example of integration with libc via ctypes

Author: andreax79

README.md

@@ -36,6 +36,7 @@ Examples
 * [Read the DOS-type (MBR) partition table](https://python-cstruct.readthedocs.io/en/latest/examples/fdisk/)
 * [Print information about logged uses](https://python-cstruct.readthedocs.io/en/latest/examples/who/)
 * [Flexible Array Member (FAM)](https://python-cstruct.readthedocs.io/en/latest/examples/flexible_array/)
+* [libc integration (using ctypes)](https://python-cstruct.readthedocs.io/en/latest/examples/dir/)
 
 
 Features

docs/examples/dir.md

@@ -0,0 +1,6 @@
+The following program prints the names of the files in a directory,
+calling the libc functions `getcwd`, `opendir`, `readdir`, and `closedir`:
+
+```
+{!examples/dir.py!}
+```

examples/dir.c

@@ -0,0 +1,22 @@
+/* https://www.gnu.org/software/libc/manual/html_mono/libc.html#Simple-Directory-Lister */
+
+#include <stdio.h>
+#include <sys/types.h>
+#include <dirent.h>
+
+int main (void) {
+    DIR *dp;
+    struct dirent *ep;
+
+    dp = opendir(".");
+    if (dp != NULL) {
+        while (ep = readdir (dp)) {
+            puts(ep->d_name);
+        }
+        closedir(dp);
+    } else {
+        perror("Couldn't open the directory");
+    }
+
+    return 0;
+}

examples/dir.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python
+import cstruct
+import ctypes
+import sys
+
+libc = ctypes.cdll.LoadLibrary("libc.so.6")
+# opendir
+libc.opendir.argtypes = [ctypes.c_char_p]
+libc.opendir.restype = ctypes.c_void_p
+# readdir
+libc.readdir.argtypes = [ctypes.c_void_p]
+libc.readdir.restype = ctypes.c_void_p
+# closedir
+libc.closedir.argtypes = [ctypes.c_void_p]
+libc.closedir.restype = ctypes.c_int
+
+
+class DType(cstruct.CEnum):
+    __size__ = 1
+    __def__ = """
+        enum d_type {
+            DT_UNKNOWN = 0x0,
+            DT_FIFO = 0x1,
+            DT_CHR = 0x2,
+            DT_DIR = 0x4,
+            DT_BLK = 0x6,
+            DT_REG = 0x8,
+            DT_LNK = 0xa,
+            DT_SOCK = 0xc
+        };
+    """
+
+    def __str__(self):
+        return {
+            DType.DT_UNKNOWN: "<unknown>",
+            DType.DT_FIFO: "<fifo>",
+            DType.DT_CHR: "<char>",
+            DType.DT_DIR: "<dir>",
+            DType.DT_BLK: "<block>",
+            DType.DT_REG: "<regular>",
+            DType.DT_LNK: "<link>",
+            DType.DT_SOCK: "<socket>",
+        }[self]
+
+
+class Dirent(cstruct.MemCStruct):
+    __def__ = """
+        #define PATH_MAX 4096
+
+        typedef long ino_t;
+        typedef long off_t;
+
+        struct dirent {
+            ino_t          d_ino;       /* Inode number */
+            off_t          d_off;       /* Not an offset */
+            unsigned short d_reclen;    /* Length of this record */
+            unsigned char  d_type;      /* Type of file; not supported
+                                           by all filesystem types */
+            char           d_name[256]; /* Null-terminated filename */
+        };
+    """
+
+    @property
+    def name(self):
+        return ctypes.c_char_p(self.d_name).value.decode("ascii")
+
+    @property
+    def type(self):
+        return DType(self.d_type)
+
+
+def main():
+    if len(sys.argv) > 1:
+        cwd = ctypes.create_string_buffer(sys.argv[1].encode("ascii"))
+    else:
+        # Get current dir
+        cwd = ctypes.create_string_buffer(cstruct.getdef("PATH_MAX") + 1)
+        assert libc.getcwd(cwd, ctypes.sizeof(cwd)) != 0
+    # Open dir
+    dp = libc.opendir(cwd)
+    assert dp != 0
+    # Read dir entries
+    ep = libc.readdir(dp)
+    while ep:
+        contents = ctypes.cast(ep, ctypes.POINTER(ctypes.c_char * Dirent.size)).contents
+        dirent = Dirent(contents)
+        print(f"{dirent.d_ino:8} {dirent.type:10} {dirent.name}")
+        ep = libc.readdir(dp)
+    # Close dir
+    libc.closedir(dp)
+
+
+if __name__ == "__main__":
+    main()

examples/dir.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env sh
+cd "$(dirname "$0")/.." || exit
+python -m examples.dir $*

mkdocs.yml

@@ -20,6 +20,7 @@ nav:
   - Code of Conduct: CODE_OF_CONDUCT.md
   - Source Code Repository: "https://github.com/andreax79/python-cstruct"
   - Examples:
+    - "dir.py": examples/dir.md
     - "fdisk.py": examples/fdisk.md
     - "flexible_array.py": examples/flexible_array.md
     - "who.py": examples/who.md