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C语言中使用scanf的一些注意事项

(1)scanf读取double类型数据应该使用%lf;而使用printf打印double时,要注意%lf用在printf中是C99才支持的,因此如果编译器不支持C99则要使用%f

double d;
scanf("%lf", &d);
printf("%f", d);

参考资料:
Reading in double values with scanf in c
Why does scanf() need “%lf” for doubles, when printf() is okay with just “%f”?

(2)

char str[10];
scanf ("%[^\n]%*c", str);

%[^\n]含义是从stdin读取输入保存到str,直到遇到第一个\n;而%*c则丢弃掉这个\n
参考资料:
scanf: “%[^\n]” skips the 2nd input but “ %[^\n]” does not. why?
What does scanf(“%*[^\n]%*c”) mean?

(3)限制输入字符串的长度,预留结尾的NUL

char str[10]
scanf("%9s", str);

得到一个有符号数的符号位

Compute the sign of an integer描述了如何得到一个有符号数的符号位:

int v;      // we want to find the sign of v
int sign;   // the result goes here 

// CHAR_BIT is the number of bits per byte (normally 8).
sign = -(v < 0);  // if v < 0 then -1, else 0. 
// or, to avoid branching on CPUs with flag registers (IA32):
sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
// or, for one less instruction (but not portable):
sign = v >> (sizeof(int) * CHAR_BIT - 1); 

关于方法1

sign = -(v < 0);  // if v < 0 then -1, else 0. 

参考C规范6.5.8:6

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.92) The result has type int.

因此(v < 0)会得到10

关于方法23

// or, to avoid branching on CPUs with flag registers (IA32):
sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
// or, for one less instruction (but not portable):
sign = v >> (sizeof(int) * CHAR_BIT - 1); 

参考stackoverflow这篇帖子。因为有符号数类型中的负数右移是C规范中未定义行为,取决与具体体系结构的实现,因此需要先把它转化成无符号数进行右移操作:

((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1))

Linux系统中locale相关的笔记

 下列笔记摘自The Linux programming interface

(1)

The actual locales that are defined on a system can vary. SUSv3 doesn’t make any requirements about this, except that a standard locale called POSIX (and synonymously, C, a name that exists for historical reasons) must be defined. This locale mirrors the historical behavior of UNIX systems. Thus, it is based on an ASCII character set, and uses English for names of days and months, and for yes/no responses. The monetary and numeric components of this locale are undefined.

The locale command displays information about the current locale environment (within the shell). The command locale –a lists the full set of locales defined on the system.

(2)

There are two different methods of setting the locale using setlocale(). The locale argument may be a string specifying one of the locales defined on the system (i.e., the name of one of the subdirectories under /usr/lib/locale), such as de_DE or en_US. Alternatively, locale may be specified as an empty string, meaning that locale settings should be taken from environment variables:

setlocale(LC_ALL, “”);

We must make this call in order for a program to be cognizant of the locale environment variables. If the call is omitted, these environment variables will have no effect on the program.

以下摘自stackoverflow.com

A C program inherits its locale environment variables when it starts up. This happens automatically. However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard ‘C’ locale.

理解C语言中关于数组地址值的两个程序

程序1:

#include <stdio.h>

char ga[] = "abcdefghijklm";

void my_array_func(char ca[10]) {
    printf("%#x, %d\n", &ca, sizeof(ca));
    printf("%#x\n", &(ca[0]));
    printf("%#x\n", &(ca[1]));
    printf("%#x\n\n", ++ca);
}

void my_poiner_func(char *pa) {
    printf("%#x, %d\n", &pa, sizeof(pa));
    printf("%#x\n", &(pa[0]));
    printf("%#x\n", &(pa[1]));
    printf("%#x\n\n", ++pa);
}

int main(void) {
    printf("%#x\n", &ga);
    printf("%#x\n", &(ga[0]));
    printf("%#x\n\n", &(ga[1]));
    my_array_func(ga);
    my_poiner_func(ga);
    return 0;
}

运行结果:

0x804987c
0x804987c
0x804987d

0xff95b040, 4
0x804987c
0x804987d
0x804987d

0xff95b040, 4
0x804987c
0x804987d
0x804987d

参考Passing an array as an argument to a function in C

程序2:

#include <stdio.h>

int main(void) {
    // your code goes here
    int array[5];
    printf("%p, %d\n", array, sizeof(array));
    printf("%p, %d\n", &array[0], sizeof(&array[0]));
    printf("%p, %d\n", &array, sizeof(&array));
    printf("%p\n", array + 1);
    printf("%p\n", &array[0] + 1);
    printf("%p\n", &array + 1);
    return 0;
}

运行结果:

0xffd062ec, 20
0xffd062ec, 4
0xffd062ec, 4
0xffd062f0
0xffd062f0
0xffd06300

参考How come an array’s address is equal to its value in C?

