当我将0作为getline的第二个参数传递时会发生什么？

Question

cplusplus.com states that the second parameter of the getline function is the

Maximum number of characters to write to s

However, I've seen code like this:

size_t linecap = 0;
ssize_t linelen;
linelen = getline(&line, &linecap, fp);

Won't this be reading 0 bytes from source? Or is there something else going on?

Answer 1

No, it's not correct. From the man page , ( emphasis mine )

If *lineptr is NULL , then getline() will allocate a buffer for storing the line, which should be freed by the user program. (In this case, the value in *n is ignored.)

Alternatively, before calling getline() , *lineptr can contain a pointer to a malloc(3)-allocated buffer *n bytes in size. If the buffer is not large enough to hold the line, getline() resizes it with realloc(3) , updating *lineptr and *n as necessary.

In either case, on a successful call, *lineptr and *n will be updated to reflect the buffer address and allocated size respectively.

So, the initial value stored in the memory pointed to by the second argument has no effect on the actual scanning. After the value is scanned and filled into the buffer,

• the function return value will tell you the size of the scanned input in bytes.
• the value of *n will tell you the size of the buffer which was allocated to store the input (which is usually bigger than the size of the scanned input).

Answer 2

The idea of getline is that there is as few as possible reallocations, as calls to malloc tend to be expensive. Hence if you use getline repeatedly to read the lines in a file, reusing the same buffer and length, the buffer will be eventually grown to the size of the longest line in the file, and no reallocations will be needed for the lines succeeding the longest line.

But for that to work certain contracts must be followed - namely if *lineptr is non-NULL then it

• must be a pointer returned by malloc
• must have allocation size of at least *n bytes

Corollary: passing 0 in *n is fine under these 2 circumstances:

• if *lineptr is NULL
• *lineptr is any live pointer returned by malloc (as any pointer returned by malloc will have 0 bytes space).

in both cases *n will be updated to the length of the line, and the return value of realloc(*lineptr, new_line_length_with_terminator) (if successful) will have been assigned to *lineptr .

Of course