fread not advancing the pointer

Question

I am working with fread() in C. I am trying to read content of a binary file name pds_demo.bin but somehow my fread function is not advancing. I checked the returned value of fread() in gdb it is returning 0.

There are totally 3 files I am working with pds_functions.c, pds_test.c and the pds_demo.bin (contains the data to be read).

pds_test.c is calling a function named pds_search_by_key() in pds_functions.c to check for a particular. This function checks the content of demo file and returns the status whether the record is present or not. I have included all the files below.

Any kind of help would be appreciated. Thanks.

pds_test.c:

void test_search() {
    int status;
    struct Contact c3;
    int key;
    key = 101;
    status = pds_search_by_key(key, &c3);
    if (status != PDS_SUCCESS) {
        fprintf(stderr, "pds_search_by_key failed for key %d: errorcode %d\n", key, status);
    } else {
        printContact(&c3);
    }
    key = 102;
    status = pds_search_by_key(key, &c3);
    if (status != PDS_SUCCESS) {
        fprintf(stderr, "pds_search_by_key failed for key %d: errorcode %d\n", key, status);
    } else {
        printContact(&c3);
    }
    key = 1020000;
    status = pds_search_by_key(key, &c3);
    if (status != PDS_SUCCESS) {
        fprintf(stderr, "pds_search_by_key failed for key %d: errorcode %d\n", key, status);
    } else {
        printContact(&c3);
    }
}

pds_functions.c:

int pds_search_by_key(int key, struct Contact *c) {

    fseek(repo_fptr, 0L, 0);

    if (pdsInfo.repo_status == 1) {
        while (!feof(repo_fptr)) {
            fread(c, sizeof(struct Contact), 1, repo_fptr);
            if (c->contact_id == key) {
                return PDS_SUCCESS;
            }
        }
        return PDS_REC_NOT_FOUND;
    }
    return PDS_REPO_NOTOPEN;
}

pds_demo.bin:

101 Contact #1 Phone #1 Email #1 102 Contact #2 Phone #2 Email #2 102 Contact #2 Phone #2 Email #2

There is also one structure which define the structure Contact.

struct Contact {
    int contact_id;
    char cname[MAX_NAME_LEN];
    char mphone[MAX_PHONE_LEN];
    char email[MAX_EMAIL_LEN];
};

Also when I debug the program using gdb the content of *c is as follows:

(gdb) p *c
$1 = {contact_id = 540094513, cname = "Contact #1 Phone #1 Email #1 102 
Contact #2 Phone ", mphone = "#2 Email #2", 
  email = " 102 Contact #2 Phone #2 Email #2 \377\377\177\000\000\t\t@\000\000\000\000\000\000\000"}

[`while (!feof(...))` is wrong](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong). — melpomene, Jul 23 '17 at 04:12
A point to note is that `fread` reads `c` which is a type of `struct Contact`, but the `size_t` argument is `1`. — phoxis, Jul 23 '17 at 05:41
`fread(c, 1, 1, repo_fptr);` reads a single byte, and you don't check that it was successful. If `c` is not a `char` — and it isn't — you need something more like `if (fread(c, sizeof(*c), 1, repo_fptr) != 1) { /* oops — nothing read */ }`. — Jonathan Leffler, Jul 23 '17 at 05:52
my bad actually i was just doing trial and error stuff and forgot to change the code before uploading here. Anyway i tried with fread(c,sizeof(struct Contact),1,repo_fptr) but still the result is same. — Coding bat, Jul 23 '17 at 06:17
I suggest the loop should be like `while(fread(c, sizeof *c, 1, repo_fptr) == 1) { if(c->contact_id == key) { return PDS_SUCCESS; }}` — Weather Vane, Jul 23 '17 at 06:31
Notice also that `fread` will return `0` if it cannot read one full record — Antti Haapala -- Слава Україні, Jul 23 '17 at 06:38
It seems that you are writing as text without using `fwrite`. — BLUEPIXY, Jul 23 '17 at 07:25
@BLUEPIXY You are right i was actually using fprintf(). But problem still persists. fread() is not advancing the pointer. — Coding bat, Jul 23 '17 at 08:42
Your reading loop is incorrect: Learn why here: https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong — chqrlie, Jul 23 '17 at 12:24

chqrlie · Accepted Answer · 2017-07-23T14:17:56.330

There are major problems in your code:

Your reading loop is incorrect, lean why here: Why is “while ( !feof (file) )” always wrong?
You are reading fixed length structures from a file, you must read and write the file in binary mode, fopen must be passed "rb" or "wb" respectively to open the file in binary mode.

Provided the file is opened in binary mode, the reading loop could be modified this way:

int pds_search_by_key(int key, struct Contact *c) {

    fseek(repo_fptr, 0L, 0);

    if (pdsInfo.repo_status == 1) {
        while (fread(c, sizeof(*c), 1, repo_fptr) == 1) {
            if (c->contact_id == key) {
                return PDS_SUCCESS;
            }
        }
        return PDS_REC_NOT_FOUND;
    }
    return PDS_REPO_NOTOPEN;
}

fread not advancing the pointer

1 Answers1