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dlsym() returns 0 for an existing function


 
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# 1  
Old 06-09-2010
dlsym() returns 0 for an existing function

Sometimes I observe this in gdb:
Code:
(gdb) br my_function
Breakpoint .. at 0x...: file ..., line ...

i.e., "my_function" does exist in the current executable.
however, dlsym does not find it:
Code:
(gdb) p dlsym(0,"my_function")
$6 = 0

This is a C program; dlsym does find other defined functions and variables.
(this question stems from this bug report)

What could cause this? How do I avoid this?

---------- Post updated 2010-06-09 at 08:59 AM ---------- Previous update was 2010-06-08 at 03:05 PM ----------

the presence of my_function is also obvious from
Code:
(gdb) p my_function
$21 = {void (void)} 0x486f9c <my_function>

# 2  
Old 06-09-2010
Code:
dlsym(0, "myfunc");
/* should be */
dlsym(RTLD_SELF, "myfunc");

At least on systems I've used....

RTLD_SELF usually expands to something unusual like (void *)-3
# 3  
Old 06-09-2010
it turned out that one has to link the executable with
Code:
ld -export-dynamic

or
Code:
cc -Wl,-export-dynamic



---------- Post updated at 12:29 PM ---------- Previous update was at 12:22 PM ----------

Quote:
Originally Posted by jim mcnamara
Code:
dlsym(0, "myfunc");
/* should be */
dlsym(RTLD_SELF, "myfunc");

At least on systems I've used....

RTLD_SELF usually expands to something unusual like (void *)-3
RTLD_SELF is not defined on linux (2.6.18, CentOS 5.4)
 

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