readdir与readdir_r简要说明
readdir可以用来遍历指定目录路径下的所有文件。不过，不包含子文件，如果要递归遍历，可以使用深度遍历，或者广度遍历算法。
readdir_r 是readdir的可重入版本，线程安全。readdir非线程安全。
readdir如何遍历目录子文件？
1. opendir打开目录
opendir有2个版本：opendir，fopendir。前者参数为目录对应字符串，后者参数为目录对应已打开文件描述符。
#include <sys/types.h>
#include <dirent.h>
DIR *opendir(const char *name);
DIR *fdopendir(int fd);
用法模型：
DIR *dirp;
const char *base_dir = "/home/martin/document";
if ((dirp = opendir(base_dir)) != NULL) {
perror("opendir error");
return -1;
}
// 调用readdir遍历目录子文件
...
closedir(base_dir);
2. readdir遍历目录子文件
readdir需要一个已打开（调用opendir）的DIR对象作为参数。
#include <dirent.h>
struct dirent *readdir(DIR *dirp);
int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result);
dirent 结构定义
struct dirent {
ino_t d_ino; /* inode number i节点编号 */
off_t d_off; /* not an offset; see NOTES 早期文件系统中，telldir返回文件在目录内的偏移 */
unsigned short d_reclen; /* length of this record dirent 记录的实际长度 */
unsigned char d_type; /* type of file; not supported
by all filesystem types 文件类型 */
char d_name[256]; /* filename 文件名 */
};
d_ino i节点编号，操作系统用来识别文件的，每个文件都有一个inode number（参见Inode详解）
d_off 早期文件系统中，文件系统使用平面表格，telldir返回文件在目录内的偏移，而d_off就代表这个偏移的缓存。应用把该值当做一个不透明的值即可。
d_reclen 记录的实际长度，并非sizeof(struct dirent)的值，而是sizeof(struct dirent) - sizeof(name) + {strlen(name) 补齐8字节对齐}。
d_type 文件类型，并非所有文件系统都支持。另外一种支持更好的查看文件类型的方式是，使用stat系统调用的st_mode。
d_name 文件名。在代码中可能会看到后面有注释/* We must not include limits.h! */，这是因为POSIX.1时，未规定路径名长度NAME_MAX，而POSIX.1要求d_name最多支持255个字符，255这个魔法数字是一个难以移植的特定数字。limits.h中定义的NAME_MAX是在POSIX.1-2001加入的。
遍历子文件模型：
DIR *dirp;
const char *base_dir = "/home/martin/document";
if ((dirp = opendir(base_dir)) != NULL) {
perror("opendir error");
return -1;
}
// 调用readdir遍历目录子文件
struct dirent *dp;
while ((dp = readdir(dirp)) != NULL) {
// 读取、打印文件名、文件类型等信息
printf dp->d_name, d_type
}
closedir(base_dir);
3. readdir完整示例
要遍历的目录：/home/martin/documents
执行$ tree -L 1
执行$ls -al
readdir遍历指定目录，并打印示例代码
int list_file(const char *base_dir)
{
DIR *dirp;
struct dirent* dp;
// 打开目录
if ((dirp = opendir(base_dir)) == NULL) {
perror("opendir error");
return -1;
}
printf("sizeof(dirent) = %ld\n", sizeof(struct dirent) - 256);
printf("%-20s %10s %25s %15s %15s %-s\n", "type", "d_ino", "d_off", "d_reclen", "len", "filename");
while ((dp = readdir(dirp)) != NULL) {
// 忽略当前目录"."和上一级目录".."（父目录）
if (0 == strcmp(dp->d_name, ".") || 0 == strcmp(dp->d_name, ".."))
