glibc detected, realloc(): invalid next size:X [duplicate]

Question

This question already has answers here:

Facing an error "*** glibc detected *** free(): invalid next size (fast)" (2 answers)

Closed 8 years ago.

I'm writing a function which to get the contents of one line of a text file at a time for parsing, without knowing the size of the line. The function works if all I'm doing is getting the lines and printing them, but as soon as I try to allocate memory to another local variable I get the glibc error:

glibc detected *** ./mention: realloc(): invalid next size: 0x00000000010ac250 ***

Specifically, SEARCH_LENGTH is defined as 40 and adding (char * word) causes the error.

void testF(FILE * stream){
    char * line = calloc(SEARCH_LENGTH , sizeof(char));
    int lineScalar = 1;

    char * word = calloc(SEARCH_LENGTH, sizeof(char)); // adding this line causes the error

    while(fgets(line, SEARCH_LENGTH + 1, stream)!= NULL){ 

            while((*(line + (SEARCH_LENGTH*lineScalar -1)) != '\0') && (*(line + (SEARCH_LENGTH*lineScalar -1)) !='\n')){
                lineScalar++;

                line = realloc(line, sizeof(char)*SEARCH_LENGTH*lineScalar);
                assert(line);

                fgets(line + SEARCH_LENGTH*(lineScalar - 1), SEARCH_LENGTH + 1, stream);

            }// inner while

            lineScalar = 1;
            printf("Line:%s\n",line);
        }


        free(line);
        free(word);
}

Thanks

Answer 1

You allocate SEARCH_LENGTH characters in line, but then immediately tell fgets() that it's OK to read SEARCH_LENGTH+1; if it does so, it will have corrupted the heap, giving you precisely the sort of error messages you're seeing. Don't let fgets() write past the ends of your blocks!