Thread: does strcat reallocs ?

  1. #1
    Registered User
    Join Date
    Mar 2006

    does strcat reallocs ?

    I'm experiencing something strange here:
    In the code below I malloced filename to the strlen of the category name, + 4 to add ".dat". When I do that I get stuffing in my string.

    Now my question: When I malloc filename to 1 (just to test) the filename string gets filled correctly. This is kinda strange to me, does strcat reallocs or something ?

    int get_questions(char*** stringarray, char* category)
        char* filename = NULL;
        int i = 0;
        filename = malloc(strlen(category)+4);
        //filename = malloc(1);
        getfilename("rubrieken/", category, filename);
        printf("%s", filename);
        return i;
    void getfilename(char* dir, char* string, char* filename)
        strcat(filename, dir);
        strcat(filename, string);
        strcat(filename, ".dat");

  2. #2
    Registered User
    Join Date
    Aug 2005
    no, strcat() does not change the length of the string. you have to do that yourself.

    >> strcat(filename, dir);
    that will crash because strcat looks for the first null terminator in the string then appends the new string at that point. Your string is unitialized, so strcat() will probably start appending way beyond the allocated memory.

    you can fix that problem by setting the first byte of the string to 0 before doing the first strcat();
    Last edited by Ancient Dragon; 03-26-2006 at 12:35 PM.

  3. #3
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    > When I malloc filename to 1 (just to test) the filename string gets filled correctly
    That just means you got lucky.
    The problems would start to appear on the next call to malloc or free, when the memory you overwrote would be used for it's real purpose.

    > filename = malloc(strlen(category)+4);
    In fact you need to count the \0 as well, so its
    filename = malloc(strlen(category)+4+1);

    > strcat(filename, dir);
    malloc does not guarantee that the memory will contain any particular value, so you should begin with a strcpy(), not a strcat()
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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