language

Do you really master c language

##1. Array

  1. Two dimensional array:
    If you have an 2-d array like char buf[50][255] = {0};, and use it for function

    void test(char *buf[])
    {

    printf("buf[0] = %s\n", buf[0]);

    }

It would appear warning as below: 

[dennis@localhost test]$ gcc -o t t.c
t.c: In function ‘main’:
t.c:179:2: warning: passing argument 1 of ‘test’ from incompatible pointer type [enabled by default]
t.c:146:6: note: expected ‘char **’ but argument is of type ‘char (*)[255]’

The correct function format is : void test(char (*buf)[255])

  1. Array initial:

    / This code is from strace’s source code /
    / strace is tool use for trace all the system call of linux binary /
    #include
    #include

    #define PTRACE_EVENT_CLONE 3
    #define PTRACE_EVENT_FORK 1
    #define PTRACE_EVENT_VFORK 2
    #define PTRACE_EVENT_VFORK_DONE 5
    #define PTRACE_EVENT_EXEC 4
    #define PTRACE_EVENT_EXIT 6

    #define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))

    int main(int argc, char* argv[])
    {

    static const char *const event_names[] = {
    [PTRACE_EVENT_CLONE] = "CLONE",
    [PTRACE_EVENT_FORK]  = "FORK",
    [PTRACE_EVENT_VFORK] = "VFORK",
    [PTRACE_EVENT_VFORK_DONE] = "VFORK_DONE",
    [PTRACE_EVENT_EXEC]  = "EXEC",
    [PTRACE_EVENT_EXIT]  = "EXIT",
};

int i=0; 
for (i=0; i<ARRAY_SIZE(event_names); i++)
    printf("event_name[%d] = %s\n", i, event_names[i]);
return 0;

    }

  2. Arrays of Length Zero

    #include
    #include

    struct line {

    int length;
char contents[0]; // C99的玩法是：char contents[]; 没有指定数组长度

    };

    int main(){

    int this_length=10;
struct line *thisline = (struct line *)
                 malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
memset(thisline->contents, 'a', this_length);
free(thisline);
return 0;

    }

##2. point

2.1 void test(char *a)

if want to change the value, use this.

2.2 void test(char **a)

if want to allocate memory, use this.

##3. structure

3.1 initial by bit

struct pid_tag {
    unsigned int inactive : 1;
    unsigned int          : 1; /* fill 1 bit */
    unsigned int refcount : 6;
    unsigned int          : 0; /* fill to next word */
    short pid_id;
    struct pid_tag *link;
}

3.2 alignment

struct st_a {
    int a;
    char b;
    short c;
};

struct st_b {
    char b;
    short c;
    int a;
};

struct st_c {
    char b;
    int a;
    short c;
};

the value of sizeof(st_a) is 8  (address: 0 1 2 3 | 4 . | 6 7)
the value of sizeof(st_b) is 8  (address: 0 . | 2 3 | 4 5 6 7)
the value of sizeof(st_c) is 12 (address: 0 . . . | 4 5 6 7 | 8 9 . .)

Therefore, you should careful for doing structure copy, and socket programming.

##4. Function

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