add comments in makefs.c

1 files changed, 25 insertions, 5 deletions

diff --git a/makefs.c b/makefs.c
index 31831a3..c122267 100644
--- a/makefs.c
+++ b/makefs.c
@@ -1,3 +1,12 @@
+// Take files given on stdin and make them into a ramfs image of our
+// own, (stupid) simple format.
+// In the format: for each file comes the null-terminated string
+// with filename, then null-padding until a 4-aligned offset, then
+// 4-byte little-endian size of the file and then the contents
+// of the file and then another null-padding until a 4-aligned offset.
+// Files encoded this way go one after another (so it's easy to add
+// something at the end or at the beginning).
+
 #include <stdint.h>
 #include <sys/types.h>
 #include <sys/wait.h>
@@ -13,6 +22,7 @@
 
 int main(int argc, char **argv)
 {
+  // process files in the order they are provided on the command line
   for (int i = 1; i < argc; i++)
     {
       struct stat fileinfo;
@@ -25,26 +35,35 @@ int main(int argc, char **argv)
 	errx(-1, ANSI_FG_RED "%s" ANSI_FG_DEFAULT
 	     " is not a regular file.", argv[i]);
 
+      // don't allow files with size so big, that it can't be encoded
+      // in a 4-byte unsigned int... In practice even smaller files
+      // won't fit on the rpi.
       if (fileinfo.st_size > UINT32_MAX)
 	errx(-1, ANSI_FG_RED "%s" ANSI_FG_DEFAULT
 	     " is too big.", argv[i]);
       
       uint32_t file_size = fileinfo.st_size;
-      uint32_t name_size = strlen(argv[i]) + 1;
+      uint32_t name_size = strlen(argv[i]) + 1; // 1 for null-byte
       
       if (fwrite(argv[i], 1, name_size, stdout) != name_size)
 	errx(-1, "error writing to stdout");
 
-      for (int j = 0; (j + (name_size & 0b11)) & 0b11; j++)
+      // pad with null-bytes until a 4-aligned offset
+      for (unsigned int j = 0; (j + (name_size & 0b11)) & 0b11; j++)
 	if (putchar('\0'))
 	  errx(-1, "error writing to stdout");	  
-      
+
+      // TODO convert file_size to little endian first (in case our
+      // host is be).
       if (fwrite(&file_size, 4, 1, stdout) != 1)
 	errx(-1, "error writing to stdout");
 
+      // flush b4 running cat, so that stuff we've written comes
+      // b4 the actual file contents in the output
       if (fflush(stdout))
 	err(-1, "couldn't flush stdout");
-      
+
+      // we don't copy the actual file ourselves - we run cat for that
       pid_t pid;
       int wstatus;
       switch (pid = fork())
@@ -62,9 +81,10 @@ int main(int argc, char **argv)
 	    exit(-1);
 	}
 
+      // again, pad with null-bytes until a 4-aligned offset
       for (int j = 0; (j + (file_size & 0b11)) & 0b11; j++)
 	if (putchar('\0'))
-	  errx(-1, "error writing to stdout");	  
+	  errx(-1, "error writing to stdout");
     }
   
   return 0;