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