Contents|Index|Previous|Next

Options for Debugging Your Program or GNU CC

GNU CC has the following special options that are used for debugging either your program or GCC.

-g
Produce debugging information in the operating systems native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program.
If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, -gdwarf+1, or -gdwarf-1 (see the explanations for each option in the following discussion).
Unlike most other C compilers, GNU CC allows you to use -g with
-O. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GNU CC is generated with the capability for more than one debugging format.

-ggdb
Produce debugging information in the native format (if that is supported), including GDB extensions if at all possible.

-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gcoff
Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf-1
Produce debugging information in DWARF version 1 format (if that is supported). This is the format used by SDB on most System V Release 4 systems.

-gdwarf-1+
Produce debugging information in DWARF version 1 format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6.

-gdwarf
Produce debugging information in DWARF version 1 format, if that is the default debugging format for the target, or version 2 if that is supported. This is the format used by SDB on most System V Release 4 systems.

-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gdwarflevel
Request debugging information and also use level to specify how much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you dont plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.

-p
Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg
Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-a
Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed, the basic block start address, and the function name containing the basic block. If
-g is used, the line number and filename of the start of the basic block will also be recorded. If not overridden by the machine description, the default action is to append to the text file bb.out.
This data could be analyzed by a program like tcov. However, the format of the data is not what tcov expects. Eventually GNU gprof should be extended to process this data.

-ax
Generate extra code to profile basic blocks. Your executable will produce output that is a superset of that produced when -a is used. Additional output is the source and target address of the basic blocks where a jump takes place, the number of times a jump is executed, and (optionally) the complete sequence of basic blocks being executed. The output is appended to file bb.out.
You can examine different profiling aspects without recompilation. Your executable will read a list of function names from file bb.in. Profiling starts when a function on the list is entered and stops when that invocation is exited. To exclude a function from profiling, use a hyphen (-), to prefix its name.
If a function name is not unique, specify its location, for example, /path/filename.d:functionname. Your executable will write the available paths and filenames in file bb.out.
The following function names have a special meaning.

- __bb_jumps__
  Write source, target and frequency of jumps to file bb.out.
- __bb_hidecall__
  Exclude function calls from frequency count.
- __bb_showret__
  Include function returns in frequency count.
- __bb_trace__
  Write the sequence of basic blocks to file bbtrace.gz. The file will be compressed using the program gzip, which must exist in your PATH. On systems without the popen function, the file will be named bbtrace and will not be compressed.

Profiling for even a few seconds on these systems will produce a very large file.
Note:
__bb_hidecall__ and __bb_showret__ will not affect the sequence written to bbtrace.gz.

What follows is a short example using different profiling parameters in file bb.in. Assume function foo consists of basic blocks 1 and 2 and is called twice from block 3 of function main. After the calls, block 3 transfers control to block 4 of main.
With __bb_trace__ and main contained in file bb.in, the following sequence of blocks is written to file bbtrace.gz: 0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because the return is to a point inside the block and not to the top. The block address 0 always indicates that control is transferred to the trace from somewhere outside the observed functions. With -foo added to bb.in, the blocks of function foo are removed from the trace, so only 0 3 4 remains.
With __bb_jumps__ and main contained in file bb.in, jump frequencies will be written to file b.out. The frequencies are obtained by constructing a trace of blocks and incrementing a counter for every neighboring pair of blocks in the trace. The trace 0 3 1 2 1 2 4 displays the following frequencies.

       Jump from block 0x0 to block 0x3 executed 1 time(s)
       Jump from block 0x3 to block 0x1 executed 1 time(s)
       Jump from block 0x1 to block 0x2 executed 2 time(s)
       Jump from block 0x2 to block 0x1 executed 1 time(s)
       Jump from block 0x2 to block 0x4 executed 1 time(s)

With __bb_hidecall__, due to call instructions, control transfer is removed from the trace; that is, the trace is cut into three parts: 0 3 4, 0 1 2, and 0 1 2. With __bb_showret__, control transfer is added to the trace. The trace becomes: 0 3 1 2 3 1 2 3 4.

