通过stat()读取指定文件状态, 然后通过宏测试struct stat的st_mode成员得到文件类型.
stat函数有3个版本, 它们的区别是stat参数需要一个文件路径, fstat需要一个已打开文件描述符, lstat不跟随符号链接(当文件是符号链接时, 获得符号链接自身文件状态, 而不是获得其指向的文件状态)

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int stat(const char *pathname, struct stat *statbuf);
int fstat(int fd, struct stat *statbuf);
int lstat(const char *pathname, struct stat *statbuf);

struct stat定义

struct stat {
   dev_t     st_dev;         /* ID of device containing file */
   ino_t     st_ino;         /* Inode number */
   mode_t    st_mode;        /* File type and mode */
   nlink_t   st_nlink;       /* Number of hard links */
   uid_t     st_uid;         /* User ID of owner */
   gid_t     st_gid;         /* Group ID of owner */
   dev_t     st_rdev;        /* Device ID (if special file) */
   off_t     st_size;        /* Total size, in bytes */
   blksize_t st_blksize;     /* Block size for filesystem I/O */
   blkcnt_t  st_blocks;      /* Number of 512B blocks allocated */

   /* Since Linux 2.6, the kernel supports nanosecond
      precision for the following timestamp fields.
      For the details before Linux 2.6, see NOTES. */

   struct timespec st_atim;  /* Time of last access */
   struct timespec st_mtim;  /* Time of last modification */
   struct timespec st_ctim;  /* Time of last status change */

#define st_atime st_atim.tv_sec      /* Backward compatibility */
#define st_mtime st_mtim.tv_sec
#define st_ctime st_ctim.tv_sec
};

可以通过宏确定文件类型

也可以通过宏确定IPC(进程间通信)对象类型

文件类型宏测试实质是按位与运算, 我们也可以手动按位与运算. 以S_ISDIR为例, 在unistd.h中, 其定义:

#define S_ISDIR(mode) ((mode) & S_IFMT) == S_IFDIR)

示例: 创建2个文件, 分别是普通文件类型和目录, 判断并打印出文件类型
更详细的示例 参见man 2 stat

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

#define FILE_NUM  2

void printFileType(char *path) {
    struct stat stat_buf;
    int ret = stat(path, &stat_buf);
    if (ret < 0) {
        perror("stat error");
        return;
    }

    if (S_ISREG(stat_buf.st_mode)) {
        printf("%s: general file\n", path);
    }
    else if(S_ISDIR(stat_buf.st_mode)) {
        printf("%s: direcotry\n", path);
    }
    else {
        printf("%s: other file type\n", path);
    }
}

int main() {
    char *filepaths[FILE_NUM] = {
        "./file", "./dir"
    };

    int ret = -1;
    for (int i = 0; i < FILE_NUM; ++i){
        if (access(filepaths[i], O_RDONLY) < 0) {
            ret = creat(filepaths[i], 0664);

            if (ret < 0) {
                perror("creat error");
                exit(1);
            }
        }
    }

    for (int i = 0; i < FILE_NUM; ++i) {
        printFileType(filepaths[i]);

    }
    return 0;
}

运行结果

$ touch file
$ mkdir dir
$ ./a.out 
./file: general file
./dir: direcotry

Linux C如何判断文件类型?