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11. Third Example: Fortune
This example requires some knowledge of C programming. The majority of UNIX/Linux software is written in C, and learning at least a little bit of C would certainly be an asset for anyone serious about software installation.
The notorious fortune program displays up a humorous saying, a "fortune cookie", every time Linux boots up. Unfortunately (pun intended), attempting to build fortune on a Red Hat distribution with a 2.0.30 kernel generates fatal errors.
~/fortune# make all
gcc -O2 -Wall -fomit-frame-pointer -pipe -c fortune.c -o
fortune.o
fortune.c: In function `add_dir':
fortune.c:551: structure has no member named `d_namlen'
fortune.c:553: structure has no member named `d_namlen'
make[1]: *** [fortune.o] Error 1
make[1]: Leaving directory `/home/thegrendel/for/fortune/fortune'
make: *** [fortune-bin] Error 2
Looking at fortune.c, the pertinent lines are these.
if (dirent->d_namlen == 0)
continue;
name = copy(dirent->d_name, dirent->d_namlen);
We need to find the structure dirent, but it is not declared in
the fortune.c file, nor does a grep dirent show it in
any of the other source files. However, at the top of
fortune.c, there is the following line.
#include <dirent.h>
This appears to be a system library include file, therefore, the logical
place to look for dirent.h is in /usr/include.
Indeed, there does exist a dirent.h file in
/usr/include, but that file does not contain the declaration of
the dirent structure. There is, however, a reference to
another dirent.h file.
#include <linux/dirent.h>
At last, going to /usr/include/linux/dirent.h, we find the structure declaration we need.
struct dirent {
long d_ino;
__kernel_off_t d_off;
unsigned short d_reclen;
char d_name[256]; /* We must not include
limits.h! */
};
Sure enough, the structure declaration contains no d_namelen, but there are a couple of "candidates" for its equivalent. The most likely of these is d_reclen, since this structure member probably represents the length of something and it is a short integer. The other possibility, d_ino, could be an inode number, judging by its name and type. As a matter of fact, we are probably dealing with a "directory entry" structure, and these elements represent attributes of a file, its name, inode, and length (in blocks). This would seem to validate our guess.
Let us edit the file fortune.c, and change the two
d_namelen references in lines 551 and 553 to d_reclen.
Try a make all again. Success. It builds without
errors. We can now get our "cheap thrills" from fortune.
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