C语言中的“#”和“##”

参考stackoverflow上的这篇帖子

#include <stdio.h>
#define f(a,b) a##b
#define g(a)   #a
#define h(a) g(a)

int main()
{
        printf("%s\n",h(f(1,2)));
        printf("%s\n",g(f(1,2)));
        return 0;
}

运行结果如下:

12
f(1,2)

#a会创建一个字符串:"a"a##b会创建一个新的tokenab。关于macro的处理,参加下列解释:

An occurrence of a parameter in a function-like macro, unless it is the operand of # or ##, is expanded before substituting it and rescanning the whole for further expansion. Because g’s parameter is the operand of #, the argument is not expanded but instead immediately stringified (“f(1,2)”). Because h’s parameter is not the operand of # nor ##, the argument is first expanded (12), then substituted (g(12)), then rescanning and further expansion occurs (“12”).

总结一下,对于function-like macro的参数,如果是#或者##的操作数,则会马上扩展为相应的字符串;否则会先扩展,再替换,循环此操作,直到完成。因此对于g(f(1,2)),会马上替换成"f(1,2)"这个字符串。而对于h(f(1,2)),由于其参数不是#..或者..#..,所以会先把f(1,2)扩展成12,然后替换变成g(12),最后变成"12"

FreeBSD kernel 笔记(9)——modeventtype_t定义

modeventtype_t定义如下:

typedef enum modeventtype {
    MOD_LOAD,
    MOD_UNLOAD,
    MOD_SHUTDOWN,
    MOD_QUIESCE
} modeventtype_t;
typedef int (*modeventhand_t)(module_t, int /* modeventtype_t */, void *);

MOD_LOADMOD_UNLOADMOD_SHUTDOWN都好理解。分别是在加载,卸载模块,还有关机时传入模块处理函数的值。而关于MOD_QUIESCE可以参考FreeBSD Device Drivers

When one issues the kldunload(8) command, MOD_QUIESCE is run before MOD_UNLOAD . If MOD_QUIESCE returns an error, MOD_UNLOAD does not get executed. In other words, MOD_QUIESCE verifies that it is safe to unload your module.

NOTE The kldunload -f command ignores every error returned by MOD_QUIESCE . So you can always unload a module, but it may not be the best idea.

另外,关于MOD_QUIESCEMOD_SHUTDOWN区别,也可参考FreeBSD Kernel Developer’s Manual

The difference between MOD_QUIESCE and MOD_UNLOAD is that the module should fail MOD_QUIESCE if it is currently in use, whereas MOD_UNLOAD should only fail if it is impossible to unload the module, for instance because there are memory references to the module which cannot be revoked.

FreeBSD kernel 笔记(8)——双向链表

FreeBSD kernel提供了对双向链表的支持(定义在sys/sys/queue.h中):

/*
 * List declarations.
 */
#define LIST_HEAD(name, type)                       \
struct name {                               \
    struct type *lh_first;  /* first element */         \
}

#define LIST_CLASS_HEAD(name, type)                 \
struct name {                               \
    class type *lh_first;   /* first element */         \
}

#define LIST_HEAD_INITIALIZER(head)                 \
    { NULL }

#define LIST_ENTRY(type)                        \
struct {                                \
    struct type *le_next;   /* next element */          \
    struct type **le_prev;  /* address of previous next element */  \
}

#define LIST_CLASS_ENTRY(type)                      \
struct {                                \
    class type *le_next;    /* next element */          \
    class type **le_prev;   /* address of previous next element */  \
}

#define LIST_EMPTY(head)    ((head)->lh_first == NULL)

#define LIST_FIRST(head)    ((head)->lh_first)

#define LIST_FOREACH(var, head, field)                  \
    for ((var) = LIST_FIRST((head));                \
        (var);                          \
        (var) = LIST_NEXT((var), field))

#define LIST_NEXT(elm, field)   ((elm)->field.le_next)

#define LIST_INSERT_HEAD(head, elm, field) do {             \
    QMD_LIST_CHECK_HEAD((head), field);             \
    if ((LIST_NEXT((elm), field) = LIST_FIRST((head))) != NULL) \
        LIST_FIRST((head))->field.le_prev = &LIST_NEXT((elm), field);\
    LIST_FIRST((head)) = (elm);                 \
    (elm)->field.le_prev = &LIST_FIRST((head));         \
} while (0)

......

