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I am trying to read characters from a socket; here is the function I created for this, largely inspired from Here, where I remove some unrelevant parts here (this sample is compiling correctly).

int process(int socketfd, void *buffer, int n)
{
  int totread = 0;
  char ch;
  int numread;

  for(;;)
  {
    numread = read(socketfd, &ch, 1);

    if(numread == 0) {
        if (totread == 0) {
            return 0;
        } else {
            break;
        }
    } else {
        if(totread < n-1) {
            totread++;
            printf("totread: %d\n", totread);
            printf("Here is OK, ch value gets printed %c\n", ch);
            *buffer++ = ch;
            printf("With help provided, this line gets printed\n");
            printf("But the value <%s> of buffer, is blank (<> output)\n, buffer);
        }
        if (ch == '\n') {
            printf("%c\n", ch);
            printf("%s\n", buffer);
            break;
        }
    }
}
return totread;
}

I can't understand why the second line does not get printed. Obviously, the *buf++ = ch instruction is faulty; But it looks like correct. It simply affects the character read to the next value of the array of characters buf. I don't see errors or warnings at compile time, the client disconnects after the first line gets printed, and the second one is not reached.

EDIT

Here is how I initialize my buffer:

char *buffer = "";
int l = process(newsockfd, buffer, 100);
printf("Number read (function network) %d\n", l);

This is probably not the appropriate way to do it; I have also tried specifying a fixed length such as char buffer = [255]; The function does not exit then but nothing get printed. I'm quite a newbie in C programming, many thanks for your help!

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9
  • When you say the second line does not get printed, I presume you mean because the program crashes. This is likely to be a segv caused by writing to buf when buf is either NULL or invalid memory. What happens when you run it under gdb, having compiled with -g to include debugging symbols? Commented May 6, 2015 at 22:05
  • 1
    Is buffer big enough? Also, I believe buf[0] is never written to, seems like a potential problem waiting to happen. Commented May 7, 2015 at 0:10
  • Please also show us how you call this function and expecially how the buffer passed in is defined and initialised. Commented May 7, 2015 at 6:44
  • Also: Did you leave out the code for error checking by intention to make this example better readbale? Commented May 7, 2015 at 6:45
  • What is the n (passed in) for? Commented May 7, 2015 at 6:46

2 Answers 2

Reset to default
2 
int process(..., void * b, ...
{
  ...

  *b++ = ...

Dereferencing and/or incrementing a void-pointer is not valid C. The compiler should have at least warned you about this.

Recompile your code with all warnings on (for gcc pass the options -Wall -Wextra -pedantic for this). Then fix the code until no more warning are issued.

Referring your edit on how the function in question is called:

This

char *buffer = "";

does not declare a buffer, but just a pointer. This pointer points to a string literal of size 1. String literals a constant by definition.

To summarise: You provide to your process()-function a pointer to constant memory of size 1.

Issues with this:

  1. You cannnot read into constant memory.
  2. An even if 1. would not apply you could only read in 1 byte.

To fix this change the code as follows:

char buffer[100] = ""; /* Define a buffer of 100 characters size and 
                          initialise it to all `0`s. */
int l = process(newsockfd, buffer, 100);

As per process() blocking, you need to look at the read() function: If it gets called but the sender does not send anything it simply does not return as long as the connection is still up.

And: Make process() handle the value passed in as n.

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5 Comments

I am using eclipse as IDE; maybe this is not the best choice for C programming :/
@philippe: Eclipse works fine for C programming, at least if you properly .. ;-) If using CDT you are using gcc anyways. Eclipse isn't a compiler/linker, but just the IDE.
I have added in Eclipse -Wall -Wextra -Werror -pedantic -pedantic-errors; I corrected every errors, the code compile now. Thanks for your advice for compilation options!
Not yet handled the value passed in as n, but edited my code taking in account your answer. Now, the program does not crash, as it at least reach the next printf; but the buffervalue does not get affected.
OK, I handle the value passed in as n now; furthermore, I handle the newline break; At this time, I know for sure buffer get instantiated; as it gets printed in when entering \n
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Try changing the first line to char *buf = (char *) buffer; and compile using gcc with -Werror on

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1 Comment

This does not chance anything :/

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