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authorGravatar Denis Vlasenko <vda.linux@googlemail.com>2007-03-15 00:57:01 +0000
committerGravatar Denis Vlasenko <vda.linux@googlemail.com>2007-03-15 00:57:01 +0000
commit972288e62fa0798f59caf209ab58c911d289a8b2 (patch)
tree4d11a46c5e62a35d357fce9659577cecec8ea117 /docs/keep_data_small.txt
parentdd2b2f75ae3a826731ae40a79812ebe94c58ac88 (diff)
downloadbusybox-972288e62fa0798f59caf209ab58c911d289a8b2.tar.gz
busybox-972288e62fa0798f59caf209ab58c911d289a8b2.tar.bz2
modify ptr_to_globals trick so that we do not violate
type safety (well, sort of ;))
Diffstat (limited to 'docs/keep_data_small.txt')
-rw-r--r--docs/keep_data_small.txt17
1 files changed, 10 insertions, 7 deletions
diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt
index 272aa21c5..ec13b4d3f 100644
--- a/docs/keep_data_small.txt
+++ b/docs/keep_data_small.txt
@@ -59,30 +59,33 @@ archival/libunarchive/decompress_unzip.c:
This example completely eliminates globals in that module.
Required memory is allocated in inflate_gunzip() [its main module]
-and then passed down to all subroutines which need to access globals
+and then passed down to all subroutines which need to access 'globals'
as a parameter.
Example 2
In case you don't want to pass this additional parameter everywhere,
take a look at archival/gzip.c. Here all global data is replaced by
-singe global pointer (ptr_to_globals) to allocated storage.
+single global pointer (ptr_to_globals) to allocated storage.
In order to not duplicate ptr_to_globals in every applet, you can
reuse single common one. It is defined in libbb/messages.c
-as void *ptr_to_globals, but is NOT declared in libbb.h.
-You first define a struct:
+as struct globals *ptr_to_globals, but the struct globals is
+NOT defined in libbb.h. You first define your own struct:
-struct my_globals { int a; char buf[1000]; };
+struct globals { int a; char buf[1000]; };
and then declare that ptr_to_globals is a pointer to it:
-extern struct my_globals *ptr_to_globals;
#define G (*ptr_to_globals)
-Linker magic enures that these two merge into single pointer object.
+Linker magic ensures that these two merge into single pointer object.
Now initialize it in <applet>_main():
ptr_to_globals = xzalloc(sizeof(G));
and you can reference "globals" by G.a, G.buf and so on, in any function.
+
+The drawback is that now you have to initialize it by hand. xzalloc()
+can be helpful in clearing allocated storage to 0, but anything more
+must be done by hand.