FreeBSD Device Drivers代码为例:

(1)race_softc结构体定义:

struct race_softc {
    LIST_ENTRY(race_softc) list;
    int unit;
};

展开以后变成如下代码:

struct race_softc {
    struct { \
        struct race_softc *le_next; /* next element */          \
        struct race_softc **le_prev;    /* address of previous next element */  \
    } list;
    int unit;
};

(2)双向链表头定义:

static LIST_HEAD(, race_softc) race_list = LIST_HEAD_INITIALIZER(&race_list);

展开以后变成如下代码:

struct {struct race_softc *lh_first;} race_list = {NULL};

(3)插入一个元素:

sc = (struct race_softc *)malloc(sizeof(struct race_softc), M_RACE, M_WAITOK | M_ZERO);
sc->unit = unit;    
LIST_INSERT_HEAD(&race_list, sc, list);

展开以后变成如下代码:

sc = (struct race_softc *)malloc(sizeof(struct race_softc), M_RACE, M_WAITOK | M_ZERO);
sc->unit = unit;
do {                \
    QMD_LIST_CHECK_HEAD((race_list), list);             \
    if ((LIST_NEXT((sc), list) = LIST_FIRST((race_list))) != NULL)  \
        LIST_FIRST((race_list))->list.le_prev = &LIST_NEXT((sc), list);\
    LIST_FIRST((race_list)) = (sc);                 \
    (sc)->list.le_prev = &LIST_FIRST((race_list));          \
} while (0)

展开以后变成如下代码:

do { 
    if (((((sc))->list.le_next) = (((&race_list))->lh_first)) != ((void *)0)) (((&race_list))->lh_first)->list.le_prev = &(((sc))->list.le_next); 
    (((&race_list))->lh_first) = (sc); 
    (sc)->list.le_prev = &(((&race_list))->lh_first); 
} while (0);

即把元素插在链表头部。因为sc位于链表头部,所以其list.le_prev指向它自己。

FreeBSD kernel 笔记(6)——设备通信和控制

FreeBSD系统上,设备通信和控制主要通过sysctlioctl接口:

Generally, sysctls are employed to adjust parameters, and ioctls are used for everything else—that’s why ioctls are the catchall of I/O operations.

ioctl比较简单,不在这里赘述。

要在kernel模块中增加对sysctl的支持,首先要调用sysctl_ctx_init初始化一个sysctl_ctx_list结构体(使用完,通过sysctl_ctx_free来进行释放);然后使用SYSCTL_ADD_*系列函数加入系统支持的参数。需要注意的是,SYSCTL_ADD_*系列函数的第二个参数用来指定新加入参数属于哪个parent node,可以使用下面两个macro来指定其位置:SYSCTL_STATIC_CHILDRENSYSCTL_CHILDREN(如果SYSCTL_STATIC_CHILDREN没有参数,则会新增加一个系统的top-level category)。

另外,SYSCTL_ADD_PROC会增加一个处理函数。其参数是SYSCTL_HANDLER_ARGS

#define SYSCTL_HANDLER_ARGS struct sysctl_oid *oidp, void *arg1,    \
    intptr_t arg2, struct sysctl_req *req

arg1指向sysctl命令需要处理的数据,arg2指向数据的长度。

参考资料:
FreeBSD Device Drivers

Freecolor工具简介

FreeBSD没有free命令,但是提供了一个freecolor命令,用来查看还有多少可用的内存。Freecolor官网在这里,其代码逻辑比较简单。如果系统提供/proc/meminfo,就直接从这个虚拟文件系统获取信息,否则就透过libstatgrab这个第三方库。因为FreeBSD默认不会挂载/proc文件系统,所以会使用libstatgrab

