Seems fine, works for me, thank you. On Sat, Aug 20, 2016 at 6:32 AM, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> wrote: > Hello Vladimir, > > On 08/18/2016 05:30 PM, Ursache Vladimir wrote: >> Hi, >> the readlink() example from: >> >> http://man7.org/linux/man-pages/man2/readlink.2.html >> >> relies on lstat()'s st_size, but that doesn't work for files contents >> created dynamically by the Linux kernel which often report a zero >> size, for example the link at: >> /sys/block/sda >> >> the example code will fail because stat.st_size reports zero and you >> try to read (stat.st_size + 1) which will succeed, which will generate >> the error : "symlink increased in size between lstat() and >> readlink()". >> >> Somewhat related, the same issue is true for reading regular text >> files, e.g: "/proc/filesystems" which will report stat.st_size = 0. >> >> My quick workaround: >> >> if (stat.st_size != 0) >> // work as usual >> else if (file_is_a_link) >> // malloc 4K of ram and try to readlink() into it >> else if (is_regular_file) >> // read() into a byte array that grows accordingly >> >> In case it matters, I'm using Ubuntu 16.04 amd64. > > Thanks for this report. I agree that the page needs to be fixed. > I modified the example program to be as follows: > > #include <sys/types.h> > #include <sys/stat.h> > #include <limits.h> > #include <stdio.h> > #include <stdlib.h> > #include <unistd.h> > > int > main(int argc, char *argv[]) > { > struct stat sb; > char *linkname; > ssize_t r, bufsiz; > > if (argc != 2) { > fprintf(stderr, "Usage: %s <pathname>\n", argv[0]); > exit(EXIT_FAILURE); > } > > if (lstat(argv[1], &sb) == -1) { > perror("lstat"); > exit(EXIT_FAILURE); > } > > bufsiz = sb.st_size + 1; > > /* Some magic symlinks under (for example) /proc and /sys > report 'st_size' as zero. In that case, take PATH_MAX as > a "good enough" estimate */ > > if (sb.st_size == 0) > bufsiz = PATH_MAX; > > printf("%zd\n", bufsiz); > > linkname = malloc(bufsiz); > if (linkname == NULL) { > perror("malloc"); > exit(EXIT_FAILURE); > } > > r = readlink(argv[1], linkname, bufsiz); > if (r == -1) { > perror("readlink"); > exit(EXIT_FAILURE); > } > > linkname[r] = '\0'; > > printf("'%s' points to '%s'\n", argv[1], linkname); > > if (r == bufsiz) > printf("(Returned buffer may have been truncated)\n"); > > free(linkname); > exit(EXIT_SUCCESS); > } > > Seem okay? > > 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
