Strange behavior with gdb printf - gdb

Question

I have a issue with gdb printf in version 9.1
echo -e '#include<stdio.h> \n int main(){ \n printf("Hello"); \n }' > test.c
gcc -g test.c -o test
echo 'break test.c:4' > test.gdb
echo 'run' >> test.gdb
echo 'set $aux = (char*)malloc(256)' >> test.gdb
echo 'set $e = strcpy($aux, "abc")' >> test.gdb
echo 'printf "%s", $aux' >> test.gdb
gdb --batch --command=test.gdb test
Output with gdb 9.1:
Breakpoint 1 at 0x1167: file test.c, line 4.
Breakpoint 1, main () at test.c:4
4 }
�e���
Expected output (same as gdb v8):
Breakpoint 1 at 0x1167: file test.c, line 4.
Breakpoint 1, main () at test.c:4
4 }
abc
I've checked charset, but it seems ok.
Any idea about that?

Answer 1

This is Bug 25650 - GDB can't 'printf' a convenience variable holding an inferior address, fixed in gdb 9.2.
If you can't upgrade to gdb 9.2 but can recompile your existing distro's gdb 9.1, there is a two-line patch.
On Ubuntu 20.04, which comes with gdb 9.1-0ubuntu1:
run apt build-dep gdb to haul in the packages needed to build gdb from source
run apt install dpkg-dev (to get /usr/bin/dpkg-source)
uncomment the deb-src lines in /etc/apt/sources.list
run apt update
cd to an empty directory and run apt source gdb . Doing this in a fresh new directory will make it easier to clean things up after the compilation.
cd to gdb-9.1 and apply the patch to gdb/printcmd.c
build and install gdb. For example, to put it in /usr/local/bin, the default, you'd run
mkdir build
cd build
../configure
make
make install