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| <h3 class="section">11.3 Interoperation</h3> |
| |
| <p>This section lists various difficulties encountered in using GCC |
| together with other compilers or with the assemblers, linkers, |
| libraries and debuggers on certain systems. |
| |
| <ul> |
| <li>On many platforms, GCC supports a different ABI for C++ than do other |
| compilers, so the object files compiled by GCC cannot be used with object |
| files generated by another C++ compiler. |
| |
| <p>An area where the difference is most apparent is name mangling. The use |
| of different name mangling is intentional, to protect you from more subtle |
| problems. |
| Compilers differ as to many internal details of C++ implementation, |
| including: how class instances are laid out, how multiple inheritance is |
| implemented, and how virtual function calls are handled. If the name |
| encoding were made the same, your programs would link against libraries |
| provided from other compilers—but the programs would then crash when |
| run. Incompatible libraries are then detected at link time, rather than |
| at run time. |
| |
| <li>On some BSD systems, including some versions of Ultrix, use of profiling |
| causes static variable destructors (currently used only in C++) not to |
| be run. |
| |
| <li>On some SGI systems, when you use <samp><span class="option">-lgl_s</span></samp> as an option, |
| it gets translated magically to ‘<samp><span class="samp">-lgl_s -lX11_s -lc_s</span></samp>’. |
| Naturally, this does not happen when you use GCC. |
| You must specify all three options explicitly. |
| |
| <li>On a SPARC, GCC aligns all values of type <code>double</code> on an 8-byte |
| boundary, and it expects every <code>double</code> to be so aligned. The Sun |
| compiler usually gives <code>double</code> values 8-byte alignment, with one |
| exception: function arguments of type <code>double</code> may not be aligned. |
| |
| <p>As a result, if a function compiled with Sun CC takes the address of an |
| argument of type <code>double</code> and passes this pointer of type |
| <code>double *</code> to a function compiled with GCC, dereferencing the |
| pointer may cause a fatal signal. |
| |
| <p>One way to solve this problem is to compile your entire program with GCC. |
| Another solution is to modify the function that is compiled with |
| Sun CC to copy the argument into a local variable; local variables |
| are always properly aligned. A third solution is to modify the function |
| that uses the pointer to dereference it via the following function |
| <code>access_double</code> instead of directly with ‘<samp><span class="samp">*</span></samp>’: |
| |
| <pre class="smallexample"> inline double |
| access_double (double *unaligned_ptr) |
| { |
| union d2i { double d; int i[2]; }; |
| |
| union d2i *p = (union d2i *) unaligned_ptr; |
| union d2i u; |
| |
| u.i[0] = p->i[0]; |
| u.i[1] = p->i[1]; |
| |
| return u.d; |
| } |
| </pre> |
| <p class="noindent">Storing into the pointer can be done likewise with the same union. |
| |
| <li>On Solaris, the <code>malloc</code> function in the <samp><span class="file">libmalloc.a</span></samp> library |
| may allocate memory that is only 4 byte aligned. Since GCC on the |
| SPARC assumes that doubles are 8 byte aligned, this may result in a |
| fatal signal if doubles are stored in memory allocated by the |
| <samp><span class="file">libmalloc.a</span></samp> library. |
| |
| <p>The solution is to not use the <samp><span class="file">libmalloc.a</span></samp> library. Use instead |
| <code>malloc</code> and related functions from <samp><span class="file">libc.a</span></samp>; they do not have |
| this problem. |
| |
| <li>On the HP PA machine, ADB sometimes fails to work on functions compiled |
| with GCC. Specifically, it fails to work on functions that use |
| <code>alloca</code> or variable-size arrays. This is because GCC doesn't |
| generate HP-UX unwind descriptors for such functions. It may even be |
| impossible to generate them. |
| |
| <li>Debugging (<samp><span class="option">-g</span></samp>) is not supported on the HP PA machine, unless you use |
| the preliminary GNU tools. |
| |
| <li>Taking the address of a label may generate errors from the HP-UX |
| PA assembler. GAS for the PA does not have this problem. |
| |
| <li>Using floating point parameters for indirect calls to static functions |
| will not work when using the HP assembler. There simply is no way for GCC |
| to specify what registers hold arguments for static functions when using |
| the HP assembler. GAS for the PA does not have this problem. |
| |
| <li>In extremely rare cases involving some very large functions you may |
| receive errors from the HP linker complaining about an out of bounds |
| unconditional branch offset. This used to occur more often in previous |
| versions of GCC, but is now exceptionally rare. If you should run |
| into it, you can work around by making your function smaller. |
| |
| <li>GCC compiled code sometimes emits warnings from the HP-UX assembler of |
| the form: |
| |
| <pre class="smallexample"> (warning) Use of GR3 when |
| frame >= 8192 may cause conflict. |
| </pre> |
| <p>These warnings are harmless and can be safely ignored. |
| |
| <li>In extremely rare cases involving some very large functions you may |
| receive errors from the AIX Assembler complaining about a displacement |
| that is too large. If you should run into it, you can work around by |
| making your function smaller. |
| |
| <li>The <samp><span class="file">libstdc++.a</span></samp> library in GCC relies on the SVR4 dynamic |
| linker semantics which merges global symbols between libraries and |
| applications, especially necessary for C++ streams functionality. |
| This is not the default behavior of AIX shared libraries and dynamic |
| linking. <samp><span class="file">libstdc++.a</span></samp> is built on AIX with “runtime-linking” |
| enabled so that symbol merging can occur. To utilize this feature, |
| the application linked with <samp><span class="file">libstdc++.a</span></samp> must include the |
| <samp><span class="option">-Wl,-brtl</span></samp> flag on the link line. G++ cannot impose this |
| because this option may interfere with the semantics of the user |
| program and users may not always use ‘<samp><span class="samp">g++</span></samp>’ to link his or her |
| application. Applications are not required to use the |
| <samp><span class="option">-Wl,-brtl</span></samp> flag on the link line—the rest of the |
| <samp><span class="file">libstdc++.a</span></samp> library which is not dependent on the symbol |
| merging semantics will continue to function correctly. |
| |
| <li>An application can interpose its own definition of functions for |
| functions invoked by <samp><span class="file">libstdc++.a</span></samp> with “runtime-linking” |
| enabled on AIX. To accomplish this the application must be linked |
| with “runtime-linking” option and the functions explicitly must be |
| exported by the application (<samp><span class="option">-Wl,-brtl,-bE:exportfile</span></samp>). |
| |
| <li>AIX on the RS/6000 provides support (NLS) for environments outside of |
| the United States. Compilers and assemblers use NLS to support |
| locale-specific representations of various objects including |
| floating-point numbers (‘<samp><span class="samp">.</span></samp>’ vs ‘<samp><span class="samp">,</span></samp>’ for separating decimal |
| fractions). There have been problems reported where the library linked |
| with GCC does not produce the same floating-point formats that the |
| assembler accepts. If you have this problem, set the <samp><span class="env">LANG</span></samp> |
| environment variable to ‘<samp><span class="samp">C</span></samp>’ or ‘<samp><span class="samp">En_US</span></samp>’. |
| |
| <li><a name="index-fdollars_002din_002didentifiers-3199"></a>Even if you specify <samp><span class="option">-fdollars-in-identifiers</span></samp>, |
| you cannot successfully use ‘<samp><span class="samp">$</span></samp>’ in identifiers on the RS/6000 due |
| to a restriction in the IBM assembler. GAS supports these |
| identifiers. |
| |
| </ul> |
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