Write char[128] array to file in C

Question

I wrote a program to test writing a char[128] array to file using write() function in C. The following is my code, however, after writing, I can see that the string "testseg" is followed by a "d" or "È" in the testFile.txt file. Is this a proper way of writing char[] array to file?

int main()
{
    char pathFile[MAX_PATHNAME_LEN];
    sprintf(pathFile, "testFile.txt");
    int filedescriptor = open(pathFile, O_RDWR | O_APPEND | O_CREAT, 0777);

    int num_segs = 10;
    int mods = 200;
    const char *segname = "testseg";  /* */
    char real_segname[128];
    strcpy(real_segname, segname);

    write(filedescriptor, &num_segs, sizeof(int));
    write(filedescriptor, real_segname, strlen(real_segname));
    printf("real_segname length is %d \n", (int) strlen(real_segname));
    write(filedescriptor, &mods, sizeof(int));

    close(filedescriptor);


    return 0;
}

After wwriting srting, your writing 200(mod) into file. that 200 is "d" or "È" — gangadhars
– gangadhars, Commented Apr 24, 2014 at 3:54
Succinctly, yes; it is one proper way to do it. However, you may have some problems reading the data back; you won't know how long the string is. This is often fixed by using a fixed-length write (for example, using strncpy(real_segname, segname, sizeof(real_segname); and then write(filedescriptor, real_segname, sizeof(real_segname)); — or by including a length before the string: short s = strlen(segname); write(filedescriptor, &s, sizeof(s)); write(filedescriptor, real_segname, s);. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 24, 2014 at 3:54
Is the "d" or "E" the very next byte after testseg? Because you're writing mods after that. Could that be it? — ooga
– ooga, Commented Apr 24, 2014 at 3:54
As @SGG said, you are writing that data. The write function writes binary data. If you want the numbers to be human readable you should print them to a string using sprintf and then write that string. You may also want to add a new line character '\n' — austinmarton
– austinmarton, Commented Apr 24, 2014 at 3:56

chux · Accepted Answer · 2014-04-24 04:50:15Z

2

...writing a char[128] array to file ...I can see that the string "testseg" ...
is a contradiction.

In C, a string is an array of char followed by and including a '\0' and a char[128] is a fixed 128 char in length.

When code does write(filedescriptor, real_segname, strlen(real_segname));, it does neither. It is not writing a C string, 7 char of "testseg" terminated with a '\0'. Instead it just wrote the 7 char and no terminating '\0'. Neither did it write 128 char.

One could instead perform write(filedescriptor, real_segname, strlen(real_segname)+1); to write the 7 char and the terminating '\0'. Or write the length and then the interesting parts of the arry. Or write the entire 128 char array`. Need to identify how you want to read data back and other coding goals to well advise.

As @SGG suggests, the unusually char are simply the result of write(filedescriptor, &mods, sizeof(int)); and are not part of your unterminated array.

answered Apr 24, 2014 at 4:50

chux

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1 Comment

gangadhars Over a year ago

yes, you're right. It won't write NULL character. I didn't cared that :(.

gangadhars · Accepted Answer · 2014-04-24 04:25:28Z

after writing, I can see that the string "testseg" is followed by a "d" or "È" in the testFile.txt file

Why it is showing "d" or "È"??

Only try below write function (in your code, comment remaining write calls except below call)

write(filedescriptor, &mods, sizeof(int));

Now see the contents of testFile.txt (cat testFile.txt). It shows some junk value(s).

Because, all .txt files will show you in the form of ASCII text format. It converts every byte into ASCII charcter. String and characters you're writing in ASCII format and reading them as ASCII. So no problem. But here you're writing mods and num_segs as integers and reading them as ASCII format. So you got those junk values.

Is this a proper way of writing char[] array to file?

Yes, according to man pages you're writing them in proper way. And please make sure to validate your function calls(write). Where to write, what to write in a file depends upon your requirement.

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