Libstatgrab获得FreeBSD系统内存使用状况的代码:

#elif defined(FREEBSD) || defined(DFBSD)
    /*returns pages*/
    size = sizeof(total_count);
    if (sysctlbyname("vm.stats.vm.v_page_count", &total_count, &size, NULL, 0) < 0) {
        RETURN_WITH_SET_ERROR_WITH_ERRNO("mem", SG_ERROR_SYSCTLBYNAME, "vm.stats.vm.v_page_count");
    }

    /*returns pages*/
    size = sizeof(free_count);
    if (sysctlbyname("vm.stats.vm.v_free_count", &free_count, &size, NULL, 0) < 0) {
        RETURN_WITH_SET_ERROR_WITH_ERRNO("mem", SG_ERROR_SYSCTLBYNAME, "vm.stats.vm.v_free_count");
    }

    size = sizeof(inactive_count);
    if (sysctlbyname("vm.stats.vm.v_inactive_count", &inactive_count , &size, NULL, 0) < 0) {
        RETURN_WITH_SET_ERROR_WITH_ERRNO("mem", SG_ERROR_SYSCTLBYNAME, "vm.stats.vm.v_inactive_count");
    }

    size = sizeof(cache_count);
    if (sysctlbyname("vm.stats.vm.v_cache_count", &cache_count, &size, NULL, 0) < 0) {
        RETURN_WITH_SET_ERROR_WITH_ERRNO("mem", SG_ERROR_SYSCTLBYNAME, "vm.stats.vm.v_cache_count");
    }

    /* Of couse nothing is ever that simple :) And I have inactive pages to
     * deal with too. So I'm going to add them to free memory :)
     */
    mem_stats_buf->cache = (size_t)cache_count;
    mem_stats_buf->cache *= (size_t)sys_page_size;
    mem_stats_buf->total = (size_t)total_count;
    mem_stats_buf->total *= (size_t)sys_page_size;
    mem_stats_buf->free = (size_t)free_count + inactive_count + cache_count;
    mem_stats_buf->free *= (size_t)sys_page_size;
    mem_stats_buf->used = mem_stats_buf->total - mem_stats_buf->free;
#elif defined(WIN32)

可以看到,所有free_countinactive_countcache_count都算作free,即可用的内存。

而获得swap使用率则通过kvm接口:

#elif defined(ALLBSD)
    /* XXX probably not mt-safe! */
    kvmd = kvm_openfiles(NULL, NULL, NULL, O_RDONLY, NULL);
    if(kvmd == NULL) {
        RETURN_WITH_SET_ERROR("swap", SG_ERROR_KVM_OPENFILES, NULL);
    }

    if ((kvm_getswapinfo(kvmd, &swapinfo, 1,0)) == -1) {
        kvm_close( kvmd );
        RETURN_WITH_SET_ERROR("swap", SG_ERROR_KVM_GETSWAPINFO, NULL);
    }

    swap_stats_buf->total = (long long)swapinfo.ksw_total;
    swap_stats_buf->used = (long long)swapinfo.ksw_used;
    kvm_close( kvmd );

    swap_stats_buf->total *= sys_page_size;
    swap_stats_buf->used *= sys_page_size;
    swap_stats_buf->free = swap_stats_buf->total - swap_stats_buf->used;
#elif defined(WIN32)

C语言中的XOR运算符

Stackoverflow上一个回答很好地解释了XOR运算符的作用:

If you know how XOR works, and you know that ^ is XOR in C, then this should be pretty simple. You should know that XOR will flip bits where 1 is set, bits 2 and 5 of 0b00100100 are set, therefore it will flip those bits.

From an “during the test” standpoint, let’s say you need to prove this to yourself, you really don’t need to know the initial value of star to answer the question, If you know how ^ works then just throw anything in there:

 00100100
^10101010  (star's made up value)
---------
 10001110  (star's new value)

 bit position: | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0  
   |---|---|---|---|---|---|---|---
 star's new v: | 1 | 0 | 0 | 0 | 1 | 1 | 1 | 0
   |---|---|---|---|---|---|---|---
 star's old v: | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 0

总结一下,就是第二个操作数用来控制第一个操作数:第二个操作数中bit1会导致第一个操作数中相应的bit发生反转,而bit0则会让第一个操作数中相应的bit不变。

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