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Ansi c and posix

ANSI-C & POSIX

memmove

与memcpy区别在于允许from和to区域重叠时的行为是可预期的, 不会出现错误.

size为0时行为未定义.

dup2实现重定向

把标准输出重定向到fd:

int fd = open("output.txt", O_RDWR|O_CREAT, 0666);
int replaced = dup2(fd, STDOUT_FILENO);

printf格式

フォーマット
ssize_t %zd
size_t %zu
int_max_t %jd
intmax_t %ju
signed char %hhd
unsigned char %hhu
short int %hd
unsigned short int %hu
int %d
unsigned int %u
long int %ld
unsigned long int %lu
long long int %lld
unsigned long long int %llu

getopt()

    #include <unistd.h>

    const char *optstring = "ab:c"; // 参数有-a -b -c其中-b必须跟参数
    while ((opt = getopt(argc, argv, optstring)) != -1) {
        printf("opt = %c\n", opt);  // 命令参数,亦即 -a -b -c -d
        printf("optarg = %s\n", optarg); // 参数内容
        printf("optind = %d\n", optind); // 下一个被处理的下标值
        printf("argv[optind - 1] = %s\n\n",  argv[optind - 1]); // 参数内容 
    }

getopt_long()

参考:25.2.4 Example of Parsing Long Options with getopt_long

getopt()的加强版,同时支持长短参数,当匹配到长参数时,会返回struct option项中的val值,如下面例子中的-h

#include <getopt.h>


static void print_usage(const char *prog)
{
    printf("usage: %s [Options]\n\n", prog);
    printf(
        "Options:\n"
        "  --dump-video=<file>          dump video stream to file\n"
        "  -h                           print help\n"
    );
}

#define ARG_DUMP_VIDEO    1
static struct option long_options[] = {
    {"dump-video", required_argument, 0, ARG_DUMP_VIDEO},
    {"help",       no_argument,       0, 'h'},
    {0, 0, 0, 0}
};

int main(int argc, char **argv)
{   
    int c, opt_index = 0;

    while (1) {
        c = getopt_long(argc, argv, "h", long_options, &opt_index);
        if (c == -1)
            break;

        switch (c) {
            case ARG_DUMP_VIDEO:
                printf("dump-video!!! %s\n", optarg);
                break;
            case 'h':
                print_usage(argv[0]);
                exit(0);
                break;
            default:
                exit(1);
                break;
        }
    }