這個15 年前的mmap 教程在 Google 搜索中排名靠前，但它實際上在我的 Linux 系統上運行時出現了微妙的錯誤。
mmap_write.c：
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#define FILEPATH "/tmp/mmapped.bin"
#define NUMINTS (1000)
#define FILESIZE (NUMINTS * sizeof(int))
int main(int argc, char *argv[])
{
int i;
int fd;
int result;
int *map; /* mmapped array of int's */
/* Open a file for writing.
* - Creating the file if it doesn't exist.
* - Truncating it to 0 size if it already exists. (not really needed)
*
* Note: "O_WRONLY" mode is not sufficient when mmaping.
*/
fd = open(FILEPATH, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
if (fd == -1) {
perror("Error opening file for writing");
exit(EXIT_FAILURE);
}
/* Stretch the file size to the size of the (mmapped) array of ints
*/
result = lseek(fd, FILESIZE-1, SEEK_SET);
if (result == -1) {
close(fd);
perror("Error calling lseek() to 'stretch' the file");
exit(EXIT_FAILURE);
}
/* Something needs to be written at the end of the file to
* have the file actually have the new size.
* Just writing an empty string at the current file position will do.
*
* Note:
* - The current position in the file is at the end of the stretched
* file due to the call to lseek().
* - An empty string is actually a single '\0' character, so a zero-byte
* will be written at the last byte of the file.
*/
result = write(fd, "", 1);
if (result != 1) {
close(fd);
perror("Error writing last byte of the file");
exit(EXIT_FAILURE);
}
/* Now the file is ready to be mmapped.
*/
map = mmap(0, FILESIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (map == MAP_FAILED) {
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
/* Now write int's to the file as if it were memory (an array of ints).
*/
for (i = 1; i <=NUMINTS; i) {
map[i] = 2 * i;
}
/* Don't forget to free the mmapped memory
*/
if (munmap(map, FILESIZE) == -1) {
perror("Error un-mmapping the file");
/* Decide here whether to close(fd) and exit() or not. Depends... */
}
/* Un-mmaping doesn't close the file, so we still need to do that.
*/
close(fd);
return 0;
}
mmap_read.c：
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#define FILEPATH "/tmp/mmapped.bin"
#define NUMINTS (1000)
#define FILESIZE (NUMINTS * sizeof(int))
int main(int argc, char *argv[])
{
int i;
int fd;
int *map; /* mmapped array of int's */
fd = open(FILEPATH, O_RDONLY);
if (fd == -1) {
perror("Error opening file for reading");
exit(EXIT_FAILURE);
}
map = mmap(0, FILESIZE, PROT_READ, MAP_SHARED, fd, 0);
if (map == MAP_FAILED) {
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
/* Read the file int-by-int from the mmap
*/
for (i = 1; i <=NUMINTS; i) {
printf("%d: %d\n", i, map[i]);
}
if (munmap(map, FILESIZE) == -1) {
perror("Error un-mmapping the file");
}
close(fd);
return 0;
}
如果檔案不存在，則輸出mmap_read為
...
998: 1996
999: 1998
1000: 2000
但如果是這樣，輸出是
...
998: 1996
999: 1998
1000: 0
作者應該重繪 write嗎？還是 GCC 錯誤編譯了代碼？
編輯：我注意到檔案的先前存在或不存在會產生影響，而不是編譯標志。
uj5u.com熱心網友回復：
您從第二個元素開始，并在地圖結束后寫入 2000。
for (i = 1; i <=NUMINTS; i) {
map[i] = 2 * i;
}
應該
for (i = 0; i < NUMINTS; i) {
map[i] = 2 * ( i 1 );
}
演示
這不是緩沖問題。write是系統呼叫，所以資料直接傳遞給OS。這并不意味著資料在write回傳時已寫入磁盤，而是在作業系統的手中，因此就作業系統功能而言，它就像在磁盤上一樣，包括其記憶體映射功能。
uj5u.com熱心網友回復：
在 C 中，索引從零開始。寫入和讀取索引 1000 您呼叫未定義的行為
更改為寫入。：
for (i = 1; i <=NUMINTS; i) {
map[i - 1] = 2 * i;
}
并閱讀：
for (i = 1; i <=NUMINTS; i) {
printf("%d: %d\n", i, map[i-1]);
}
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