From 1168c080af2f2f901a08886b670228a05ce74b77 Mon Sep 17 00:00:00 2001 From: Wojtek Kosior Date: Sat, 28 Dec 2019 12:29:08 +0100 Subject: add comments in makefs.c --- makefs.c | 30 +++++++++++++++++++++++++----- 1 file changed, 25 insertions(+), 5 deletions(-) (limited to 'makefs.c') 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 #include #include @@ -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; -- cgit v1.2.3