Weird behaviour of C strcpy - c

Question

I wrote a tiny program in C. It compile perfectly and works, when I compile it through my Makefile and clang, however, in Xcode this function behaves not like it should (or it behaves like it should and clang is ignoring it).
size_t getUrlForArgAndPlatform(char **dest, const char *arg, const char *platform) {
int isLinux = strcmp(platform, "Linux");
int isOSX = strcmp(platform, "Darwin");
char *platformUrlDelimiter = malloc(6 + 1);
if (isLinux == 0) {
strcpy(platformUrlDelimiter, "linux");
} else if (isOSX == 0) {
strcpy(platformUrlDelimiter, "osx");
} else {
strcpy(platformUrlDelimiter, "common");
}
int length = (int) strlen(kBaseUrl);
length += strlen(platformUrlDelimiter);
length += strlen(arg);
length += 5;
char *buffer = (char *) malloc(length);
if (buffer == NULL) {
exit(EXIT_FAILURE);
}
strcpy(buffer, kBaseUrl);
strcat(buffer, "/");
strcat(buffer, platformUrlDelimiter);
strcat(buffer, "/");
strcat(buffer, arg);
strcat(buffer, ".md");
*dest = malloc(strlen(buffer) + 1);
strcpy(*dest, buffer);
free(platformUrlDelimiter);
free(buffer);
return strlen(buffer) + 1;
}
It works 4 times out of 10. In the other 6 times, Xcode is telling me it's failing at strcpy(*dest, buffer) with a SIGBRT. If I take a look at the debugger, I see that buffer contains the same string twice. Why?

Answer 1

The size you have calculated for buffer is not quite correct:
int length = (int) strlen(kBaseUrl);
length += strlen(platformUrlDelimiter);
length += strlen(arg);
length += 5;
The last part should be '+6' because you need space for 2 times "/", for ".md" AND for the terminating NUL.

Answer 2

(The incorrect length has already been mentioned.)
There is a lot of pointless copying going on in your code. Since you don't modify platformUrlDelimiter, there is no need to copy it twice. This is much simpler:
const char *platformUrlDelimiter = "common";
if (isLinux == 0) {
platformUrlDelimiter = "linux";
} else if (isOSX == 0) {
platformUrlDelimiter = "osx";
}
And remove the call to free (platformUrlDelimiter).
Every call to strcat() (probably) has to needlessly loop through the buffer. I would write:
int length = strlen(kBaseUrl)
+ strlen(platformUrlDelimiter)
+ strlen(arg)
+ 6;
*dest = malloc(length);
if (*dest) {
sprintf (*dest, "%s/%s/%s.md", kBaseUrl, platformUrlDelimiter, arg);
} else {
/* Error */
}
return length;
Now there is no need to use buffer at all.
Also, there's a real bug right at the end of your code:
free(buffer);
return strlen(buffer) + 1;
You must not use the value of buffer in any way after it has been freed.