This question already has answers here:
C++: how to get fprintf results as a std::string w/o sprintf
(8 answers)
Closed 5 years ago.
Does anyone know a good safe way to redirect the output of a printf-style function to a string? The obvious ways result in buffer overflows.
Something like:
string s;
output.beginRedirect( s ); // redirect output to s
... output.print( "%s%d", foo, bar );
output.endRedirect();
I think the problem is the same as asking, "how many characters will print produce?"
Ideas?
You can use:
std::snprintf if you are working with a char*
std::stringstream if you want to use strings (not same as printf but will allow you to easily manipulate the string using the normal stream functions).
boost::format if you want a function similar to printf that will work with streams. (as per jalf in comments)
fmt::format which is has been standardized since c++20 std::format
The snprintf() function prints to a string, but only as much as the length given to it.
Might be what you're looking for...
The fmt library provides fmt::sprintf function that performs printf-compatible formatting (including positional arguments according to POSIX specification) and returns the result as an std::string:
std::string s = fmt::sprintf( "%s%d", foo, bar );
Disclaimer: I'm the author of this library.
Since you've tagged this as C++ (rather than just C), I'll point out that the typical way to do this sort of thing in C++ is to use stringstream, not the printf family. No need to worry about buffer overflows with stringstreams.
The Boost Format library is also available if you like printf-style format strings but want something safer.
snprintf() returns the number of bytes needed to write the whole string.
So, as a tiny example:
#include <strings.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv)
{
char* buf = 0;
size_t bufsize = 0;
size_t sz;
const char* format="%s and %s.";
const char* str_1 ="string 1";
const char* str_2 ="string 2";
sz = snprintf(buf, bufsize, format, str_1, str_2);
printf("The buffer needs to be %d bytes.\n", sz);
buf=malloc(sz+1);
if(!buf) {
printf("Can't allocate buffer!\n");
return 1;
}
bufsize = sz+1;
buf[bufsize-1] = '\0';
sz = snprintf(buf, bufsize, format, str_1, str_2);
printf("Filled buffer with %d bytes.\n", sz);
printf("The buffer contains:\n'%s'\n", buf);
return 0;
}
output:
The buffer needs to be 22 bytes.
Filled buffer with 22 bytes.
The buffer contains:
'string 1 and string 2.'
This StackOverflow question has a similar discussion. Also in that question I present my favorite solution, a "format" function that takes identical arguments to printf and returns a std::string.
Old school:
snprintf()
allows you to put a limit on the number written, and return the actual written size, and
asprintf()
allocate (with malloc()) a sufficient buffer which then becomes your problem to free(). `asprintf is a GNU libc function now reimplemented in the BSD libc.
With C99 you have the snprintf-function which takes the size of the buffer as a parameter. The GNU C-library has asprintf which allocates a buffer for you. For c++ though, you might be better of using iostream.
Wikipedia has more info.
I find the printf formatting to be very helpful and easier to use than streams. On the other hand, I do like std::string a lot too. The solution is to use sprintf, but that cannot handle arbitrary buffer size.
I've found that I need to handle common case (say, buffer limited to 256 chars) w/o
overhead, and yet handle the large buffer safely. To do that, I have a buffer of 256 chars alocated in my class as a member, and I use snprinf, passing that buffer and its size. If snprintf succeeds, I can immediately retunr the formatted string. If it fails, I allocate the buffer and call snprinf again. The buffer is deallocated in the class' destructor.
On Windows:
StringCchPrintf
StringCbPrintf
from strsafe.h/lib.
Microsoft introduce the 'safe' crt functions for this.
You could use printf_s()
Related
In my Arduino sketch I use sprintf to format a reply. The formatted data will be send using SPI.
I have to set the buffer length, but I don't know how large would be good. Here is a piece of the code:
uint8_t myValue = 4;
// Set buffer
unsigned char data[1000];
// Reset this buffer
memset(data, 0, sizeof(data));
// Format the reply
sprintf((char *)data, "This is the int value: %d", \
myValue);
I can also set the buffer to 0 (unsigned char data[0];), The code compiles and the reply is the same as using a large buffer. I can't explain this?
It seems malloc() and free() are pretty rare in the Arduino world...
What is the right way of using a buffer in Arduino?
I would recommend using snprintf instead, where you specify how large the buffer is. With the current approach sprintf assumes that the buffer is large enough, so if you pass it a small buffer it will stomp over some other memory. Not good (and may explain the weird results you sometimes get)!
You can use a fixed buffer as in your current code. Allocating also works, but it has more overhead.
I can also set the buffer to 0 (unsigned char data[0];),
If this is true, you're just getting lucky (or unlucky, depending on how you look at it). The behaviour of your code doing this is undefined, in both C and C++. Which, among other things, means it is not guaranteed to work (e.g. it might break if your compiler is ever updated).
As to better options .... Your question is tagged C++ and C, and different approaches are preferable in different languages.
Assuming you are using C, and your implementation complies with the 1999 standard or later, you can use snprintf().
char *buffer;
int length = snprintf((char *)NULL, 0, "This is the int value: %d", myValue);
buffer = malloc(length + 1); /* +1 allows for terminator */
snprintf(buffer, length, "This is the int value: %d", myValue);
/* use buffer */
free(buffer);
The first call of sprintf() is used to recover the buffer length. The second to actually write to the allocated buffer. Remember to release the dynamically allocated buffer when done. (The above also does not check for errors - snprintf() returns a negative value if an error occurs).
If your C library does not include snprintf() then (AFAIK) there is no standard way to work out the length automatically - you will need to estimate an upper bound on length by hand (e.g. work out the length of output of the largest negative or positive int, and add that to the length of other content in your format string).
In C++, use a string stream
#include <sstream>
#include <string>
std::ostringstream oss;
oss << "This is the int value: " << myValue);
std::string str = oss.str();
const char *buffer = str.data();
// readonly access to buffer here
// the objects above will be cleaned up when they pass out of scope.
If you insist on using the C approach in C++, the approach above using snprintf() can be used in C++ if your implementation complies with the 2011 standard (or later). However, in general terms, I would NOT recommend this.
Is it possible to use an std::string for read() ?
Example :
std::string data;
read(fd, data, 42);
Normaly, we have to use char* but is it possible to directly use a std::string ? (I prefer don't create a char* for store the result)
Thank's
Well, you'll need to create a char* somehow, since that's what the
function requires. (BTW: you are talking about the Posix function
read, aren't you, and not std::istream::read?) The problem isn't
the char*, it's what the char* points to (which I suspect is what
you actually meant).
The simplest and usual solution here would be to use a local array:
char buffer[43];
int len = read(fd, buffer, 42);
if ( len < 0 ) {
// read error...
} else if ( len == 0 ) {
// eof...
} else {
std::string data(buffer, len);
}
If you want to capture directly into an std::string, however, this is
possible (although not necessarily a good idea):
std::string data;
data.resize( 42 );
int len = read( fd, &data[0], data.size() );
// error handling as above...
data.resize( len ); // If no error...
This avoids the copy, but quite frankly... The copy is insignificant
compared to the time necessary for the actual read and for the
allocation of the memory in the string. This also has the (probably
negligible) disadvantage of the resulting string having an actual buffer
of 42 bytes (rounded up to whatever), rather than just the minimum
necessary for the characters actually read.
(And since people sometimes raise the issue, with regards to the
contiguity of the memory in std:;string: this was an issue ten or more
years ago. The original specifications for std::string were designed
expressedly to allow non-contiguous implementations, along the lines of
the then popular rope class. In practice, no implementor found this
to be useful, and people did start assuming contiguity. At which point,
the standards committee decided to align the standard with existing
practice, and require contiguity. So... no implementation has ever not
been contiguous, and no future implementation will forego contiguity,
given the requirements in C++11.)
No, you cannot and you should not. Usually, std::string implementations internally store other information such as the size of the allocated memory and the length of the actual string. C++ documentation explicitly states that modifying values returned by c_str() or data() results in undefined behaviour.
If the read function requires a char *, then no. You could use the address of the first element of a std::vector of char as long as it's been resized first. I don't think old (pre C++11) strings are guarenteed to have contiguous memory otherwise you could do something similar with the string.
No, but
std::string data;
cin >> data;
works just fine. If you really want the behaviour of read(2), then you need to allocate and manage your own buffer of chars.
Because read() is intended for raw data input, std::string is actually a bad choice, because std::string handles text. std::vector seems like the right choice to handle raw data.
Using std::getline from the strings library - see cplusplus.com - can read from an stream and write directly into a string object. Example (again ripped from cplusplus.com - 1st hit on google for getline):
int main () {
string str;
cout << "Please enter full name: ";
getline (cin,str);
cout << "Thank you, " << str << ".\n";
}
So will work when reading from stdin (cin) and from a file (ifstream).
