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 source, and also get the same problem. (Also note that MacPorts 7.9.1 gdb has a startup problem which may be related to this issue, see the end of this message.)

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 gdb, the type of the global float gf is unknown to gdb from within main():

% /usr/local/bin/gdb m
GNU gdb (GDB) 7.7.1
...
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
Breakpoint 1, main () at m0.c:5
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
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 of using two source files m[01].c you combine them 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 issue 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?

With regard to MacPorts gdb 7.9.1, I am also seeing the following startup problem (as noted at the end of the first paragraph of this message) when the executable is actually run in gdb. Note the references to the various missing files "/BinaryCache/coreTLS/...". There is no /BinaryCache directory on my system.

% gdb m
GNU gdb (GDB) 7.9.1
...
(gdb) break main
Breakpoint 1 at 0x100000eee: file m0.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"

Thanks,

Roger Davis
Univ. of Hawaii