gdb 7.9.1 cannot determine type of primitive global variables

[rbdavis (Roger Davis)]

Hi all,

I am having problems with gdb being unable to recognize the type of primitive global variables within an executable compiled from multiple source files under MacOS Yosemite 10.10.4 and Xcode 6.4 (6E35b). The same problem occurs whether I compile with either /usr/bin/cc or /usr/bin/gcc, both of which appear to be using Apple LLVM version 6.1.0 (clang-602.0.53) based on LLVM 3.6.0svn. (I have not tried with a self-built gcc compiled from gcc source.) I first noticed this a few days ago after a MacPorts update which installed gdb 7.9.1, but the same problem occurred when I deactivated MacPorts gdb 7.9.1 and reverted to MacPorts gdb 7.7.1. I also tried building gdb 7.7.1 from GNU source and still get the same problem. (Curiously, I am fairly sure that this problem did not exist with MacPorts gdb 7.7.1 before I upgraded Xcode from 6.3 to 6.4, but I upgraded gdb at the same time from 7.7.1 to 7.9.1 and so cannot be certain of exactly what triggered this bug.)

This problem is *not* exhibited by the current MacPorts gdb-apple port, but since (i) gdb-apple is based on a relatively old gdb revision (6.3) and thus possibly of questionable future lifespan, and (ii) it seems pointless to have a 'standard' (i.e., 7.9.1) gdb port that does not work, I am very ardently hoping that this issue will be addressed in the gdb 7.9.1 port.

You can reproduce this bug by creating the following two files m0.c and m1.c:

% cat m0.c
#include <stdio.h>
float gf;
extern void func();
int main() {

gf= 1.23;
printf("gf: %4.2f\n", gf);
func();

}

% cat m1.c
#include <stdio.h>
extern float gf;
void func() {

float lf;
lf= 4.56;
printf("lf: %4.2f\n", lf);

}

Then compile as follows:

% cc -g -c m0.c -o m0.o
% cc -g -c m1.c -o m1.o
% cc -g -o m m0.o m1.o

When you debug with MacPorts gdb 7.9.1, the type of the global float gf is unknown to gdb from within main():

% ggdb m
GNU gdb (GDB) 7.9.1
...
This GDB was configured as "x86_64-apple-darwin14.3.0".
...
Reading symbols from m...done.
(gdb) break main
Breakpoint 1 at 0x100000eee: file m0.c, line 5.
(gdb) break func
Breakpoint 2 at 0x100000f27: file m1.c, line 5.
(gdb) run
Starting program: /private/tmp/m
warning: `/BinaryCache/coreTLS/coreTLS-35.30.2~2/Objects/coretls.build/coretls.build/Objects-normal/x86_64/system_coretls_vers.o': can't open to read symbols: No such file or directory.
warning: Could not open OSO archive file "/BinaryCache/coreTLS/coreTLS-35.30.2~2/Symbols/BuiltProducts/libcoretls_ciphersuites.a"
warning: Could not open OSO archive file "/BinaryCache/coreTLS/coreTLS-35.30.2~2/Symbols/BuiltProducts/libcoretls_handshake.a"
warning: Could not open OSO archive file "/BinaryCache/coreTLS/coreTLS-35.30.2~2/Symbols/BuiltProducts/libcoretls_record.a"
warning: Could not open OSO archive file "/BinaryCache/coreTLS/coreTLS-35.30.2~2/Symbols/BuiltProducts/libcoretls_stream_parser.a"
Breakpoint 1, main () at m0.c:5[[BR]] 5 gf= 1.23;
(gdb) s
6 printf("gf: %4.2f\n", gf);
(gdb) s
gf: 1.23
7 func();
(gdb) ptype gf
type = <data variable, no debug info>
(gdb) print gf
$1 = 1067282596

However, the type of local float lf within func is known:

(gdb) continue
Continuing.
Breakpoint 2, func () at m1.c:5[[BR]] 5 lf= 4.56;
(gdb) s
6 printf("lf: %4.2f\n", lf);
(gdb) s
lf: 4.56
7 }
(gdb) ptype lf
type = float
(gdb) print lf
$2 = 4.55999994

The type of the global float gf is also unknown to gdb from within func():

(gdb) ptype gf
type = <data variable, no debug info>
(gdb) print gf
$3 = 1067282596

If instead you combine the two files m[01].c into the single file s.c with the contents

#include <stdio.h>
float gf;
int main() {

void func();
gf= 1.23;
printf("gf: %4.2f\n", gf);
func();

}

void func() {

float lf;
lf= 4.56;
printf("lf: %4.2f\n", lf);

}

and compile with

% cc -o s -g s.c

then gdb works perfectly well, recognizing the type of gf and correctly printing its floating point value within either main() or func().

Naturally, the actual program I am having a problem with is much larger and really needs to be in multiple files, but I think this example shows the problem about as succinctly as possible. The problem is not limited to float variables -- a global int variable shows the same problem of unknown type.

I have noticed that when I compile the above single-file s.c code, a directory s.dSYM is created. This is supposedly done when a dsymutil symbol utility is implictly run by cc. I have read that dsymutil is *not* run when the source files are separately compiled into .o files and the latter are then linked with a separate command (as in my multi-file cc command sequence shown above). Perhaps this has some bearing on the problem?

Please note also all of the references in the gdb output above to the various missing files "/BinaryCache/coreTLS/..." when gdb first starts the executable. There is no /BinaryCache directory on my system, and as these errors seem to be related to symbol issues I wonder if this is connected to the global variable type recognition problem.

Thanks,

Roger Davis