grub tips

grub install for specific drive without BIOS drive name mapping grub --device-map=/dev/null
grub> device (hd0) /dev/sda

CentOS/RHEL rootfs on tmpfs

1: remember to disable selinux first
2: update mount process in /usr/share/dracut/modules.d/95rootfs-block/mount-root.sh

            mkdir /mytmpfs
            mount -t "$rootfs" -o "$rflags","$rootopts" \
                "${root#block:}" "/mytmpfs" 2>&1 | vinfo
            mount -t tmpfs -o size=100% none "$NEWROOT"
            cp -rfa /mytmpfs/* "$NEWROOT"
            sed -i '/\/boot\ /d' "$NEWROOT/etc/fstab"
            sed -i '/\/boot\t/d' "$NEWROOT/etc/fstab"
            sed -i '/\/ /d' "$NEWROOT/etc/fstab"
            sed -i '/\/\t/d' "$NEWROOT/etc/fstab"
            cat /dev/null > "$NEWROOT/root/.bash_history"
            umount /mytmpfs

3: create new initrd
4: update grub boot menu
5: boot parameter parsing
imgfile=$(getarg imgfile=)
[ -f "$imgfile" ] && tar zxvf $imgfile -C $NEWROOT

http://linux.die.net/man/1/bash
Conditional Expressions boot process stage https://www.kernel.org/pub/linux/utils/boot/dracut/dracut.html#stages

hexdump - format output

hexdump -s 0x2f -n 7 -e '7 1 "%c" ' 1 7  repeat 7 times 1  each time for 1 byte

Linux C dev tips

errno.h strerror (errno) dlfcn.h void *handle; handle = dlopen("libkido.so", RTLD_LAZY); if (!handle) { fprintf(stderr, "%s \n", dlerror()); exit(EXIT_FAILURE); } int (*test)() = dlsym (handle,"test3"); if (!test) { fprintf(stderr, "%s \n", dlerror()); exit(EXIT_FAILURE); } int a; a=(*test)();

Linux C program tips

compile object files gcc -c 1.c --> automatically generate 1.o file link object files and generate binary file gcc -o app 1.o 2.o main.o build archives -- static library ar cr libkido.a 1.o 2.o main.o gcc -L . -lkido -o app build shared library gcc -c -fPIC 1.c --> automatically generate 1.o file gcc -shared -fPIC -o libkido.so main.o 1.o 2.o gcc -o app -L . -lkido -Wl,-rpath,./ <-- if rpath is not defined, LD_LIBRARY_PATH need to be defined in environment variable to have the binary locate library file

PMbus access

1st byte of rtn is length and last byte is for nothing if you want to read 9bytes data, you need to specify length 11 in i2c master write-read command, when length is set to 0, BMC will perform i2c write.

gcc object file linking

gcc -c xxx.c --> automatically generate xxx.o gcc yyy.c xxx.o --> automatically generate a.out You can use functions defined in xxx.c without including any header files in yyy.c