Realloc returns invalid pointer when char array is passed into another function

Question

I have a short snippet where I am passing a char array into another function and changing the size in memory.

// in main
char *str = argv[1];
find(&str);

void func(char **str){
  // some code
  *str = realloc(*str, 10+1);
}

This gives the error

realloc(): invalid pointer
Aborted (core dumped)

What did I do wrong here? In order to change the value of str in another function, I am using a double pointer.

You can only reallocate an array that was allocated with malloc. argv[1] is not dynamically allocated. — Barmar
– Barmar, Commented Feb 20, 2020 at 3:06
@Barmar being pedantic, it might be dynamically allocated, but not in a way that it can be realloc()ed, or argv[] might be in read-only memory, etc, etc. — Ken Y-N
– Ken Y-N, Commented Feb 20, 2020 at 3:07
@KenY-N It's not specified to be dynamically allocated, so you can't assume it is. So as far as being able to call realloc, it should be considered not dynamically allocated. — Barmar
– Barmar, Commented Feb 20, 2020 at 3:20
Note on realloc (not your primary issue, but...). Do NOT realloc to the original pointer, e.g. don't do *str = realloc(*str, 10+1); instead, use a temporary pointer, e.g. void *tmp = realloc(*str, 10+1);, then validate the reallocation if (tmp == NULL) { /* handle error - return or exit */ }; *str = tmp; That way when realloc fails, you don't overwrite your original pointer with NULL losing the address to the prior allocated block creating a memory leak. — David C. Rankin
– David C. Rankin, Commented Feb 20, 2020 at 3:30
@KenY-N: it can't be in read-only memory. Sect 5.1.2.2.1; "The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination." — rici
– rici, Commented Feb 20, 2020 at 5:38

3 revs, 2 users 93% · Accepted Answer · 2020-02-20 15:31:35Z

3

What did I do wrong here?

*str in realloc(*str, 10+1) was not allocated by this process. @Barmar

Your code could have used the below or equivalent to allocate its own copy to later reallocate it. As strdup() is not in C STL as of C17, a sample implementation.

char *str = strdup(argv[1]);

edited Feb 20, 2020 at 15:31

community wiki

3 revs, 2 users 93%
chux - Reinstate Monica

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3 Comments

M.M Over a year ago

strdup is not in ISO C

Ken Y-N Over a year ago

Apparently it will be in C20.

chux Over a year ago

@M.M Post amended to include that.

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