CODEWITHSUNDEEP
csystem-calls
mrQWERTY
mrQWERTY

Reputation: 4149

Getting write() system call to write in file

I am having trouble getting write() system call to write in a file. So far, I manage to only print gibberish into a file that is said to be a binary file. I know that we must somehow preformat write().

How do I actually get write() to create/write in a text file?

void writeNumbers(int array[], int size, char* fname, int hexFlag) {

    int fp;
    int i;
    char buf[512];


    fp = open(fname, O_WRONLY | O_CREAT | O_APPEND);         


    if (fp < 0) {
            close(fp);
            char msg[] = "Cannot write";
            write(STDOUT_FILENO, msg, sizeof(msg) - 1);
            exit(1);
    }


    if (hexFlag == 0) {
            for (i = 0; i < size; i++) { 
                    snprintf(buf, sizeof(int), "%d ", array[i]);
                    write(fp, buf, sizeof(buf));
            }
    } else {
            for (i = 0; i < size; i++) { 
                    snprintf(buf, sizeof(int), "%x ", array[i]);
                    write(fp, buf, sizeof(int));
            }
    }
    write(fp, "message", sizeof(buf));


    close(fp);
}

Here I am trying to print an array of ints either in hexadecimal or decimal formats.

Upvotes: 0

Views: 1165

Answers (2)

alk
alk

Reputation: 70981

snprintf() returns the number of characters printed into the buffer passed. Also it takes the maximum number of characters to print to the buffer as second argument, that typically is the size of the buffer, but also could be few.

You want to write out exactly the number of characters, as returned by snprintf().

        if (hexFlag == 0) {
            for (i = 0; i < size; i++) { 
                int result = snprintf(buf, sizeof(buf), "%d ", array[i]);
                if (0 > result)
                {
                  write(STDERR_FILENO, "snprintf() failed.\n", strlen("snprintf() failed.\n"));
                }
                else
                {
                  write(fp, buf, result);
                }
            }
        }
        else 
        ...

As general advise: Carefully read a function's documentation.

Upvotes: 2

JS1
JS1

Reputation: 4767

Each call to snprintf should take sizeof(buffer) instead of sizeof(int). Each call to write should take strlen(buf) as its 3rd argument.

Upvotes: 2

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