { .y = 23, .name = "awesome", .x = -38 };
}

Answer 12

anonymous structures and arrays is my favourite one. (cf. http://www.run.montefiore.ulg.ac.be/~martin/resources/kung-f00.html)

setsockopt(yourSocket, SOL_SOCKET, SO_REUSEADDR, (int[]){1}, sizeof(int));

or

void myFunction(type* values) {
    while(*values) x=*values++;
}
myFunction((type[]){val1,val2,val3,val4,0});

it can even be used to instanciate linked lists...

Answer 13

gcc has a number of extensions to the C language that I enjoy, which can be found here. Some of my favorites are function attributes. One extremely useful example is the format attribute. This can be used if you define a custom function that takes a printf format string. If you enable this function attribute, gcc will do checks on your arguments to ensure that your format string and arguments match up and will generate warnings or errors as appropriate.

int my_printf (void *my_object, const char *my_format, ...)
            __attribute__ ((format (printf, 2, 3)));

Answer 14

the (hidden) feature that "shocked" me when I first saw is about printf. this feature allows you to use variables for formatting format specifiers themselves. look for the code, you will see better:

#include <stdio.h>

int main() {
    int a = 3;
    float b = 6.412355;
    printf("%.*f\n",a,b);
    return 0;
}

the * character achieves this effect.

Answer 15

Well... I think that one of the strong points of C language is its portability and standardness, so whenever I find some "hidden trick" in the implementation I am currently using, I try not to use it because I try to keep my C code as standard and portable as possible.

Answer 16

Compile-time assertions, as already discussed here.

//--- size of static_assertion array is negative if condition is not met
#define STATIC_ASSERT(condition) \
    typedef struct { \
        char static_assertion[condition ? 1 : -1]; \
    } static_assertion_t

//--- ensure structure fits in 
STATIC_ASSERT(sizeof(mystruct_t) <= 4096);

Answer 17

Constant string concatenation

I was quite surprised not seeing it allready in the answers, as all compilers I know of support it, but many programmers seems to ignore it. Sometimes it's really handy and not only when writing macros.

Use case I have in my current code: I have a #define PATH "/some/path/" in a configuration file (really it is setted by the makefile). Now I want to build the full path including filenames to open ressources. It just goes to:

fd = open(PATH "/file", flags);

Instead of the horrible, but very common:

char buffer[256];
snprintf(buffer, 256, "%s/file", PATH);
fd = open(buffer, flags);

Notice that the common horrible solution is:

• three times as long
• much less easy to read
• much slower
• less powerfull at it set to an arbitrary buffer size limit (but you would have to use even longer code to avoid that without constant strings contatenation).
• use more stack space

Answer 18

Well, I've never used it, and I'm not sure whether I'd ever recommend it to anyone, but I feel this question would be incomplete without a mention of Simon Tatham's co-routine trick.

Answer 19

When initializing arrays or enums, you can put a comma after the last item in the initializer list. e.g:

int x[] = { 1, 2, 3, };

enum foo { bar, baz, boom, };

This was done so that if you're generating code automatically you don't need to worry about eliminating the last comma.

Answer 20

Struct assignment is cool. Many people don't seem to realize that structs are values too, and can be assigned around, there is no need to use memcpy(), when a simple assignment does the trick.

For example, consider some imaginary 2D graphics library, it might define a type to represent an (integer) screen coordinate:

typedef struct {
   int x;
   int y;
} Point;

Now, you do things that might look "wrong", like write a function that creates a point initialized from function arguments, and returns it, like so:

Point point_new(int x, int y)
{
  Point p;
  p.x = x;
  p.y = y;
  return p;
}

This is safe, as long (of course) as the return value is copied by value using struct assignment:

Point origin;
origin = point_new(0, 0);

In this way you can write quite clean and object-oriented-ish code, all in plain standard C.

Answer 21

Strange vector indexing:

int v[100]; int index = 10; 
/* v[index] it's the same thing as index[v] */

Answer 22

C compilers implement one of several standards. However, having a standard does not mean that all aspects of the language are defined. Duff's device, for example, is a favorite 'hidden' feature that has become so popular that modern compilers have special purpose recognition code to ensure that optimization techniques do not clobber the desired effect of this often used pattern.

In general hidden features or language tricks are discouraged as you are running on the razor edge of whichever C standard(s) your compiler uses. Many such tricks do not work from one compiler to another, and often these kinds of features will fail from one version of a compiler suite by a given manufacturer to another version.

Various tricks that have broken C code include:

1. Relying on how the compiler lays out structs in memory.
2. Assumptions on endianness of integers/floats.
3. Assumptions on function ABIs.
4. Assumptions on the direction that stack frames grow.
5. Assumptions about order of execution within statements.
6. Assumptions about order of execution of statements in function arguments.
7. Assumptions on the bit size or precision of short, int, long, float and double types.

Other problems and issues that arise whenever programmers make assumptions about execution models that are all specified in most C standards as 'compiler dependent' behavior.

Answer 23

When using sscanf you can use %n to find out where you should continue to read:

sscanf ( string, "%d%n", &number, &length );
string += length;

Apparently, you can't add another answer, so I'll include a second one here, you can use "&&" and "||" as conditionals:

#include <stdio.h>
#include <stdlib.h>

int main()
{
   1 || puts("Hello\n");
   0 || puts("Hi\n");
   1 && puts("ROFL\n");
   0 && puts("LOL\n");

   exit( 0 );
}

This code will output:

Hi
ROFL

Answer 24

using INT(3) to set break point at the code is my all time favorite

Answer 25

My favorite "hidden" feature of C, is the usage of %n in printf to write back to the stack. Normally printf pops the parameter values from the stack based on the format string, but %n can write them back.

Check out section 3.4.2 here. Can lead to a lot of nasty vulnerabilities.

Answer 26

Compile-time assumption-checking using enums: Stupid example, but can be really useful for libraries with compile-time configurable constants.

#define D 1
#define DD 2

enum CompileTimeCheck
{
    MAKE_SURE_DD_IS_TWICE_D = 1/(2*(D) == (DD)),
    MAKE_SURE_DD_IS_POW2    = 1/((((DD) - 1) & (DD)) == 0)
};

Answer 27

Gcc (c) has some fun features you can enable, such as nested function declarations, and the a?:b form of the ?: operator, which returns a if a is not false.

Answer 28

I discoverd recently 0 bitfields.

struct {
  int    a:3;
  int    b:2;
  int     :0;
  int    c:4;
  int    d:3;
};

which will give a layout of

000aaabb 0ccccddd

instead of without the :0;

0000aaab bccccddd

The 0 width field tells that the following bitfields should be set on the next atomic entity (char)

Answer 29

C99-style variable argument macros, aka

#define ERR(name, fmt, ...)   fprintf(stderr, "ERROR " #name ": " fmt "\n", \
                                  __VAR_ARGS__)

which would be used like

ERR(errCantOpen, "File %s cannot be opened", filename);

Here I also use the stringize operator and string constant concatentation, other features I really like.

Answer 30

Variable size automatic variables are also useful in some cases. These were added i nC99 and have been supported in gcc for a long time.

void foo(uint32_t extraPadding) {
    uint8_t commBuffer[sizeof(myProtocol_t) + extraPadding];

You end up with a buffer on the stack with room for the fixed-size protocol header plus variable size data. You can get the same effect with alloca(), but this syntax is more compact.

You have to make sure extraPadding is a reasonable value before calling this routine, or you end up blowing the stack. You'd have to sanity check the arguments before calling malloc or any other memory allocation technique, so this isn't really unusual.