OWASP says:
"C library functions such as strcpy
(), strcat (), sprintf () and vsprintf
() operate on null terminated strings
and perform no bounds checking."
sprintf writes formatted data to string
int sprintf ( char * str, const char * format, ... );
Example:
sprintf(str, "%s", message); // assume declaration and
// initialization of variables
If I understand OWASP's comment, then the dangers of using sprintf are that
1) if message's length > str's length, there's a buffer overflow
and
2) if message does not null-terminate with \0, then message could get copied into str beyond the memory address of message, causing a buffer overflow
Please confirm/deny. Thanks
You're correct on both problems, though they're really both the same problem (which is accessing data beyond the boundaries of an array).
A solution to your first problem is to instead use std::snprintf, which accepts a buffer size as an argument.
A solution to your second problem is to give a maximum length argument to snprintf. For example:
char buffer[128];
std::snprintf(buffer, sizeof(buffer), "This is a %.4s\n", "testGARBAGE DATA");
// std::strcmp(buffer, "This is a test\n") == 0
If you want to store the entire string (e.g. in the case sizeof(buffer) is too small), run snprintf twice:
int length = std::snprintf(nullptr, 0, "This is a %.4s\n", "testGARBAGE DATA");
++length; // +1 for null terminator
char *buffer = new char[length];
std::snprintf(buffer, length, "This is a %.4s\n", "testGARBAGE DATA");
(You can probably fit this into a function using va or variadic templates.)
Both of your assertions are correct.
There's an additional problem not mentioned. There is no type checking on the parameters. If you mismatch the format string and the parameters, undefined and undesirable behavior could result. For example:
char buf[1024] = {0};
float f = 42.0f;
sprintf(buf, "%s", f); // `f` isn't a string. the sun may explode here
This can be particularly nasty to debug.
All of the above lead many C++ developers to the conclusion that you should never use sprintf and its brethren. Indeed, there are facilities you can use to avoid all of the above problems. One, streams, is built right in to the language:
#include <sstream>
#include <string>
// ...
float f = 42.0f;
stringstream ss;
ss << f;
string s = ss.str();
...and another popular choice for those who, like me, still prefer to use sprintf comes from the boost Format libraries:
#include <string>
#include <boost\format.hpp>
// ...
float f = 42.0f;
string s = (boost::format("%1%") %f).str();
Should you adopt the "never use sprintf" mantra? Decide for yourself. There's usually a best tool for the job and depending on what you're doing, sprintf just might be it.
Yes, it is mostly a matter of buffer overflows. However, those are quite serious business nowdays, since buffer overflows are the prime attack vector used by system crackers to circumvent software or system security. If you expose something like this to user input, there's a very good chance you are handing the keys to your program (or even your computer itself) to the crackers.
From OWASP's perspective, let's pretend we are writing a web server, and we use sprintf to parse the input that a browser passes us.
Now let's suppose someone malicious out there passes our web browser a string far larger than will fit in the buffer we chose. His extra data will instead overwrite nearby data. If he makes it large enough, some of his data will get copied over the webserver's instructions rather than its data. Now he can get our webserver to execute his code.
Your 2 numbered conclusions are correct, but incomplete.
There is an additional risk:
char* format = 0;
char buf[128];
sprintf(buf, format, "hello");
Here, format is not NULL-terminated. sprintf() doesn't check that either.
Your interpretation seems to be correct. However, your case #2 isn't really a buffer overflow. It's more of a memory access violation. That's just terminology though, it's still a major problem.
The sprintf function, when used with certain format specifiers, poses two types of security risk: (1) writing memory it shouldn't; (2) reading memory it shouldn't. If snprintf is used with a size parameter that matches the buffer, it won't write anything it shouldn't. Depending upon the parameters, it may still read stuff it shouldn't. Depending upon the operating environment and what else a program is doing, the danger from improper reads may or may not be less severe than that from improper writes.
It is very important to remember that sprintf() adds the ASCII 0 character as string terminator at the end of each string. Therefore, the destination buffer must have at least n+1 bytes (To print the word "HELLO", a 6-byte buffer is required, NOT 5)
In the example below, it may not be obvious, but in the 2-byte destination buffer, the second byte will be overwritten by ASCII 0 character. If only 1 byte was allocated for the buffer, this would cause buffer overrun.
char buf[3] = {'1', '2'};
int n = sprintf(buf, "A");
Also note that the return value of sprintf() does NOT include the null-terminating character. In the example above, 2 bytes were written, but the function returns '1'.
In the example below, the first byte of class member variable 'i' would be partially overwritten by sprintf() (on a 32-bit system).
struct S
{
char buf[4];
int i;
};
int main()
{
struct S s = { };
s.i = 12345;
int num = sprintf(s.buf, "ABCD");
// The value of s.i is NOT 12345 anymore !
return 0;
}
I pretty much have stated a small example how you could get rid of the buffer size declaration for the sprintf (if you intended to, of course!) and no snprintf envolved ....
