CODEWITHSUNDEEP
cstringfile
TonyW
TonyW

Reputation: 18875

Write char[128] array to file in C

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;
}

Upvotes: 1

Views: 2164

Answers (2)

chux
chux

Reputation: 153348

...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.

Upvotes: 2

gangadhars
gangadhars

Reputation: 2728

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.

Upvotes: 1

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