Question

编译时我收到错误：

cc     holetest.c   -o holetest
holetest.c: In function ‘test_seek’:
holetest.c:48:19: error: ‘SEEK_HOLE’ undeclared (first use in this function)
holetest.c:48:19: note: each undeclared identifier is reported only once for each function it appears in
holetest.c:51:19: error: ‘SEEK_DATA’ undeclared (first use in this function)
make: *** [holetest] Error 1

如果我删除了SEEK_HOLE和SEEK_DATA，我没有任何问题。

我错过了包含或图书馆吗？

生成文件：

all: holetest

holetest: holetest.c
    rm -f holetest
    gcc holetest.c -o holetest

holetest.c：

#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>

#define FILENAME "/tmp/partly.sparse"
#define FILE_SIZE (1<<30)
#define START_STRING "start of file\n"
#define START_LEN strlen(START_STRING)
#define END_STRING "\nend of file\n"
#define END_LEN strlen(END_STRING)

#define debug(M, ...) fprintf(stderr, "%i: DEBUG %10.10s:%3.0d: " M "\n", getpid(), __FILE__, __LINE__, ##__VA_ARGS__); fflush(stderr);
#define log_err(M, ...) fprintf(stderr, "%i: ERROR errno:%i %10.10s:%3.0d: " M "\n", getpid(), errno, __FILE__, __LINE__, ##__VA_ARGS__); fflush(stderr);

#define quit_if(COND, ...) do { \
        if(COND) { \
                log_err(__VA_ARGS__); \
                perror(NULL); \
                exit(errno); \
        } \
} while(0);

int make_partly_sparse(const char *filename, off_t size) {
        int r, fd;
        fd = open(filename, O_RDWR|O_CREAT, 0755);
        quit_if(fd < 1, "unable to create %s", filename);
        r = write(fd, START_STRING, START_LEN);
        quit_if(r < START_LEN, "unable to write %s", filename);
        r = lseek(fd, FILE_SIZE - END_LEN, SEEK_SET);
        quit_if(r < 0, "unable to seek %s", filename);
        r = write(fd, END_STRING, END_LEN);
        quit_if(r < END_LEN, "unable to write %s", filename);
        r = close(fd);
        quit_if(r < 0, "unable to close %s", filename);
        return 0;
}

int test_seek(const char *filename) {
        int r, fd;
        fd = open(filename, O_RDWR|O_CREAT, 0755);
        quit_if(fd < 1, "unable to open %s", filename);
        debug("seeking hole at %li", START_LEN);
        r = lseek(fd, 0, SEEK_HOLE);
        quit_if(r < 0, "unable to seek %s", filename);
        quit_if(r != START_LEN, "SEEK_HOLE unsupported %i", r);
        r = lseek(fd, 0, SEEK_DATA);
        quit_if(r < 0, "unable to seek %s", filename);
        quit_if(r != (FILE_SIZE - END_LEN), "SEEK_DATA unsupported %i", r);
        r = close(fd);
        quit_if(r < 0, "unable to close %s", filename);
        return 0;
}

int main(int argc, char *argv[]) {
        debug("making sparse file: %s", FILENAME);
        make_partly_sparse(FILENAME, FILE_SIZE);
        test_seek(FILENAME);
        return 0;
}

系统：

$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=12.04
DISTRIB_CODENAME=precise
DISTRIB_DESCRIPTION="Ubuntu 12.04.2 LTS"

$ uname -a
Linux tux 3.2.0-45-generic #70-Ubuntu SMP Wed May 29 20:12:06 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

$ grep "ext" /etc/fstab
UUID=be3aacb3-6457-4ba1-92bb-0f63ad514f40 /               ext4    errors=remount-ro 0       1

更新

它现在编译但不起作用，SEEK_HOLE跳过洞并寻找到文件的末尾。

$ make
rm -f holetest
gcc -D_GNU_SOURCE holetest.c -o holetest

$ ./holetest 
18731: DEBUG holetest.c: 60: making sparse file: /tmp/partly.sparse
18731: DEBUG holetest.c: 47: seeking hole at 14
18731: ERROR errno:0 holetest.c: 50: SEEK_HOLE unsupported 1073741824
Success

$ du /tmp/partly.sparse 
8   /tmp/partly.sparse

$ ls -lA /tmp/partly.sparse 
-rwxr-xr-x 1 chris chris 1073741824 Aug 16 14:08 /tmp/partly.sparse

Answer 1

stdio.h将其定义为;

/* The possibilities for the third argument to `fseek'.
   These values should not be changed.  */
#define SEEK_SET        0       /* Seek from beginning of file.  */
#define SEEK_CUR        1       /* Seek from current position.  */
#define SEEK_END        2       /* Seek from end of file.  */
#ifdef __USE_GNU
# define SEEK_DATA      3       /* Seek to next data.  */
# define SEEK_HOLE      4       /* Seek to next hole.  */
#endif

__USE_GNU是您定义_GNU_SOURCE时设置的内部符号，这意味着要使用它们，您需要使用-D_GNU_SOURCE进行编译。

$ gcc test.c 
test.c: In function ‘test_seek’:
test.c:48:26: error: ‘SEEK_HOLE’ undeclared (first use in this function)
test.c:48:26: note: each undeclared identifier is reported only once for each function it appears in
test.c:51:26: error: ‘SEEK_DATA’ undeclared (first use in this function)
$ gcc -D_GNU_SOURCE test.c 
$

Answer 2

允许您现在遇到的行为：来自lseek linux manpage：

在最简单的实现中，文件系统可以通过使SEEK_HOLE始终返回文件末尾的偏移量来支持操作，并使SEEK_DATA始终返回偏移量（即，即使offset指向的位置是一个洞，它可以被认为是由零序列组成的数据。

Answer 3

请注意，lseek联机帮助页

但是，文件系统没有义务报告漏洞，所以这些操作不是保证映射的机制实际分配给文件的存储空间。

在3.8 kernel的Linux中添加了对ext4文件系统中的SEEK_HOLE支持，Ubuntu 12.04使用了比这更老的内核 - 你的内核版本是3.2.0-45。

SEEK_HOLE和SEEK_DATA在Ubuntu 12.04.2 LTS中不起作用

3 个答案: