Why would one add 1 or 2 to the second argument of snprintf?

What is the role of 1 and 2 in these snprintf functions? Could anyone please explain it

snprintf(argv[arg++], strlen(pbase) + 2 + strlen("i开发者_JAVA百科vlpp"), "%s%ccivlpp", pbase, sep);
snprintf(argv[arg++], strlen(defines_path) + 1, "-F\"%s\"", defines_path);

The role of the +2 is to allow for a terminal null and the embedded character from the %c format, so there is exactly the right amount of space for formatting the first string. but (as 6502 points out), the actual string provided is one space shorter than needed because the strlen("ivlpp") doesn't match the civlpp in the format itself. This means that the last character (the second 'p') will be truncated in the output.

The role of the +1 is also to cause snprintf() to truncate the formatted data. The format string contains 4 literal characters, and you need to allow for the terminal null, so the code should allocate strlen(defines)+5. As it is, the snprintf() truncates the data, leaving off 4 characters.

I'm dubious about whether the code really works reliably...the memory allocation is not shown, but will have to be quite complex - or it will have to over-allocate to ensure that there is no danger of buffer overflow.

Since a comment from the OP says:

I don't know the use of snprintf()

int snprintf(char *restrict s, size_t n, const char *restrict format, ...);

The snprintf() function formats data like printf(), but it writes it to a string (the s in the name) instead of to a file. The first n in the name indicates that the function is told exactly how long the string is, and snprintf() therefore ensures that the output data is null terminated (unless the length is 0). It reports how long the string should have been; if the reported value is longer than the value provided, you know the data got truncated.

So, overall, snprintf() is a relatively safe way of formatting strings, provided you use it correctly. The examples in the question do not demonstrate 'using it correctly'.

One gotcha: if you work on MS Windows, be aware that the MSVC implementation of snprintf() does not exactly follow the C99 standard (and it looks a bit as though MS no longer provides snprintf() at all; only various alternatives such as _snprintf()). I forget the exact deviation, but I think it means that the string is not properly null-terminated in all circumstances when it should be longer than the space provided.

With locally defined arrays, you normally use:

nbytes = snprintf(buffer, sizeof(buffer), "format...", ...);

With dynamically allocated memory, you normally use:

nbytes = snprintf(dynbuffer, dynbuffsize, "format...", ...);

In both cases, you check whether nbytes contains a non-negative value less than the size argument; if it does, your data is OK; if the value is equal to or larger, then your data got chopped (and you know how much space you needed to allocate).

The C99 standard says:

The snprintf function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null-terminated output has been completely written if and only if the returned value is nonnegative and less than n.

The programmer whose code you are reading doesn't know how to use snprintf properly. The second argument is the buffer size, so it should almost always look like this:

snprintf(buf, sizeof buf, "..." ...);

The above is for situations where buf is an array, not a pointer. In the latter case you have to pass the buffer size along:

snprintf(buf, bufsize, "...", ...);

Computing the buffer size is unneeded.

By the way, since you tagged the question as qt-related. There is a very nice QString class that you should use instead.

At a first look both seem incorrect.

1. In the first case the correct computation would be path + sep + name + NUL so 2 would seem ok, but for the name the strlen call is using ilvpp while the formatting code is using instead cilvpp that is one char longer.

2. In the second case the number of chars added is 4 (-L"") so the number to add should be 5 because of the ending NUL.