[PATCH] user-cr: add --output-fd to write output to a specific fd

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at librato.com):
> This is useful if the user would like redirect the output to
> e.g, a socket or any other already open file descriptor when
> invoking 'checkpoint'.
> 
> Also useful if the user would like to append an existing file.
> 
> Signed-off-by: Oren Laadan <orenl at cs.columbia.edu>

Acked-by: Serge Hallyn <serue at us.ibm.com>

> ---
>  checkpoint.c |   47 +++++++++++++++++++++++++++++++++++++++++------
>  1 files changed, 41 insertions(+), 6 deletions(-)
> 
> diff --git a/checkpoint.c b/checkpoint.c
> index c116daf..aef954b 100644
> --- a/checkpoint.c
> +++ b/checkpoint.c
> @@ -32,12 +32,14 @@ static char usage_str[] =
>  "\tOptions:\n"
>  "  -h,--help             print this help message\n"
>  "  -o,--output=FILE      write data to FILE instead of standard output\n"
> +"     --output-fd=FD     write data to file descriptor FD instead of stdout\n"
>  "  -c,--container        require the PID is a container-init\n"
>  "  -v,--verbose          verbose output\n"
>  "";
> 
>  struct args {
>  	char *output;
> +	int outputfd;
>  	int container;
>  	int verbose;
>  };
> @@ -53,17 +55,33 @@ static void usage(char *str)
>  	exit(1);
>  }
> 
> +/* negative retval means error */
> +static int str2num(char *str)
> +{
> +	char *nptr;
> +	int num;
> +
> +	num = strtol(str, &nptr, 10);
> +	if (nptr - str != strlen(str))
> +		num = -1;
> +	return num;
> +}
> +
>  static void parse_args(struct args *args, int argc, char *argv[])
>  {
>  	static struct option opts[] = {
>  		{ "help",	no_argument,		NULL, 'h' },
>  		{ "output",	required_argument,	NULL, 'o' },
> +		{ "output-fd",	required_argument,	NULL, 1 },
>  		{ "container",	no_argument,		NULL, 'c' },
>  		{ "verbose",	no_argument,		NULL, 'v' },
>  		{ NULL,		0,			NULL, 0 }
>  	};
>  	static char optc[] = "hvco:";
> 
> +	/* defaults */
> +	args->outputfd = -1;
> +
>  	while (1) {
>  		int c = getopt_long(argc, argv, optc, opts, NULL);
>  		if (c == -1)
> @@ -76,6 +94,13 @@ static void parse_args(struct args *args, int argc, char *argv[])
>  		case 'o':
>  			args->output = optarg;
>  			break;
> +		case 1:
> +			args->outputfd = str2num(optarg);
> +			if (args->outputfd < 0) {
> +				printf("checkpoint: invalid file descriptor\n");
> +				exit(1);
> +			}
> +			break;
>  		case 'c':
>  			args->container = 1;
>  			break;
> @@ -86,6 +111,12 @@ static void parse_args(struct args *args, int argc, char *argv[])
>  			usage(usage_str);
>  		}
>  	}
> +
> +	if (args->output && args->outputfd >= 0) {
> +		printf("Invalid used of both -o/--output and --output-fd\n");
> +		exit(1);
> +	}
> +	
>  }
> 
>  int main(int argc, char *argv[])
> @@ -111,19 +142,23 @@ int main(int argc, char *argv[])
>  		exit(1);
>  	}
> 
> -	/* output file (default: stdout) */
> +	/* output file */
>  	if (args.output) {
> -		ret = open(args.output, O_RDWR | O_CREAT, 0);
> -		if (ret < 0) {
> +		args.outputfd = open(args.output, O_RDWR | O_CREAT, 0);
> +		if (args.outputfd < 0) {
>  			perror("open output file");
>  			exit(1);
>  		}
> -		if (dup2(ret, STDOUT_FILENO) < 0) {
> +	}
> +
> +	/* output file descriptor (default: stdout) */
> +	if (args.outputfd >= 0) {
> +		if (dup2(args.outputfd, STDOUT_FILENO) < 0) {
>  			perror("dup2 output file");
>  			exit(1);
>  		}
> -		if (ret != STDOUT_FILENO)
> -			close(ret);
> +		if (args.outputfd != STDOUT_FILENO)
> +			close(args.outputfd);
>  	}
> 
>  	ret = checkpoint(pid, STDOUT_FILENO, flags);
> -- 
> 1.6.0.4
> 
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