MacOS下pwrite无法O_APPEND的问题

这个问题来自Swoole的一个issue。

有如下代码：

#include <sys/file.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char const *argv[]) {
    int flags = 0;
    int fd;

    flags = O_CREAT | O_WRONLY;
    fd = open("test.txt", flags, 0644);
    pwrite(fd, "first line\n", strlen("first line\n"), 0);

    flags = O_APPEND | O_CREAT | O_WRONLY;
    fd = open("test.txt", flags, 0644);
    pwrite(fd, "second line\n", strlen("second line\n"), 0);

    flags = O_APPEND | O_CREAT | O_WRONLY;
    fd = open("test.txt", flags, 0644);
    pwrite(fd, "third line\n", strlen("third line\n"), 0);

    return 0;
}

此时，test.txt文件里面的内容是：

third line

我们发现，这实际上没有追加，而是覆盖了之前写入的内容。也就意味着pwrite的offset和O_APPEND没有一起起到作用。

我们换成write来测试追加：

#include <sys/file.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char const *argv[]) {
    int flags = 0;
    int fd;

    flags = O_CREAT | O_WRONLY;
    fd = open("test.txt", flags, 0644);
    write(fd, "first line\n", strlen("first line\n"));

    flags = O_APPEND | O_CREAT | O_WRONLY;
    fd = open("test.txt", flags, 0644);
    write(fd, "second line\n", strlen("second line\n"));

    flags = O_APPEND | O_CREAT | O_WRONLY;
    fd = open("test.txt", flags, 0644);
    write(fd, "third line\n", strlen("third line\n"));

    return 0;
}

此时，test.txt文件里面的内容是：

first line
second line
third line

追加成功了。