Hi Heinrich Thanks for the revision. Please see comments below. On 02/09/2015 07:57 PM, Heinrich Schuchardt wrote: > An example code is provided for listxattr and getxattr. > > Changes from version 1: > > As Michael correctly pointed out, the buffer length returned by > listxattr does not contain any byte after the 0x00 indicating > the end of the last list entry. > > Furthermore getxattr does not append 0x00. > > Signed-off-by: Heinrich Schuchardt <xypron.glpk@xxxxxx> > --- > man2/listxattr.2 | 141 ++++++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 140 insertions(+), 1 deletion(-) > > diff --git a/man2/listxattr.2 b/man2/listxattr.2 > index a67592f..3fa6d20 100644 > --- a/man2/listxattr.2 > +++ b/man2/listxattr.2 > @@ -1,5 +1,6 @@ > .\" Copyright (C) Andreas Gruenbacher, February 2001 > .\" Copyright (C) Silicon Graphics Inc, September 2001 > +.\" Copyright (C) 2015 Heinrich Schuchardt <xypron.glpk@xxxxxx> > .\" > .\" %%%LICENSE_START(GPLv2+_DOC_FULL) > .\" This is free documentation; you can redistribute it and/or > @@ -22,7 +23,7 @@ > .\" <http://www.gnu.org/licenses/>. > .\" %%%LICENSE_END > .\" > -.TH LISTXATTR 2 2014-02-06 "Linux" "Linux Programmer's Manual" > +.TH LISTXATTR 2 2015-02-09 "Linux" "Linux Programmer's Manual" (No need to update the timestamp these days; my scripts do it automatically) > .SH NAME > listxattr, llistxattr, flistxattr \- list extended attribute names > .SH SYNOPSIS > @@ -150,6 +151,144 @@ These system calls are Linux-specific. > .\" and the SGI XFS development team, > .\" .RI < linux-xfs@xxxxxxxxxxx >. > .\" Please send any bug reports or comments to these addresses. > +.SH EXAMPLE > +The following program demonstrates the usage of > +.BR listxattr () > +and > +.BR getxattr (2). > +For a path provided, it lists all extended file attributes and their values. > + > +The following output was recorded when first creating a file, setting > +some extended file attributes, and then listing them with the example code. > + > +.SS Example output > +.in +4n > +.nf > +$ touch /tmp/foo > +$ setfattr -n user.fred -v chocolate /tmp/foo > +$ setfattr -n user.frieda -v bar /tmp/foo > +$ setfattr -n user.empty /tmp/foo > +$ ./listxattr /tmp/foo > +user.fred: chocolate > +user.frieda: bar > +user.empty: <no value> > +.fi > +.in > +.SS Program source > +.nf > +#include <malloc.h> > +#include <stdio.h> > +#include <stdlib.h> > +#include <attr/xattr.h> > +#include <sys/types.h> > + > +int > +main(int argc, char *argv[]) > +{ > + ssize_t keylen, vallen; > + char *key, *p, *val; > + int i; 'i' is unused. > + > + if (argc != 2) { > + fprintf(stderr, "usage: %s path\\n", argv[0]); s/u/U/ > + exit(EXIT_FAILURE); > + } > + > + /* > + * determine the length of the buffer needed I think it's a little better to begin each comment with a capitalized word. > + */ > + keylen = listxattr(argv[1], NULL, 0); > + if (keylen == \-1) { > + perror("listxattr"); > + exit(EXIT_FAILURE); > + } > + if (keylen == 0) { > + printf("%s has no attibutes.\\n", argv[1]); > + exit(EXIT_SUCCESS); > + } > + > + /* > + * allocate the buffer > + */ > + key = malloc(keylen); > + if (key == NULL) { > + perror("malloc"); > + exit(EXIT_FAILURE); > + } > + > + /* > + * copy the attribute list to the buffer > + */ > + keylen = listxattr(argv[1], key, keylen); > + if (keylen == \-1) { > + perror("listxattr"); > + exit(EXIT_FAILURE); > + } > + > + /* > + * The end of the list is marked by zero terminated string > + * of length zero. > + */ I think the above comment is stale, no? > + p = key; > + while (keylen > 0) { > + > + /* > + * output attribute name > + */ > + printf("%s: ", p); > + > + /* > + * determine length of value > + */ > + vallen = getxattr(argv[1], p, NULL, 0); > + if (vallen == \-1) > + perror("getxattr"); > + > + if (vallen > 0) { > + /* > + * allocate value buffer > + * one extra byte to append 0x00 > + */ > + ++vallen; > + val = malloc(vallen); Replace two previous lines with val = malloc(vallen + 1); > + if (val == NULL) { > + perror("malloc"); > + exit(EXIT_FAILURE); > + } > + > + /* > + * read value to buffer > + */ > + vallen = getxattr(argv[1], p, val, vallen); > + if (vallen == \-1) > + perror("getxattr"); > + else { > + /* > + * output attribute value > + */ > + val[vallen] = 0; > + printf("%s", val); > + } > + > + free(val); > + } else > + printf("<no value>"); > + > + printf("\\n"); > + > + /* > + *forward to next attribute name > + */ Here, I really think it would help readability to just take strlen() pf 'p', and then use that to adjust 'p' and 'keylen'. I puzzled over this a few seconds wondering if there were off-by-one errors. > + while(*++p) > + \-\-keylen; > + ++p; > + keylen \-= 2; > + } > + > + free(key); > + exit(EXIT_SUCCESS); > +} One further general point, which you need not do anything about, but might choose to. You use the 'len==0' technique to estimate the required buffer sizes. That's good. But of course the values in each case could increase by the time of the second call. One *could* implement looping logic that retries (to some loop count limit) the listxattr() and getxattr() calls in the case of an ERANGE error. > +.fi > .SH SEE ALSO > .BR getfattr (1), > .BR setfattr (1), Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
