NAME

shmcat - dump shared memory segments, files, standard input and/or text

SYNOPSIS

shmcat [ OPTION ]... [ OPERAND ]...

DESCRIPTION

Dump shared memory segments, files, standard input and/or text to standard output. If no OPERAND is given, read standard input only. If more than one OPERAND is given, dump them in order. If some of the OPERANDs cannot be read, shmcat prints an error message to standard error and tries to continue with the other OPERANDs, unless the -S or --stop option is given.

Most modern systems support two APIs for shared memory:

The older one is commonly known as the System V API. It uses numeric keys and IDs as identifiers and consists of system calls like shmget(2), shmat(2) and shmdt(2).

The newer API is called the POSIX shared memory API. It uses names in string form as identifiers. It treats shared memory similar to files, which are opened with shm_open(3) and then mapped into the program address space via mmap(2). Please note that POSIX shared memory is not supported by all versions of shmcat.

If shmcat has been compiled without getopt_long(3) support, only the short, one character OPTIONs and OPERANDs can be used. Otherwise the long ones are also available. When a long OPTION or OPERAND needs an argument, the short versions do so, as well.

OPTIONS

-h, -?, --help: Display usage information and quit without dumping anything.
-S, --stop: By default, if an OPERAND cannot be read, shmcat tries to continue with the remaining OPERANDs. This option turns of that behavior, so that the program stops immediately after the first OPERAND that cannot be read.
-V, --version: Show version information and quit without further action.

OPERANDS

-f, --file=FILENAME: Dump the contents of the given file.
-i, --stdin: Dump standard input.
-M, --shmkey=KEY: Dump the System V shared memory segment with the given key. The key can be given as a decimal (no prefix), octal (prefix 0), or hexadecimal (prefix 0x) number. It can, for example, be obtained using the ipcs(1) or the ftok(1) tool.
-m, --shmid=ID: Dump the System V shared memory segment with the given id. The id can be given as a decimal (no prefix), octal (prefix 0), or hexadecimal (prefix 0x) number. It can, for example, be obtained using the ipcs(1) tool.
-n, --newline: Add a line feed.
-p, --posix-shm=NAME: Dump the POSIX shared memory segment with the given name (not supported in all installations).
-t, --text=TEXT: Print the given text.

EXIT CODES

0: No errors, every OPERAND requested was dumped.
1: There was an error while opening or reading at least one OPERAND. If shmcat has been invoked with the -S or --stop option, the program stopped after this error, otherwise it tried to continue with the remaining OPERANDs (if given).
2: There was a fatal error, usually this means that shmcat could not write to standard output.
10: Wrong usage of the program.

NOTES

Usually, when you dump shared memory segments, you should redirect the output of shmcat to a file or a program like od(1), because shared memory data usually has a binary form and thus it may contain non-printable characters. You also probably want to suspend the processes writing to the shared memory object in order to get a consistent dump.

AUTHOR

Stefan Gast