| Debugging with GDB: Files |
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Next: File Caching, Up: GDB Files [Contents][Index]
You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB’s start-up commands (see Getting In and Out of GDB ).
Occasionally it is necessary to change to a different file during a
GDB session. Or you may run GDB and forget to
specify a file you want to use. Or you are debugging a remote target
via gdbserver (see Using the gdbserver
Program). In these situations the GDB commands to specify
new files are useful.
file filename
Use filename as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the program
executed when you use the run command. If you do not specify a
directory and the file is not found in the GDB working directory,
GDB uses the environment variable PATH as a list of
directories to search, just as the shell does when looking for a program
to run. You can change the value of this variable, for both GDB
and your program, using the path command.
You can load unlinked object .o files into GDB using
the file command. You will not be able to “run” an object
file, but you can disassemble functions and inspect variables. Also,
if the underlying BFD functionality supports it, you could use
gdb -write to patch object files using this technique. Note
that GDB can neither interpret nor modify relocations in this
case, so branches and some initialized variables will appear to go to
the wrong place. But this feature is still handy from time to time.
file
file with no argument makes GDB discard any information it
has on both executable file and the symbol table.
exec-file [ filename ]
Specify that the program to be run (but not the symbol table) is found
in filename. GDB searches the environment variable PATH
if necessary to locate your program. Omitting filename means to
discard information on the executable file.
symbol-file [ filename ]
Read symbol table information from file filename. PATH is
searched when necessary. Use the file command to get both symbol
table and program to run from the same file.
symbol-file with no argument clears out GDB information on your
program’s symbol table.
The symbol-file command causes GDB to forget the contents of
some breakpoints and auto-display expressions. This is because they may
contain pointers to the internal data recording symbols and data types,
which are part of the old symbol table data being discarded inside
GDB.
symbol-file does not repeat if you press RET again after
executing it once.
When GDB is configured for a particular environment, it
understands debugging information in whatever format is the standard
generated for that environment; you may use either a GNU compiler, or
other compilers that adhere to the local conventions.
Best results are usually obtained from GNU compilers; for example,
using
GCC
you can generate debugging information for
optimized code.
For most kinds of object files, with the exception of old SVR3 systems
using COFF, the symbol-file command does not normally read the
symbol table in full right away. Instead, it scans the symbol table
quickly to find which source files and which symbols are present. The
details are read later, one source file at a time, as they are needed.
The purpose of this two-stage reading strategy is to make GDB
start up faster. For the most part, it is invisible except for
occasional pauses while the symbol table details for a particular source
file are being read. (The set verbose command can turn these
pauses into messages if desired. See Optional
Warnings and Messages.)
We have not implemented the two-stage strategy for COFF yet. When the
symbol table is stored in COFF format, symbol-file reads the
symbol table data in full right away. Note that “stabs-in-COFF”
still does the two-stage strategy, since the debug info is actually
in stabs format.
symbol-file [ -readnow ] filename
file [ -readnow ] filename
You can override the GDB two-stage strategy for reading symbol tables by using the ‘-readnow’ option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available.
symbol-file [ -readnever ] filename
file [ -readnever ] filename
You can instruct GDB to never read the symbolic information contained in filename by using the ‘-readnever’ option. See --readnever.
core-file [
filename
]
core
Specify the whereabouts of a core dump file to be used as the “contents of memory”. Traditionally, core files contain only some parts of the address space of the process that generated them; GDB can access the executable file itself for other parts.
core-file with no argument specifies that no core file is
to be used.
Note that the core file is ignored when your program is actually running
under GDB. So, if you have been running your program and you
wish to debug a core file instead, you must kill the subprocess in which
the program is running. To do this, use the kill command
(see Killing the Child Process).
add-symbol-file filename address
add-symbol-file filename address [ -readnow | -readnever ]
add-symbol-file filename address -s section address …
The add-symbol-file command reads additional symbol table
information from the file filename. You would use this command
when filename has been dynamically loaded (by some other means)
into the program that is running. The address should give the memory
address at which the file has been loaded; GDB cannot figure
this out for itself. You can additionally specify an arbitrary number
of ‘-s section address
’ pairs, to give an explicit
section name and base address for that section. You can specify any
address as an expression.
The symbol table of the file filename is added to the symbol table
originally read with the symbol-file command. You can use the
add-symbol-file command any number of times; the new symbol data
thus read is kept in addition to the old.
