Why fwrite doesn't work for binary files in C?

Question

I want to write some data into a binary file, using C language. Apparently, the function fwrite doesn't seem to work properly. This is the code:

typedef struct tw {
    char name[40];
    char tweet[150];
    int rt, like;
} Twitter;

void createFile() {
    FILE *bin;
    Twitter user;
    if (bin = fopen("test.bin", "wb") == NULL) {
        printf("Error");
        return 0;
    }
    strcpy(user.name, "name");
    strcpy(user.tweet, "this is a tweet");
    user.rt=5;
    user.like=10;

    fwrite(&user, sizeof(Twitter), 1, bin);

    fclose(bin);

}

The fwrite function doesn't write anything into the file, and I've looked for mistakes in the code, but I couldn't find any. Also, when I tried to use the function to write into a txt file, it worked correctly.

return 0 should be return, since it's a void function. — Barmar
– Barmar, Commented Nov 8, 2014 at 19:30
And read your compiler warnings. They point out all the problems in your code. — Barmar
– Barmar, Commented Nov 8, 2014 at 19:30

nos · Accepted Answer · 2014-11-08 19:29:06Z

3

You need to change

if (bin = fopen("test.bin", "wb") == NULL)

to

 if ((bin = fopen("test.bin", "wb")) == NULL)

answered Nov 8, 2014 at 19:29

nos

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2 Comments

Barmar Over a year ago

You should explain why the original code doesn't work, i.e. how it's being grouped.

Weather Vane Over a year ago

Because == has a higher predecence than =, bin is not set to the file pointer, but to either 0 or 1, the result of the == comparison. If the file was successfully opened, fopen() != NULL and so bin == 0 thus skipping the error report. However OP is then trying to call fwrite() with a file pointer of value 0 or NULL.

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