next size error in realloc()

orion9114 · 11-07-2011, 01:31 PM

hi guys, im going crazy for this problem!!

Code:

char* read_s(FILE* fs) {
    int mem = 20;
    char* str = malloc (mem * sizeof(char));
    int i = 0, r;
    char c;
    while ((r = fread(&c, sizeof(char), 1, fs)) != 0 &&  c != '\n') {
        str[i] = c;
        i++;
        if (i > mem - 1) {
            mem += mem/2;
            str = realloc(str, sizeof(char) * mem);
        }
    }
    if (r == 0)
        return NULL;
    return str;
}

this is my code it's a simple function that read a line of a file, if the line is longer than allocated memory for buffer i do a realloc on that buffer, but at the second iteration realloc goes wrong, and i don't know how to fix it!!
thank u guys!!

jhwilliams · 11-07-2011, 02:54 PM

I do not observe that to be the case; the program works as you have specified. I tried it with the following test code:

Code:

int main(int argc, char *argv[]) {
    char *string = NULL;
    FILE *filep = NULL;

    if (argc != 2 || argv == NULL || argv[1] == NULL) {
        return EXIT_FAILURE;
    }   

    if (NULL == (filep = fopen(argv[1], "r"))) {
        return EXIT_FAILURE;
    }   

    if (NULL == (string = read_s(filep))) {
        fclose(filep);
        return EXIT_FAILURE;
    }   

    printf("%s\n", string);

    return EXIT_SUCCESS;
}

orion9114 · 11-07-2011, 02:58 PM

meybe it works becouse realloc is invoked just 1 time! in fact it crash the second time it invoke realloc with this message:
crypt: malloc.c:3096: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.

jhwilliams · 11-07-2011, 04:11 PM

A few more thoughts.

(1) When you pass in a FILE * to a zero-byte file, r == 0 happens. At this point, you have a reference to some allocated memory str, which never gets freed. You should add free(str) to that if() statement:

Code:

    if (r == 0) {
        free(str);
        return NULL;
    }

(2) Also, since you're allocating memory in read_s(), you need to make sure that you free the memory elsewhere in the program. Basically, when you're done with whatever you're doing, free() the output of read_s().

As a final note in that vein, run your program through gdb and valgrind.

orion9114 · 11-07-2011, 05:12 PM

Quote:

Originally Posted by jhwilliams

A few more thoughts.

(1) When you pass in a FILE * to a zero-byte file, r == 0 happens. At this point, you have a reference to some allocated memory str, which never gets freed. You should add free(str) to that if() statement:

Code:

    if (r == 0) {
        free(str);
        return NULL;
    }

(2) Also, since you're allocating memory in read_s(), you need to make sure that you free the memory elsewhere in the program. Basically, when you're done with whatever you're doing, free() the output of read_s().

As a final note in that vein, run your program through gdb and valgrind.

thank you very much!! now it works

it's alway free's fault -.-