Changes can be reverted using the command remove-symbol-file.
Although filename is typically a shared library file, an executable file, or some other object file which has been fully relocated for loading into a process, you can also load symbolic information from relocatable .o files, as long as:
add-symbol-file command.
Some embedded operating systems, like Sun Chorus and VxWorks, can load
relocatable files into an already running program; such systems
typically make the requirements above easy to meet. However, it’s
important to recognize that many native systems use complex link
procedures (.linkonce section factoring and C++ constructor table
assembly, for example) that make the requirements difficult to meet. In
general, one cannot assume that using add-symbol-file to read a
relocatable object file’s symbolic information will have the same effect
as linking the relocatable object file into the program in the normal
way.
add-symbol-file does not repeat if you press RET after using it.
remove-symbol-file filename
remove-symbol-file -a address
Remove a symbol file added via the add-symbol-file command. The
file to remove can be identified by its filename or by an address
that lies within the boundaries of this symbol file in memory. Example:
(gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480
add symbol table from file "/home/user/gdb/mylib.so" at
.text_addr = 0x7ffff7ff9480
(y or n) y
Reading symbols from /home/user/gdb/mylib.so...done.
(gdb) remove-symbol-file -a 0x7ffff7ff9480
Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y
(gdb)
remove-symbol-file does not repeat if you press RET after using it.
add-symbol-file-from-memory address
Load symbols from the given address in a dynamically loaded
object file whose image is mapped directly into the inferior’s memory.
For example, the Linux kernel maps a syscall DSO into each
process’s address space; this DSO provides kernel-specific code for
some system calls. The argument can be any expression whose
evaluation yields the address of the file’s shared object file header.
For this command to work, you must have used symbol-file or
exec-file commands in advance.
section section addr
The section command changes the base address of the named
section of the exec file to addr. This can be used if the
exec file does not contain section addresses, (such as in the
a.out format), or when the addresses specified in the file
itself are wrong. Each section must be changed separately. The
info files command, described below, lists all the sections and
their addresses.
info files
info target
info files and info target are synonymous; both print the
current target (see Specifying a Debugging Target),
including the names of the executable and core dump files currently in
use by GDB, and the files from which symbols were loaded. The
command help target lists all possible targets rather than
current ones.
maint info sections
Another command that can give you extra information about program sections
is maint info sections. In addition to the section information
displayed by info files, this command displays the flags and file
offset of each section in the executable and core dump files. In addition,
maint info sections provides the following command options (which
may be arbitrarily combined):
ALLOBJ
Display sections for all loaded object files, including shared libraries.
sections
Display info only for named sections.
section-flags
Display info only for sections for which section-flags are true. The section flags that GDB currently knows about are:
ALLOC
Section will have space allocated in the process when loaded. Set for all sections except those containing debug information.
LOAD
Section will be loaded from the file into the child process memory.
Set for pre-initialized code and data, clear for .bss sections.
RELOC
Section needs to be relocated before loading.
READONLY
Section cannot be modified by the child process.
CODE
Section contains executable code only.
DATA
Section contains data only (no executable code).
ROM
Section will reside in ROM.
CONSTRUCTOR
Section contains data for constructor/destructor lists.
HAS_CONTENTS
Section is not empty.
NEVER_LOAD
An instruction to the linker to not output the section.
COFF_SHARED_LIBRARY
A notification to the linker that the section contains COFF shared library information.
IS_COMMON
Section contains common symbols.
set trust-readonly-sections on
Tell GDB that readonly sections in your object file really are read-only (i.e. that their contents will not change). In that case, GDB can fetch values from these sections out of the object file, rather than from the target program. For some targets (notably embedded ones), this can be a significant enhancement to debugging performance.
The default is off.
set trust-readonly-sections off
Tell GDB not to trust readonly sections. This means that the contents of the section might change while the program is running, and must therefore be fetched from the target when needed.
show trust-readonly-sections
Show the current setting of trusting readonly sections.
All file-specifying commands allow both absolute and relative file names as arguments. GDB always converts the file name to an absolute file name and remembers it that way.
GDB supports GNU/Linux, MS-Windows, SunOS, Darwin/Mach-O, SVr4, IBM RS/6000 AIX, QNX Neutrino, FDPIC (FR-V), and DSBT (TIC6X) shared libraries.
On MS-Windows GDB must be linked with the Expat library to support shared libraries. See Expat.
