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Debugging with GDB

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2.1.1 Choosing files

When GDB starts, it reads any arguments other than options as specifying an executable file and core file (or process ID). This is the same as if the arguments were specified by the `-se' and `-c' (or `-p' options respectively. (GDB reads the first argument that does not have an associated option flag as equivalent to the `-se' option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the `-c'/`-p' option followed by that argument.) If the second argument begins with a decimal digit, GDB will first attempt to attach to it as a process, and if that fails, attempt to open it as a corefile. If you have a corefile whose name begins with a digit, you can prevent GDB from treating it as a pid by prefixing it with `./', eg. `./12345'.

If GDB has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it.

Many options have both long and short forms; both are shown in the following list. GDB also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with `--' rather than `-', though we illustrate the more usual convention.)

-symbols file
-s file
Read symbol table from file file.

-exec file
-e file
Use file file as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.

-se file
Read symbol table from file file and use it as the executable file.

-core file
-c file
Use file file as a core dump to examine.

-c number
-pid number
-p number
Connect to process ID number, as with the attach command. If there is no such process, GDB will attempt to open a core file named number.

-command file
-x file
Execute GDB commands from file file. See section Command files.

-directory directory
-d directory
Add directory to the path to search for source files.

-m
-mapped
Warning: this option depends on operating system facilities that are not supported on all systems.
If memory-mapped files are available on your system through the mmap system call, you can use this option to have GDB write the symbols from your program into a reusable file in the current directory. If the program you are debugging is called `/tmp/fred', the mapped symbol file is `/tmp/fred.syms'. Future GDB debugging sessions notice the presence of this file, and can quickly map in symbol information from it, rather than reading the symbol table from the executable program.

The `.syms' file is specific to the host machine where GDB is run. It holds an exact image of the internal GDB symbol table. It cannot be shared across multiple host platforms.

-r
-readnow
Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster.

You typically combine the -mapped and -readnow options in order to build a `.syms' file that contains complete symbol information. (See section Commands to specify files, for information on `.syms' files.) A simple GDB invocation to do nothing but build a `.syms' file for future use is:

 
gdb -batch -nx -mapped -readnow programname


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