02-14-2011
Let the process produce a core file
There are two ways you can deal with this problem.
The first, and most obvious one, is to run the program under the debugger (e.g.: dbx). When it crashes, you'll get the debugger's prompt and you can explore the location and causes of the crash.
Since running under the debugger may change the behavior of the program, you may want to let it run normally. If the program crashes due to certain signals (e.g.: SIGSEGV - indicating accessing a bad address, such as address 0), it can produce a core file, at least under Unix variants, including Linux. You can then use the debugger to look at the state of the program at the time of the crash and analyze the reason for the crash.
For example, if the name of the program is prog, then
use: dbx prog core
to explore the state of the program at the time of the crash.
The core file is not always produced even when the program crashes due to a signal such as SIGSEGV. The reason is that there is a limit on the size of the core file. Often, this limit is 0. You can change this limit to "unlimited" and then the core file will be produced when the need arises.
The command to change the core file size limit is shell dependent.
For example, under bash I am using: ulimit -c unlimited
under tcsh, I use: limit coredumpsize unlimited
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gcore(1) BSD General Commands Manual gcore(1)
NAME
gcore -- get core images of running processes
SYNOPSIS
gcore [-s] [-v] [-b size] [-o path | -c pathformat] pid
DESCRIPTION
The gcore program creates a core file image of the process specified by pid. The resulting core file can be used with a debugger, e.g.
lldb(1), to examine the state of the process.
The following options are available:
-s Suspend the process while the core file is captured.
-v Report progress on the dump as it proceeds.
-b size Limit the size of the core file to size MiBytes.
The following options control the name of the core file:
-o path
Write the core file to path.
-c pathformat
Write the core file to pathformat. The pathformat string is treated as a pathname that may contain various special characters which
cause the interpolation of strings representing specific attributes of the process into the name.
Each special character is introduced by the % character. The format characters and their meanings are:
N The name of the program being dumped, as reported by ps(1).
U The uid of the process being dumped, converted to a string.
P The pid of the process being dumped, converted to a string.
T The time when the core file was taken, converted to ISO 8601 format.
% Output a percent character.
The default file name used by gcore is %N-%P-%T. By default, the core file will be written to a directory whose name is determined from the
kern.corefile MIB. This can be printed or modified using sysctl(8).
The directory where the core file is to be written must be accessible to the owner of the target process.
gcore will not overwrite an existing file, nor will it create missing directories in the path.
EXIT_STATUS
The gcore utility exits 0 on success, and >0 if an error occurs.
FILES
/cores/%N-%P-%T default pathname for the corefile.
BUGS
With the -b flag, gcore writes out as much data as it can up to the specified limit, even if that results in an incomplete core image. Such
a partial core dump may confuse subsequent programs that attempt to parse the contents of such files.
SEE ALSO
lldb(1), core(5), Mach-O(5), sysctl(8), sudo(8).
Darwin May 31, 2019 Darwin