GDB automatically loads symbol definitions from shared libraries
when you use the run command, or when you examine a core file.
(Before you issue the run command, GDB does not understand
references to a function in a shared library, however—unless you are
debugging a core file).
There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of them.
To control the automatic loading of shared library symbols, use the commands:
set auto-solib-add mode
If mode is on, symbols from all shared object libraries
will be loaded automatically when the inferior begins execution, you
attach to an independently started inferior, or when the dynamic linker
informs GDB that a new library has been loaded. If mode
is off, symbols must be loaded manually, using the
sharedlibrary command. The default value is on.
If your program uses lots of shared libraries with debug info that takes large amounts of memory, you can decrease the GDB memory footprint by preventing it from automatically loading the symbols from shared libraries. To that end, type set auto-solib-add off before running the inferior, then load each library whose debug symbols you do need with sharedlibrary regexp , where regexp is a regular expression that matches the libraries whose symbols you want to be loaded.
show auto-solib-add
Display the current autoloading mode.
To explicitly load shared library symbols, use the sharedlibrary
command:
info share regex
info sharedlibrary regex
Print the names of the shared libraries which are currently loaded that match regex. If regex is omitted then print all shared libraries that are loaded.
info dll regex
This is an alias of info sharedlibrary.
sharedlibrary regex
share regex
Load shared object library symbols for files matching a
Unix regular expression.
As with files loaded automatically, it only loads shared libraries
required by your program for a core file or after typing run. If
regex is omitted all shared libraries required by your program are
loaded.
nosharedlibrary
Unload all shared object library symbols. This discards all symbols that have been loaded from all shared libraries. Symbols from shared libraries that were loaded by explicit user requests are not discarded.
Sometimes you may wish that GDB stops and gives you control
when any of shared library events happen. The best way to do this is
to use catch load and catch unload (see Set Catchpoints).
GDB also supports the the set stop-on-solib-events
command for this. This command exists for historical reasons. It is
less useful than setting a catchpoint, because it does not allow for
conditions or commands as a catchpoint does.
set stop-on-solib-events
This command controls whether GDB should give you control when the dynamic linker notifies it about some shared library event. The most common event of interest is loading or unloading of a new shared library.
show stop-on-solib-events
Show whether GDB stops and gives you control when shared library events happen.
Shared libraries are also supported in many cross or remote debugging configurations. GDB needs to have access to the target’s libraries; this can be accomplished either by providing copies of the libraries on the host system, or by asking GDB to automatically retrieve the libraries from the target. If copies of the target libraries are provided, they need to be the same as the target libraries, although the copies on the target can be stripped as long as the copies on the host are not.
For remote debugging, you need to tell GDB where the target libraries are, so that it can load the correct copies—otherwise, it may try to load the host’s libraries. GDB has two variables to specify the search directories for target libraries.
set sysroot path
Use path as the system root for the program being debugged. Any
absolute shared library paths will be prefixed with path; many
runtime loaders store the absolute paths to the shared library in the
target program’s memory. When starting processes remotely, and when
attaching to already-running processes (local or remote), their
executable filenames will be prefixed with path if reported to
GDB as absolute by the operating system. If you use
set sysroot to find executables and shared libraries, they need
to be laid out in the same way that they are on the target, with
e.g. a /bin, /lib and /usr/lib hierarchy under
path.
If path starts with the sequence target: and the target
system is remote then GDB will retrieve the target binaries
from the remote system. This is only supported when using a remote
target that supports the remote get command (see Sending files to a remote system). The part of path
following the initial target: (if present) is used as system
root prefix on the remote file system. If path starts with the
sequence remote: this is converted to the sequence
target: by set sysroot
14
. If you want
to specify a local system root using a directory that happens to be
named target: or remote:, you need to use some
equivalent variant of the name like ./target:.