Note:
The previous trace is not the same as the sequence written to bbtrace.gz. It is solely used for counting jump frequencies.

-fprofile-arcs
Instrument arcs during compilation. For each function of your program, GNU CC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.
Since not every arc in the program must be instrumented, programs compiled with this option run faster than programs compiled with -a, which adds instrumentation code to every basic block in the program. The tradeoff: since gcov does not have execution counts for all branches, it must start with the execution counts for the instrumented branches, and then iterate over the program flow graph until the entire graph has been solved. Hence, gcov runs a little more slowly than a program which uses information from -a.

-fprofile-arcs also makes it possible to estimate branch probabilities, and to calculate basic block execution counts. In general, basic block execution counts do not give enough information to estimate all branch probabilities. When the compiled program exits, it saves the arc execution counts to a file called sourcename.da. Use the compiler option, -fbranch-probabilities, when recompiling, to optimize using estimated branch probabilities (see Options that Control Optimization).

-ftest-coverage
Create data files for the gcov code-coverage utility (see gcov: a Test Coverage Program). The data filenames begin with the name of your source file:

- sourcename.bb
  A mapping from basic blocks to line numbers, which gcov uses to associate basic block execution counts with line numbers.
- sourcename.bbg
  A list of all arcs in the program flow graph. This allows gcov to reconstruct the program flow graph, so that it can compute all basic block and arc execution counts from the information in the sourcename.da file (this last file is the output from -fprofile-arcs).

-dletters
This option says to make debugging dumps during compilation at times specified by letters. This is used for debugging the compiler. The filenames for most of the dumps are made by appending a word to the source filename (e.g., foo.c.rtl or foo.c.jump). What follows are the possible letters for use in letters, and their meanings.

- M
  Dump all macro definitions, at the end of preprocessing, and write no output.
- N
  Dump all macro names, at the end of preprocessing.
- D
  Dump all macro definitions, at the end of preprocessing, in addition to normal output.
- y
  Dump debugging information during parsing, to standard error.
- r
  Dump after RTL generation, to file.rtl.
- x
  Just generate RTL for a function instead of compiling it. Usually used with r.
- j
  Dump after first jump optimization, to file.jump.
- s
  Dump after CSE (including the jump optimization that sometimes follows CSE), to file.cse.
- L
  Dump after loop optimization, to file.loop.
- t
  Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to file.cse2.
- f
  Dump after flow analysis, to file.flow.
- c
  Dump after instruction combination, to file.combine.
- S
  Dump after the first instruction scheduling pass, to file.sched.
- l
  Dump after local register allocation, to file.lreg.
- g
  Dump after global register allocation, to file.greg.
- R
  Dump after the second instruction scheduling pass, to file.sched2.
- J
  Dump after last jump optimization, to file.jump2.
- d
  Dump after delayed branch scheduling, to file.dbr.
- k
  Dump after conversion from registers to stack, to file.stack.
- a
  Produce all the dumps previously listed.
- m
  Print statistics on memory usage, at the end of the run, to standard error.
- p
  Annotate the assembler output with a comment indicating which pattern and alternative was used.
- A
  Annotate the assembler output with miscellaneous debugging information.

-fpretend-float
When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine.

-save-temps
Store the usual temporary intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c with -c-save-temps would produce files foo.i and foo.s, as well as foo.o.

-print-file-name=library
Print the full absolute name of the library file library that would be used when linkingand dont do anything else. With this option, GNU CC does not compile or link anything; it just prints the filename.

-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.

-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can use the following command.

    gcc -nostdlib files... 'gcc -print-libgcc-file-name'

-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc will searchand dont do anything else.
This is useful when gcc prints the following error message:

    installation problem, cannot exec cpp: No such file or directory

To resolve this you either need to put cpp and the other compiler components where gcc expects to find them, or you can set the environment variable, GCC_EXEC_ PREFIX, to the directory where you installed them. (Dont forget the trailing slash). See Environment Variables Affecting GNU CC.