dlsym() returns 0 for an existing function

[sds57]

Sometimes I observe this in gdb:

(gdb) br my_function
Breakpoint .. at 0x...: file ..., line ...

i.e., "my_functio n" does exist in the current executable.
however, dlsym does not find it:

(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?

[sds57]

the presence of my_function is also obvious from

(gdb) p my_function
$21 = {void (void)} 0x486f9c <my_function>

[sds57]

it turned out that one has to link the executable with

ld -export-dynamic

or

cc -Wl,-export-dynamic