For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, GDB tries prefixing a few variants of the target absolute file name with path. But first, on Unix hosts, GDB converts all backslash directory separators into forward slashes, because the backslash is not a directory separator on Unix:
c:\foo\bar.dll ⇒ c:/foo/bar.dll
Then, GDB attempts prefixing the target file name with path, and looks for the resulting file name in the host file system:
c:/foo/bar.dll ⇒ /path/to/sysroot/c:/foo/bar.dll
If that does not find the binary, GDB tries removing the ‘:’ character from the drive spec, both for convenience, and, for the case of the host file system not supporting file names with colons:
c:/foo/bar.dll ⇒ /path/to/sysroot/c/foo/bar.dll
This makes it possible to have a system root that mirrors a target with more than one drive. E.g., you may want to setup your local copies of the target system shared libraries like so (note ‘c’ vs ‘z’):
/path/to/sysroot/c/sys/bin/foo.dll /path/to/sysroot/c/sys/bin/bar.dll /path/to/sysroot/z/sys/bin/bar.dll
and point the system root at /path/to/sysroot, so that GDB can find the correct copies of both c:\sys\bin\foo.dll, and z:\sys\bin\bar.dll.
If that still does not find the binary, GDB tries removing the whole drive spec from the target file name:
c:/foo/bar.dll ⇒ /path/to/sysroot/foo/bar.dll
This last lookup makes it possible to not care about the drive name, if you don’t want or need to.
The set solib-absolute-prefix command is an alias for set
sysroot.
You can set the default system root by using the configure-time ‘--with-sysroot’ option. If the system root is inside GDB’s configured binary prefix (set with ‘--prefix’ or ‘--exec-prefix’), then the default system root will be updated automatically if the installed GDB is moved to a new location.
show sysroot
Display the current executable and shared library prefix.
set solib-search-path path
If this variable is set, path is a colon-separated list of directories to search for shared libraries. ‘solib-search-path’ is used after ‘sysroot’ fails to locate the library, or if the path to the library is relative instead of absolute. If you want to use ‘solib-search-path’ instead of ‘sysroot’, be sure to set ‘sysroot’ to a nonexistent directory to prevent GDB from finding your host’s libraries. ‘sysroot’ is preferred; setting it to a nonexistent directory may interfere with automatic loading of shared library symbols.
show solib-search-path
Display the current shared library search path.
set target-file-system-kind kind
Set assumed file system kind for target reported file names.
Shared library file names as reported by the target system may not
make sense as is on the system GDB is running on. For
example, when remote debugging a target that has MS-DOS based file
system semantics, from a Unix host, the target may be reporting to
GDB a list of loaded shared libraries with file names such as
c:\Windows\kernel32.dll. On Unix hosts, there’s no concept of
drive letters, so the ‘c:\’ prefix is not normally understood as
indicating an absolute file name, and neither is the backslash
normally considered a directory separator character. In that case,
the native file system would interpret this whole absolute file name
as a relative file name with no directory components. This would make
it impossible to point GDB at a copy of the remote target’s
shared libraries on the host using set sysroot, and impractical
with set solib-search-path. Setting
target-file-system-kind to dos-based tells GDB
to interpret such file names similarly to how the target would, and to
map them to file names valid on GDB’s native file system
semantics. The value of kind can be "auto", in addition
to one of the supported file system kinds. In that case, GDB
tries to determine the appropriate file system variant based on the
current target’s operating system (see Configuring the
Current ABI). The supported file system settings are:
unix
Instruct GDB to assume the target file system is of Unix kind. Only file names starting the forward slash (‘/’) character are considered absolute, and the directory separator character is also the forward slash.
dos-based
Instruct GDB to assume the target file system is DOS based. File names starting with either a forward slash, or a drive letter followed by a colon (e.g., ‘c:’), are considered absolute, and both the slash (‘/’) and the backslash (‘\\’) characters are considered directory separators.
auto
Instruct GDB to use the file system kind associated with the target operating system (see Configuring the Current ABI). This is the default.
When processing file names provided by the user, GDB
frequently needs to compare them to the file names recorded in the
program’s debug info. Normally, GDB compares just the
base names of the files as strings, which is reasonably fast
even for very large programs. (The base name of a file is the last
portion of its name, after stripping all the leading directories.)
This shortcut in comparison is based upon the assumption that files
cannot have more than one base name. This is usually true, but
references to files that use symlinks or similar filesystem
facilities violate that assumption. If your program records files
using such facilities, or if you provide file names to GDB
using symlinks etc., you can set basenames-may-differ to
true to instruct GDB to completely canonicalize each
pair of file names it needs to compare. This will make file-name
comparisons accurate, but at a price of a significant slowdown.
set basenames-may-differ
Set whether a source file may have multiple base names.
show basenames-may-differ
Show whether a source file may have multiple base names.
Historically the functionality to retrieve binaries from the remote system was provided by prefixing path with remote:
Next: File Caching, Up: GDB Files [Contents][Index]