identical strings. one is misbehaving

Question

v is read from stdin
v2 is not but they are identical.

char *v;
v = (char *) malloc(1);
fread(v,i,1,stdin);
printf("%s",v);

prints:

a=aaa&b=ooo%3C%2Ftest%3Eooo

v2 is identical to v

char *v2 = "a=aaa&b=ooo%3C%2Ftest%3Eooo"

let's try to PARSE what is after "b=" from "v"

const char *PATTERN1 = "b=";
const char *PATTERN2 = "&";

char *END = NULL;
char *START = strstr(v, PATTERN1);

if (START) {
START = START + strlen(PATTERN1);
char * END = strstr(START, PATTERN2);

if (!END){
ENDm = START + strlen(START);
}

TARGET = malloc(END - START + 1);
memcpy(TARGET, START, END - START);
TARGET[END - START] = '\0';
}

printf("--%s--",TARGET);

result :

--abc%3C%2Ftest%!--

it is missing some stuff towards the end. ( also has an extra "!" )

Now let's use the same code above on "v2"

--abc%3C%2Ftest%3Eabc--

this time nothing is missing

`v = (char *) malloc(1);` how do you expect this to accomodate your whole string? — Federico klez Culloca, Jun 22 '17 at 08:29
what should it have been ? I was suspecting something like that but I dont know much about those things yet. — Directional Forces, Jun 22 '17 at 08:30
use malloc with bigger size. v = malloc(100) should be enough in your case — CIsForCookies, Jun 22 '17 at 08:32
Usualy the [`man page of the command you are using fread`](https://linux.die.net/man/3/fread) is a good bet on how to get the length. — RedX, Jun 22 '17 at 08:32
Note that it is [redundant and potentially dangerous to cast the result of malloc and friends in C](http://stackoverflow.com/q/605845/253056). — Paul R, Jun 22 '17 at 08:40
Als, please do not use ALL UPPER CASE VARIABLE NAMES - this is C, not FORTRAN or BASIC. — Paul R, Jun 22 '17 at 08:42
@PaulR It is a common naming convention in C to name constants with all upper case. — Lundin, Jun 22 '17 at 09:08
@Lundin: yes, I'm aware of that, but the OP seems to using all upper case for local variables, e.g. `START`, `END`, `TARGET`. — Paul R, Jun 22 '17 at 09:47

unwind · Accepted Answer · 2017-06-22T08:39:28.037

5

You cannot store a string in an array of 1 characters. The only string you can store is "" (the empty string) since that requires exactly one character for the '\0' terminator.

Your code suffers from undefined behavior, so it cannot be reasoned about beyond that.

edited Jun 22 '17 at 08:39

answered Jun 22 '17 at 08:30

unwind

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identical strings. one is misbehaving

1 Answers1