How to determine whether a file exists in Linux

How to determine whether a file exists under Linux:

1. stat series functions

The stat function is used to return structural information related to files. . The stat series functions have three situations, corresponding to file names, file descriptors and symbolic link files respectively. The stat structure describes the file attributes, mainly including file type, file size, etc. The detailed stat structure is as follows:

struct stat {
    mode_t    st_mode;    // file type & mode(permissions)
    ino_t     st_ino;     // i-node number(serial number)
    dev_t     st_dev;     // device number(filesystem)
    dev_t     st_rdev;    // device number for specials files
    nlink_t   st_nlink;   // number of links
    uid_t     st_uid;     // user ID of owner
    gid_t     st_gid;     // group ID of owner
    off_t     st_size;    // size in bytes, for regular files
    time_t    st_atime;   // time of last access
    time_t    st_mtime;   // time of last modification
    time_t    st_ctime;   // time of last file status change
    long      st_blksize; // best I/O block size
    long      st_blocks;  // number of 512-byte blocks allocated
};

We can obtain information such as file type and file size through stat. File types are: ordinary files, directory files, block special files, character special files, FIFO, sockets and symbolic links. If you want to use the stat series of functions to determine whether a file or directory exists, when executing the stat function, if the file exists, you need to further determine whether the file is an ordinary file or a directory file.

The stat series function error returns -1. The error code is stored in errno. The value of errno is as follows:

1. The file specified by the ENOENT parameter file_name does not exist

2. The directory in the ENOTDIR path exists but is not a real directory

3. The file to be opened by ELOOP has too many symbolic links. The upper limit is 16 symbolic links

4. EFAULT parameter buf It is an invalid pointer, pointing to a memory space that cannot exist

5. EACCESS was denied when accessing the file

6. ENOMEM core memory is insufficient

7. The path of the ENAMETOOLONG parameter file_name The name is too long

2. Access function

The access function tests access permissions based on the actual user ID and actual group. The function prototype is:

#include <unistd.h>
int access(const char *pathname, int mode);

mode value:

F_OK Test whether the file exists

R_OK Test read permission

W_OK Test write permission

X_OK Test execution permissions

To correctly determine whether a file exists, use the access function. The implementation is as follows:

3. oepndir function

The opendir function is used to open a file directory and returns a pointer if successful or NULL if an error occurs. The implementation is as follows:

(Recommended learning: linux tutorial)

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