jjk@astro.UMD.EDU (Jim Klavetter) (10/03/89)
Here is a section of code:
if((home=getenv("HOME"))==(char *) 0)
perror("GETENV\n");
printf("%s:\n", home);
printf("%s:\n", strcpy(string,strcat(home, "/astro/data/obs.list")));
printf("%s:\n", string);
printf("%d:\n", access(string, 4));
printf("%d:\n", access(strcat(home, "/astro/data/obs.list"), 4));
And the associated output:
/a/jjk:
/a/jjk/astro/data/obs.list:
/a/jjk/astro/data/obs.list:
0:
-1:
The man page says that strcat returns a null-terminated string so that
the two calls to access (I think) should both give "0" but the second
is saying the file doesn't exist. I've included the appropriate
files. Any response to the above address would be appreciated (and
I will summarize the response if there is some good answer(s)).
jjkok@cs.mu.oz.au (Richard O'Keefe) (10/03/89)
In article <5409@umd5.umd.edu>, jjk@astro.UMD.EDU (Jim Klavetter) writes: > if((home=getenv("HOME"))==(char *) 0) perror("GETENV\n"); > printf("%s:\n", home); At this point, home points to "/a/jjk\0" followed by who knows what > printf("%s:\n", strcpy(string,strcat(home, "/astro/data/obs.list"))); ^^^^^ At this point, home points to "/a/jjk/astro/data/obs.list\0" who knows what strcat() copies its second argument into the same array that its first argument points to, starting at the first NUL it finds there. > printf("%d:\n", access(strcat(home, "/astro/data/obs.list"), 4)); At this point, home points to "/a/jjk/astro/data/obs.list/astro/data/obs.list" > The man page says that strcat returns a null-terminated string so that Yes it does; and the pointer it returns is the pointer you gave it as its first argument. Sometimes it is simpler to forget strcat() and so on and just use sprintf: sprintf(work_string, "%s/astro/data/obs.list", home); This is unlikely to be a performance bottleneck.
cpcahil@virtech.UUCP (Conor P. Cahill) (10/03/89)
In article <5409@umd5.umd.edu>, jjk@astro.UMD.EDU (Jim Klavetter) writes: > Here is a section of code: > > if((home=getenv("HOME"))==(char *) 0) > perror("GETENV\n"); Perror is useless here since getenv is not a system call. In addition, since you use home in later code, you should exit here. > printf("%s:\n", home); > printf("%s:\n", strcpy(string,strcat(home, "/astro/data/obs.list"))); ^^^^^ This modifies home. You should never do this to an environment pointer since this will mess up your environment (or may cause the program to core dump if HOME is at the end of your environment and the end of your address space). Home now points to "/a/jjk/astro/data/obs.list" > printf("%s:\n", string); > printf("%d:\n", access(string, 4)); > printf("%d:\n", access(strcat(home, "/astro/data/obs.list"), 4)); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Home now points to "/a/jjk/astro/data/obs.list/astro/data/obs.list" hence the failure code from access(2). > The man page says that strcat returns a null-terminated string so that > the two calls to access (I think) should both give "0" but the second > is saying the file doesn't exist. I've included the appropriate > files. Any response to the above address would be appreciated (and > I will summarize the response if there is some good answer(s)). You must read the man page for any routine you want to use. If you had read the man page for strcat (string(3) or maybe strings(3)) you would have seen that strcat appends the second string to the first string and returns a pointer to the first string (in this case home). Make sure you read the entire man page!!!!!!! -- +-----------------------------------------------------------------------+ | Conor P. Cahill uunet!virtech!cpcahil 703-430-9247 ! | Virtual Technologies Inc., P. O. Box 876, Sterling, VA 22170 | +-----------------------------------------------------------------------+
john@chinet.chi.il.us (John Mundt) (10/03/89)
In article <5409@umd5.umd.edu> jjk@astro.UMD.EDU writes: >Here is a section of code: > >if((home=getenv("HOME"))==(char *) 0) >printf("%d:\n", access(strcat(home, "/astro/data/obs.list"), 4)); home is a char pointer, pointing to somewhere in the environmental variable area that was passed to main since that is where getenv() looks for things like HOME. main(int argc, char **argv, char **env) When you concatenated home with the rest of the path, it would have to step on other environmental variables, probably one that was needed by access(). -- --------------------- John Mundt Teachers' Aide, Inc. P.O. Box 1666 Highland Park, IL john@chinet.chi.il.us (312) 998-5007 (Day voice) || -432-8860 (Answer Mach) && -432-5386 Modem
cpcahil@virtech.UUCP (Conor P. Cahill) (10/04/89)
In article <9733@chinet.chi.il.us>, john@chinet.chi.il.us (John Mundt) writes: > home is a char pointer, pointing to somewhere in the environmental > variable area that was passed to main since that is where getenv() > looks for things like HOME. > > When you concatenated home with the rest of the path, it would have > to step on other environmental variables, probably one that was > needed by access(). The part about stepping on other environment variables is true, but the cause of the problem is not that access() requires something in the environment. access(2) is a system call and system calls don't use any such thing as an environment variable (can you imagine the kernel searching through your data space looking for some strings?) The problem is that the original poster appended the path from his HOME directory to the file twice thereby generating a non-existent file name. -- +-----------------------------------------------------------------------+ | Conor P. Cahill uunet!virtech!cpcahil 703-430-9247 ! | Virtual Technologies Inc., P. O. Box 876, Sterling, VA 22170 | +-----------------------------------------------------------------------+
mccaugh@s.cs.uiuc.edu (10/04/89)
> Here is a section of code: > > if((home=getenv("HOME"))==(char *) 0) > perror("GETENV\n"); > printf("%s:\n", home); > printf("%s:\n", strcpy(string,strcat(home, "/astro/data/obs.list"))); > printf("%s:\n", string); > printf("%d:\n", access(string, 4)); > printf("%d:\n", access(strcat(home, "/astro/data/obs.list"), 4)); > > And the associated output: > > /a/jjk: > /a/jjk/astro/data/obs.list: > /a/jjk/astro/data/obs.list: > 0: > -1: > > The man page says that strcat returns a null-terminated string so that > the two calls to access (I think) should both give "0" but the second > is saying the file doesn't exist. Actually, I recall the 'man' page stating that strcat returns a pointer (char-type) to the null-terminated string, but in any case I suspect that the last printf contains a call to strcat which assigns to your string-variable 'home' the name of a truly non-existent file, namely: "/a/jjk/astro/data/obs.list/astro/data/obs.list", so a redundant 'strcat' appears to have occurred in variable 'home'.