Why this code freezes, when i'm trying to use write (man 2 write) function?

Question

I'm trying to write test, in which some data should be read from file descriptor, so i'm using dup and pipe functions to check this.

 int main()
{
    char    *line;
    int     out;
    int     p[2];
    char    *str;
    int     len = 50;

    str = (char *)malloc(235436);
    for (int i = 0; i < 235436; ++i)
    {
        str[i]='h';
    }
    out = dup(1);
    pipe(p);
    dup2(p[1], 1);

    write(1, str, strlen(str)); //freezes there. malloc alocates memory, i've checked this with debuger
    close(p[1]);
    dup2(out, 1);
    get_next_line(p[0], &line);


}

And for some reason this code works perfectly although it does all the same.

str = (char *)malloc(1000 * 1000);
    *str = '\0';
    while (len--)
        strcat(str, "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur in leo dignissim, gravida leo id, imperdiet urna. Aliquam magna nunc, maximus quis eleifend et, scelerisque non dolor. Suspendisse augue augue, tempus");
    out = dup(1);
    pipe(p);
    dup2(p[1], 1);

    if (str)
        write(1, str, strlen(str));
    close(p[1]);
    dup2(out, 1);
    get_next_line(p[0], &line);

it doesn't matter. I 've replaced strlen with actual size of string, and appended \0 to the end. It just stays on this line of code — Vladimir, Jan 18 '19 at 15:18
Ok, then please edit your question with the actual code with this undefined behaviour fixed. — Arkku, Jan 18 '19 at 15:19
@Vladimir every one of the 235436 bytes is filled with `'h'` but no string terminator. — Weather Vane, Jan 18 '19 at 15:19
sorry guys, i'm an idiot. My ide responded after 30 seconds of waiting and this function is working. Actual problem really was with \0 — Vladimir, Jan 18 '19 at 15:21

score 2 · Answer 1 · answered Jan 18 '19 at 15:21

2

(Assuming you fix the strlen problem - i.e. write(1, str, 235436))

You are writing to a pipe. This pipe is not being read. Therefore the write will block.

Get something to read from the pipe.

answered Jan 18 '19 at 15:21

Ed Heal

59,252
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So, how should i fix this? i need to invoke read from pipe before write? – Vladimir Jan 18 '19 at 15:40
1

@Vladimir You can't read from the pipe "before write", because there's nothing to read at that time. Either you need to use non-blocking I/O (and accept that `write` may then return without writing everything) or accept that it may block until read _by some other thread/process_. – Arkku Jan 18 '19 at 15:50
is using of another fd will improve my situation? – Vladimir Jan 18 '19 at 15:51

HmT · Answer 2 · 2019-01-18T15:46:26.020

0

The problem is in calling the strlen function!

strlen(str)

Because you didn't set the terminator at the end of the string.

Adding the following code after the initialization loop will fix the problem:

str[235435]='\0';

edited Jan 18 '19 at 15:46

answered Jan 18 '19 at 15:24

HmT

856
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`sizeof str` is not a correct replacement, since `str` is a pointer, not an array. – Arkku Jan 18 '19 at 15:32
yes you are right, sizeof was removed! – HmT Jan 18 '19 at 15:46

Govind Parmar · Answer 3 · 2019-01-18T18:00:49.943

Typical strlen implementation:

size_t strlen(char *s)
{
    size_t len = 0;
    while(*s++) len++;
    return len;
}

In other words, it determines the length of a string by incrementing a pointer to the start of the string until a null-terminator is reached. Since your buffer is just a block of the character 'h' without a null-terminator, calling strlen on it has undefined behavior.

Your second code-example, where you explicitly add a null-terminator after calling malloc and then use strcat to write to the string repeatedly does not have this problem since string literals are automatically null-terminated.

Also, don't cast the result of malloc unless you need your code to be compatible with C++ as well.

score 0 · Accepted Answer · answered Jan 18 '19 at 18:49

I'm using pipe on linux machine. Acording to this answer i'm trying to read more bytes, than pipe can contain. So, in this situation i need to create file an operate with it.

FILE *fptr;
fptr = fopen("buffer", "rb+");
if(fptr == NULL) //if file does not exist, create it
{
    fptr = fopen("buffer", "wb");
}

Why this code freezes, when i'm trying to use write (man 2 write) function?

4 Answers4