Using strcat in C

Question

Okay so I have the following Code which appends a string to another in C#, note that this is Just an example, so giving alternative string concatination methods in C# is not nessesary, this is just to simplify the example.

string Data = "";
Data +="\n\nHTTP/1.1 " + Status_code;
Data += "\nContent-Type: " + Content_Type;
Data += "\nServer: PT06";
Data += "\nContent-Length: " + Content_Lengt;
Data += "\nDate: " + Date;
Data += "\n" + HTML;

Now I'd like to do the exact same thing in C and I'm trying to do this the following way

time_t rawtime;

time ( &rawtime );

char *message = "\n\nHTTP/1.1 ";
message = strcat(message, Status_code);
message = strcat(message, "\nContent-Type: ");
message = strcat(message, Content_Type);
message = strcat(message, "\nServer: PT06");
message = strcat(message, "\nContent-Length: ");
message = strcat(message, Content_Lengt);
message = strcat(message,  "\nDate: ");
message = strcat(message, ctime(&rawtime));
message = strcat(message, "\n");
message = strcat(message, HTML);

Now, this gives me a Segment fault, I know why, I access and read on memory that i shouldn't. But the question is, how do i solve it? Could I use string.h and just do it the same way that I did in C#?

Answer 1

Change

char *message = "\n\nHTTP/1.1 ";

to

char message[1024];  
strcpy(message,"\n\nHTTP/1.1 ");

and you should be ok, up to a total message length of 1023.

Edit: (as per mjy's comment). Using strcat in this fashion is a great way of getting buffer overflows. You could readily write a small function that checks the size of the buffer and length of incoming string addition to overcome this, or use realloc on a dynamic buffer. IMO, the onus is on the programmer to check correct buffer sizes where they are used, as with sprintfs and other C strings functions. I assume that C is being used over C++ for performance reasons, and hence STL is not an option.

Edit: As per request from Filip's comment, a simple strcat implementation based on a fixed size char buffer:

char buffer[MAXSIZE] = "";

int mystrcat(char *addition)
{
   if (strlen(buffer) + strlen(addition) + sizeof(char)  >= MaxSize)
     return(FAILED);
   strcat(buffer,addition);
   return(OK);
}

Using dynamic allocation:

char *buffer = NULL;

int mystrcat(char *addition)
{
   buffer = realloc(buffer, strlen(buffer) + strlen(addition) + sizeof(char));
   if (!buffer)
     return(FAIL);
   strcat(buffer, addition);
   return(OK);
}

In this case you have to free your buffer manually when you are finished with it. (Handled by destructors in C++ equivalents)

Addendum (Pax):

Okay, since you didn't actually explain why you had to create message[1024], here it is.

With char *x = "hello", the actual bytes ('h','e','l','l','o',0) (null on the end) are stored in an area of memory separate from the variables (and quite possibly read-only) and the variable x is set to point to it. After the null, there's probably something else very important. So you can't append to that at all.

With char x[1024]; strcpy(x,"hello");, you first allocate 1K om memory which is totally dedicated to x. Then you copy "hello" into it, and still leave quite a bit of space at the end for appending more strings. You won't get into trouble until you append more than the 1K-odd allowed.

End addendum (Pax):

Answer 2

I wonder why no one mentioned snprintf() from stdio.h yet. That's the C way to output multiple values and you won't even have to convert your primitives to strings beforehand.

The following example uses a stack allocated fixed-sized buffer. Otherwise, you have to malloc() the buffer (and store its size), which would make it possible to realloc() on overflow...

char buffer[1024];
int len = snprintf(buffer, sizeof(buffer), "%s %i", "a string", 5);
if(len < 0 || len >= sizeof(buffer))
{
    // buffer too small or error
}

Edit: You might also consider using the asprintf() function. It's a widely available GNU extension and part of TR 24731-2 (which means it might make it into the next C standard). The example from above would read

char * buffer;
if(asprintf(&buffer, "%s %i", "a string", 5) < 0)
{
    // (allocation?) error
}

Remember to free() the buffer when done using it!