Added more documentation

libXmd/README.md

@@ -0,0 +1,5 @@
+# libXmd
+
+This library contains code that is common to Xmd applications. The interface to
+its functionality is divided among several header files which also contain
+documentation comments.

libXmd/include/Xmd/Exec.h

@@ -5,8 +5,18 @@
 #include <stdio.h>
 #include <sys/types.h>
 
+/* XmdVaPipedExecPath is like XmdPipedExecPath, but it takes in a vararg list to
+   use as the "args" array. The vararg list must be terminated by NULL. */
 FILE *XmdVaPipedExecPath (const String file, pid_t *child, String mode, ...);
 
+/* XmdPipedExecPath is like popen(), but takes in an explicit command name and
+   argument list instead of a shell command. The argument list must be
+   terminated by NULL. "file" is searched for in PATH, and it is executed as a
+   child process with the specified args. The PID of the child is stored at the
+   location of "child".
+   
+   To correctly close the connection to the child process, first fclose() the
+   returned file, and then call waitpid() on the child PID. */
 FILE *XmdPipedExecPath (const String file, pid_t *child, String mode, String const argv[]);
 
 #endif