좋은 시스템 프로그래밍 문서 소개

#include <stdio.h>
#include <unistd.h>
 
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
 
#include <string.h>
#include <wait.h>
 
 
int file_lock(int fd)
{
    struct flock lock;
    int rc;
 
    lock.l_whence = SEEK_SET;
    lock.l_start  = 0;
    lock.l_len    = 0;
    lock.l_pid    = 0;
    lock.l_type   = F_WRLCK;
 
    if ((rc = fcntl(fd, F_SETLKW, &lock)) != 0)
    {
        return errno;
    }
 
    return rc;
}
 
int file_unlock(int fd)
{
    struct flock lock;
    int rc;
 
    lock.l_whence = SEEK_SET;
    lock.l_start  = 0;
    lock.l_len    = 0;
    lock.l_pid    = 0;
    lock.l_type   = F_UNLCK;
 
    if ((rc = fcntl(fd, F_SETLKW, &lock)) != 0)
    {
        return errno;
    }
 
    return rc;
}   
 
#define ORIG_FILE_MSG "<CHILD>THIS IS ORIG FILE\n"
#define NEW_FILE_MSG "<CHILD>THIS IS NEW FILE\n"
 
int rename_process(void)
{
    int orig_fd;
 
    orig_fd = open("file.txt",O_WRONLY|O_CREAT, S_IRWXU);
    if (orig_fd < 0)
    {
        perror("OPEN FAIL");
        return orig_fd;
    }
    printf("CHILD OPEN file.txt\n");
 
    if (file_lock(orig_fd) < 0)
    {
        perror("LOCK FAIL");
        close(orig_fd);
    }
 
    printf("CHILD<%d> lock file\n", getpid());
 
    if (write(orig_fd, ORIG_FILE_MSG, strlen(ORIG_FILE_MSG)) <= 0)
    {
        perror("WRITE ORIG-MSG ERROR");
    }
 
    printf("CHILD RENAME FILE!!--->LOCK IS OK??\n");
 
    if (rename("file.txt", "file.bak") < 0)
    {
        perror("RENAME FAIL");
        close(orig_fd);
    }
 
    /* File descriptor is the same,
       and this message is written at file.bak */
 
    if (write(orig_fd, NEW_FILE_MSG, strlen(NEW_FILE_MSG)) <= 0)
    {
        perror("WRITE NEW-MSG ERROR");
    }
 
    /* make parent try lock file */
    sleep(10);
 
    printf("CHILD<%d> unlock file\n", getpid());
 
    if (file_unlock(orig_fd) < 0)
    {
        perror("UNLOCK FAIL");
        close(orig_fd);
    }
 
    /* wake other processes */
    sleep(3);
 
    close(orig_fd);
    return 0;
}
 
int main(void)
{
    pid_t childp;
 
    childp = fork();
    if (childp == 0)
    {
        printf("CHILD<%d> START\n", getpid());
        rename_process();
        printf("CHILD<%d> END\n", getpid());
    }
    else if (childp > 0)
    { 
        int orig_fd;
        char readbuf[128];
        int waitstat;
        struct stat orig_stat;
 
        printf("PARENT<%d> START\n", getpid());
 
        orig_fd = open("file.txt",O_WRONLY|O_CREAT|O_TRUNC, S_IRWXU);
        if (orig_fd < 0)
        {
            perror("OPEN FAIL");
            return orig_fd;
        }
        printf("PARENT OPEN file.txt\n");
 
        /* child lock&rename file first! */
        sleep(3);
 
        if (file_lock(orig_fd) < 0)
        {
            perror("PARENT LOCK FAIL");
            close(orig_fd);
        }
        printf("PARENT<%d> lock file\n", getpid());
 
        if (stat("file.txt", &orig_stat) == 0)
        {
            printf("PARENT READ FILE SIZE of file.txt: %d\n",
                   orig_stat.st_size);
        }
        else
        {
            perror("STAT FAIL");
        }
 
        if (fstat(orig_fd, &orig_stat) == 0)
        {
            printf("PARENT READ FILE SIZE of file.txt: %d\n",
                   orig_stat.st_size);
        }
        else
        {
            perror("STAT FAIL");
        }
 
 
        if (read(orig_fd, readbuf, 127) > 0)
        {
            printf("PARENT READ <%s>\n", readbuf);
        }
        else
        {
            printf("PARENT CANNOT READ\n");
        }
 
 
 
        if (write(orig_fd, "HELLO", 5) <= 0)
        {
            perror("WRITE ORIG-MSG ERROR");
        }
 
        /* File descriptor is the same,
           and this message is written at file.bak */
 
        if (write(orig_fd, "BYE", 3) <= 0)
        {
            perror("WRITE NEW-MSG ERROR");
        }
 
 
        if (read(orig_fd, readbuf, 127) > 0)
        {
            printf("PARENT READ <%s>\n", readbuf);
        }
        else
        {
            printf("PARENT CANNOT READ\n");
        }
 
        printf("PARENT<%d> unlock file\n", getpid());
 
        if (file_unlock(orig_fd) < 0)
        {
            perror("UNLOCK FAIL");
            close(orig_fd);
        }
 
        close(orig_fd);
        printf("PARENT<%d> END\n", getpid());
 
        wait(&waitstat);
    }
    else
    {
        perror("WARNING! fork failed\n");
    }
 
    return 0;
}

CHILD<20189> START
CHILD OPEN file.txt
CHILD<20189> lock file
CHILD RENAME FILE!!--->LOCK IS OK??   ===> 자식 프로세스가 파일 이름 변경
PARENT<20188> START
PARENT OPEN file.txt
PARENT<20188> lock file ==============> 자식 프로세스가 파일 락을 안풀었어도 이름이 바꼈으므로 락 가능
STAT FAIL: No such file or directory========> stat콜은 실패함
PARENT READ FILE SIZE of file.txt: 49 ======> fstat 콜은 성공??
PARENT CANNOT READ
PARENT CANNOT READ
PARENT<20188> unlock file
PARENT<20188> END
CHILD<20189> unlock file =============> 락 걸린 파일의 이름이 바뀌면 락이 깨짐!!
CHILD<20189> END
 
gio-desk:rename$ cat file.txt
HELLOBYE =========================> 부모 프로세스는 데이터를 정상적으로 쓰지만 읽지 못함
gio-desk:rename$ cat file.bak
<CHILD>THIS IS ORIG FILE =============> 자식 프로세스는 데이터를 정상적으로 씀
<CHILD>THIS IS NEW FILE
gio-desk:rename$