continue;
// 读取文件类型
char type[50];
switch (dp->d_type) {
case DT_DIR: // a directory
snprintf(type, sizeof(type), "%s", "directory");
break;
case DT_REG: // a regular file
snprintf(type, sizeof(type), "%s", "regular file");
break;
case DT_BLK: // a block device
snprintf(type, sizeof(type), "%s", "block device");
break;
case DT_CHR: // a character device
snprintf(type, sizeof(type), "%s", "character device");
break;
case DT_FIFO: // a named pipe (FIFO)
snprintf(type, sizeof(type), "%s", "named pipe (FIFO)");
break;
case DT_LNK: // a symbolic link
snprintf(type, sizeof(type), "%s", "symbolic link");
break;
case DT_SOCK: // a UNIX domain socket
snprintf(type, sizeof(type), "%s", "UNIX domain socket");
break;
default: // DT_UNKNOWN - file type unknown
snprintf(type, sizeof(type), "%s", "file type unknown");
break;
}
printf("%-20s %10lu %25ld %15u %15ld %-s\n", type, dp->d_ino, dp->d_off, dp->d_reclen, strlen(dp->d_name), dp->d_name);
}
// 关闭目录
closedir(dirp);
return 0;
}
int main(int argc, char *argv[])
{
list_file("/home/martin/Documents");
return 0;
}
运行结果
sizeof(dirent) = 24
type d_ino d_off d_reclen len filename
regular file 292970 3024556464499973779 40 13 list12.dKLpEX
directory 1180001 3470762240268728258 32 8 winshark
regular file 293625 4800707232840484128 32 8 list.txt
regular file 292872 9057525276248794967 64 37 RFC959_FTP传输协议(中文版).pdf
regular file 288738 9223372036854775807 40 14 rfc114.txt.pdf
4. readdir_r完整示例
readdir_r遍历指定目录，并打印示例代码
#define NAME_MAX_PORTABLE // 文件名称可移植
int list_file_r(const char *base_dir)
{
DIR *dirp;
#ifndef NAME_MAX_PORTABLE // 未定义 文件名称可移植
struct dirent d1, d2;
struct dirent *entryp, *res;
entryp = &d1;
res = &d2;
#else // 定义了 文件名称可移植
struct dirent *entryp, *res;
long name_max = pathconf(base_dir, _PC_NAME_MAX);
if (name_max == -1) {
name_max = 255; // guess
}
int len = offsetof(struct dirent, d_name) + name_max + 1; // + 1 for NULL terminate byte
entryp = malloc(len);
res = malloc(len);
#endif
// 打开目录
if ((dirp = opendir(base_dir)) == NULL) {
perror("opendir error");
return -1;
}
printf("%-20s %10s %25s %15s %15s %-s\n", "type", "d_ino", "d_off", "d_reclen", "len", "filename");
int ret;
while(1) {
if ((ret = readdir_r(dirp, entryp, &res)) > 0) {
fprintf(stderr, "readdir_r error: %d\n", ret);
perror("readdir_r error");
return -1;
}
if (res == NULL)
break;
if (strcmp(entryp->d_name, ".") == 0 || strcmp(entryp->d_name, "..") == 0)
continue;
// 忽略当前目录"."和上一级目录".."（父目录）
if (0 == strcmp(entryp->d_name, ".") || 0 == strcmp(entryp->d_name, ".."))
continue;
// 读取文件类型
char type[50];
switch (entryp->d_type) {
case DT_DIR: // a directory
snprintf(type, sizeof(type), "%s", "directory");
break;
case DT_REG: // a regular file
snprintf(type, sizeof(type), "%s", "regular file");
break;
case DT_BLK: // a block device
snprintf(type, sizeof(type), "%s", "block device");
break;
case DT_CHR: // a character device
snprintf(type, sizeof(type), "%s", "character device");
break;
case DT_FIFO: // a named pipe (FIFO)
snprintf(type, sizeof(type), "%s", "named pipe (FIFO)");
break;
case DT_LNK: // a symbolic link
snprintf(type, sizeof(type), "%s", "symbolic link");
break;
case DT_SOCK: // a UNIX domain socket
snprintf(type, sizeof(type), "%s", "UNIX domain socket");
break;
default: // DT_UNKNOWN - file type unknown
snprintf(type, sizeof(type), "%s", "file type unknown");
break;
}
printf("%-20s %10lu %25ld %15u %15ld %-s\n", type, entryp->d_ino, entryp->d_off, entryp->d_reclen, strlen(entryp->d_name), entryp->d_name);
}
printf("exit list_file_r\n");
#ifdef NAME_MAX_PORTABLE
// 释放资源
free(res);
free(entryp);
#endif
// 关闭目录
closedir(dirp);
return 0;
}
int main(int argc, char *argv[])
{
list_file_r("/home/martin/Documents");
return 0;
}
运行结果
type d_ino d_off d_reclen len filename
regular file 292970 3024556464499973779 40 13 list12.dKLpEX
directory 1180001 3470762240268728258 32 8 winshark
regular file 293625 4800707232840484128 32 8 list.txt
regular file 292872 9057525276248794967 64 37 RFC959_FTP传输协议(中文版).pdf
regular file 288738 9223372036854775807 40 14 rfc114.txt.pdf
exit list_file_r