🔴 短读：转着圈读，读够为止

（本条路线统一用 gcc -std=c11 -O0 -Wall 编译）⚠️ 这一节用 POSIX 的 open/read/write，在 -std=c11 下**必须**先写一行 #define _POSIX_C_SOURCE 200809L，而且要写在所有 #include 前面。

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

/* read 一次不一定读满，要转着圈读到够为止 —— 这就是"短读" */
static ssize_t read_all(int fd, char *buf, size_t want) {
    size_t got = 0;
    int rounds = 0;
    while (got < want) {
        ssize_t n = read(fd, buf + got, want - got);
        rounds++;
        if (n <= 0) break;
        got += (size_t)n;
    }
    (void)rounds;
    return (ssize_t)got;
}

int main(void) {
    char path[] = "/tmp/l6XXXXXX";
    int fd = mkstemp(path);
    if (fd < 0) { printf("建不出\n"); return 1; }

    const char *msg = "abcdefghij";           /* 10 个字节 */
    write(fd, msg, strlen(msg));
    lseek(fd, 0, SEEK_SET);

    char buf[32] = {0};
    ssize_t got = read_all(fd, buf, 10);
    close(fd);
    unlink(path);

    printf("%ld/%d\n", (long)got, strcmp(buf, msg) == 0);
    return 0;
}