#include <dirent.h> DIR *fdopendir(int fd); DIR *opendir(const char *dirname);
Upon successful return from fdopendir(), the file descriptor is under the control of the system, and if any attempt is made to close the file descriptor, or to modify the state of the associated description, other than by means of closedir(), readdir(), readdir_r(), rewinddir(), or seekdir(), the behavior is undefined. Upon calling closedir() the file descriptor shall be closed.
It is unspecified whether the FD_CLOEXEC flag will be set on the file descriptor by a successful call to fdopendir().
The opendir() function shall open a directory stream corresponding to the directory named by the dirname argument. The directory stream is positioned at the first entry. If the type DIR is implemented using a file descriptor, applications shall only be able to open up to a total of {OPEN_MAX} files and directories.
If the type DIR is implemented using a file descriptor, the descriptor shall be obtained as if the O_DIRECTORY flag was passed to open().
The opendir() function shall fail if:
The opendir() function may fail if:
The following sections are informative.
The following program fragment demonstrates how the opendir() function is used.
#include <dirent.h> ... DIR *dir; struct dirent *dp; ... if ((dir = opendir (".")) == NULL) { perror ("Cannot open ."); exit (1); } while ((dp = readdir (dir)) != NULL) { ...
The following program searches through a given directory looking for files whose name does not begin with a dot and whose size is larger than 1 MiB.
#include <stdio.h> #include <dirent.h> #include <fcntl.h> #include <sys/stat.h> #include <stdint.h> #include <stdlib.h> #include <unistd.h> int main(int argc, char *argv[]) { struct stat statbuf; DIR *d; struct dirent *dp; int dfd, ffd; if ((d = fdopendir((dfd = open("./tmp", O_RDONLY)))) == NULL) { fprintf(stderr, "Cannot open ./tmp directory\n"); exit(1); } while ((dp = readdir(d)) != NULL) { if (dp->d_name[0] == '.') continue; /* there is a possible race condition here as the file * could be renamed between the readdir and the open */ if ((ffd = openat(dfd, dp->d_name, O_RDONLY)) == -1) { perror(dp->d_name); continue; } if (fstat(ffd, &statbuf) == 0 && statbuf.st_size > (1024*1024)) { /* found it ... */ printf("%s: %jdK\n", dp->d_name, (intmax_t)(statbuf.st_size / 1024)); } close(ffd); } closedir(d); // note this implicitly closes dfd return 0; }
Based on historical implementations, the rules about file descriptors apply to directory streams as well. However, this volume of POSIX.1-2017 does not mandate that the directory stream be implemented using file descriptors. The description of closedir() clarifies that if a file descriptor is used for the directory stream, it is mandatory that closedir() deallocate the file descriptor. When a file descriptor is used to implement the directory stream, it behaves as if the FD_CLOEXEC had been set for the file descriptor.
The directory entries for dot and dot-dot are optional. This volume of POSIX.1-2017 does not provide a way to test a priori for their existence because an application that is portable must be written to look for (and usually ignore) those entries. Writing code that presumes that they are the first two entries does not always work, as many implementations permit them to be other than the first two entries, with a ``normal'' entry preceding them. There is negligible value in providing a way to determine what the implementation does because the code to deal with dot and dot-dot must be written in any case and because such a flag would add to the list of those flags (which has proven in itself to be objectionable) and might be abused.
Since the structure and buffer allocation, if any, for directory operations are defined by the implementation, this volume of POSIX.1-2017 imposes no portability requirements for erroneous program constructs, erroneous data, or the use of unspecified values such as the use or referencing of a dirp value or a dirent structure value after a directory stream has been closed or after a fork() or one of the exec function calls.
The Base Definitions volume of POSIX.1-2017, <dirent.h>, <sys_types.h>
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