copy a string to an array -- strcpy segfault

Question

I am trying to copy a string to an array and print it. It works for the first for loop, but seg faults the second time.

main (int argc, char *argv[]){
 int argcIndex;
 char **argumentArray = NULL;

 for(argcIndex=0; argcIndex < argc; argcIndex++){
     argumentArray = (char**) malloc((argc+1)*sizeof(char*));
     argumentArray[argcIndex] = (char*) malloc(((strlen(argv[argcIndex])+1)*sizeof(char)));
     strcpy(argumentArray[argcIndex], argv[argcIndex]); //Works here
     printf("argumentArray[%d]: %s \n", argcIndex, argumentArray[argcIndex]); //Works here
 }

 for(argcIndex=0; argcIndex < argc; argcIndex++){
     //strcpy(argumentArray[argcIndex], argv[argcIndex]); //This gives me a segfault
     printf("argumentArray[%d]: %s \n", argcIndex, argumentArray[argcIndex]); //This will only grab the last string in array
 }

}

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, Sep 29 '16 at 14:04
@SouravGhosh I took away the cast and the second printf statement still only grabs the last string passed in to the array — Felicia, Sep 29 '16 at 14:06

score 1 · Accepted Answer · edited May 23 '17 at 12:33

You need to move the allocation for argumentArray

 argumentArray = (char**) malloc((argc+1)*sizeof(char*));

outside the first for loop, otherwise, every iteration, you're overwriting (and finally, leaking) the memory.

The problem underneath that is, malloc() returns uninitialized memory, and for the last iteration, only one index, argc-1 is getting written, so the content of other index remains indeterminate. In the later loop, when you try to use that value, it invokes undefined behavior.

That said,

sizeof(char) is guaranteed to be 1 in C. There's no point in multiplying anything by it.
Please see this discussion on why not to cast the return value of malloc() and family in C..
instead of malloc() and strcpy(), you can make use of strdup(), if that's available.

Note: strdup() is not a standard C function, FWIW.

Thank you! I knew it was something about the malloc statement that was causing the error, but didn't know what. Thank you! — Felicia, Sep 29 '16 at 14:13

copy a string to an array -- strcpy segfault

1 Answers1