C function to read binary file doesn't work if used by other function.?

Question

I wrote this function to return an unsigned char * containing the data from any file(using rbread mode) it looks like this:

uchar * get_file_data (char *f_name, luint *len) {

    // verify file
    struct stat sBuffer;
    int iRc;

    iRc = stat(f_name, &sBuffer);
    if (iRc != 0) {
        printf("Failed to get file size\n");
        exit(3);
    }

    // read file
    FILE *fp;
    fp = fopen(f_name, "rb");
    if (fp == NULL) {
        printf("Failed to open file\n");
        exit(3);
    }

    uchar *buff = (uchar *) malloc(sBuffer.st_size);
    *len = fread(buff, 1, sBuffer.st_size, fp);
    fclose(fp);

    return buff;

}

Types:

typedef unsigned char uchar;
typedef unsigned long int luint;

Now this works when run from the int main() function like so:

luint *len;
uchar *dat = get_file_data(argv[2] /* argument congaing filename */, len);
print_uchar(dat, len);
free(dat);

However when calling has_checksum(argv[2]); from int main() i get a Segmentation fault (core dumped) though the use is seemingly the same.

int has_checksum (char *f_name) {

    luint *p_len;
    uchar *p_f_data = get_file_data(f_name, p_len); /* this fails */

    /* more code, never reached */

}

When i try running my program with the following main():

int main(int argc, char **argv) {

    // verifying other arguments (not filename) */

    luint *len;
    uchar *dat = get_file_data(argv[2], len);
    print_uchar(dat, len);
    free(dat);

    has_checksum(argv[2]);

    return 0;
}

I get this output:

$ ./md5 -add ../<filename> 
<file contents (correct)>
Segmentation fault (core dumped)

I tested if char *f_name in get_file_data() was the same each time(using printf("%s\n", f_name); immediately when entering get_file_data()), and it was.

System:
-- VM Ubuntu 17.04 (64-bit) -- VirtualBox

You're supposed declare `luint len;` and pass in `&len` - now you have an uninitialized pointer pointing to who knows where. — Antti Haapala -- Слава Україні, Oct 03 '17 at 14:10
Possible duplicate of [Crash or "segmentation fault" when data is copied/scanned/read to an uninitialized pointer](https://stackoverflow.com/questions/37549594/crash-or-segmentation-fault-when-data-is-copied-scanned-read-to-an-uninitializ) — TylerH, Oct 03 '17 at 14:21

score 0 · Accepted Answer · answered Oct 03 '17 at 14:11

0

You're passing an uninitialized pointer in both cases, but you're getting "lucky" in main() and it appears to work. You need to pass the address of an actual integer:

luint len;
uchar *dat = get_file_data(argv[2], &len);

You see, when you declare luint *len;, you've declared a pointer but nothing for it to point at. It contains an uninitialized value and may point anywhere. You may or may not be able to write to that location.

answered Oct 03 '17 at 14:11

Fred Larson

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Seems that should be it. 3 people looking to find it and u do in 3 min or less. Will try this. Also thanks to @Antti Haapala – Otard95 Oct 03 '17 at 14:33

C function to read binary file doesn't work if used by other function.?

1 Answers1