Note: This is an APPEND/CONCATENATION example, take a look at here
I'm working in C++.
I want to write a potentially very long formatted string using sprintf (specifically a secure counted version like _snprintf_s, but the idea is the same). The approximate length is unknown at compile time so I'll have to use some dynamically allocated memory rather than relying on a big static buffer. Is there any way to determine how many characters will be needed for a particular sprintf call so I can always be sure I've got a big enough buffer?
My fallback is I'll just take the length of the format string, double it, and try that. If it works, great, if it doesn't I'll just double the size of the buffer and try again. Repeat until it fits. Not exactly the cleverest solution.
It looks like C99 supports passing NULL to snprintf to get the length. I suppose I could create a module to wrap that functionality if nothing else, but I'm not crazy about that idea.
Maybe an fprintf to "/dev/null"/"nul" might work instead? Any other ideas?
EDIT: Alternatively, is there any way to "chunk" the sprintf so it picks up mid-write? If that's possible it could fill the buffer, process it, then start refilling from where it left off.
The man page for snprintf says:
Return value
Upon successful return, these functions return the number of
characters printed (not including the trailing '\0' used to end
output to strings). The functions snprintf and vsnprintf do not
write more than size bytes (including the trailing '\0'). If
the output was truncated due to this limit then the return value
is the number of characters (not including the trailing '\0')
which would have been written to the final string if enough
space had been available. Thus, a return value of size or more
means that the output was truncated. (See also below under
NOTES.) If an output error is encountered, a negative value is
returned.
What this means is that you can call snprintf with a size of 0. Nothing will get written, and the return value will tell you how much space you need to allocate to your string:
int how_much_space = snprintf(NULL, 0, fmt_string, param0, param1, ...);
As others have mentioned, snprintf() will return the number of characters required in a buffer to prevent the output from being truncated. You can simply call it with a 0 buffer length parameter to get the required size then use an appropriately sized buffer.
For a slight improvement in efficiency, you can call it with a buffer that's large enough for the normal case and only do a second call to snprintf() if the output is truncated. In order to make sure the buffer(s) are properly freed in that case, I'll often use an auto_buffer<> object that handles the dynamic memory for me (and has the default buffer on the stack to avoid a heap allocation in the normal case).
If you're using a Microsoft compiler, MS has a non-standard _snprintf() that has serious limitations of not always null terminating the buffer and not indicating how big the buffer should be.
To work around Microsoft's non-support, I use a nearly public domain snprintf() from Holger Weiss.
Of course if your non-MS C or C++ compiler is missing snprintf(), the code from the above link should work just as well.
I would use a two-stage approach. Generally, a large percentage of output strings will be under a certain threshold and only a few will be larger.
Stage 1, use a reasonable sized static buffer such as 4K. Since snprintf() can restrict how many characters are written, you won't get a buffer overflow. What you will get returned from snprintf() is the number of characters it would have written if your buffer had been big enough.
If your call to snprintf() returns less than 4K, then use the buffer and exit. As stated, the vast majority of calls should just do that.
Some will not and that's when you enter stage 2. If the call to snprintf() won't fit in the 4K buffer, you at least now know how big a buffer you need.
Allocate, with malloc(), a buffer big enough to hold it then snprintf() it again to that new buffer. When you're done with the buffer, free it.
We worked on a system in the days before snprintf() and we acheived the same result by having a file handle connected to /dev/null and using fprintf() with that. /dev/null was always guaranteed to take as much data as you give it so we would actually get the size from that, then allocate a buffer if necessary.
Keep in kind that not all systems have snprintf() (for example, I understand it's _snprintf() in Microsoft C) so you may have to find the function that does the same job, or revert to the fprintf /dev/null solution.
Also be careful if the data can be changed between the size-checking snprintf() and the actual snprintf() to the buffer (i.e., wathch out for threads). If the sizes increase, you'll get buffer overflow corruption.
If you follow the rule that data, once handed to a function, belongs to that function exclusively until handed back, this won't be a problem.
For what it's worth, asprintf is a GNU extension that manages this functionality. It accepts a pointer as an output argument, along with a format string and a variable number of arguments, and writes back to the pointer the address of a properly-allocated buffer containing the result.
You can use it like so:
#define _GNU_SOURCE
#include <stdio.h>
int main(int argc, char const *argv[])
{
char *hi = "hello"; // these could be really long
char *everyone = "world";
char *message;
asprintf(&message, "%s %s", hi, everyone);
puts(message);
free(message);
return 0;
}
Hope this helps someone!
Take a look at CodeProject: CString-clone Using Standard C++. It uses solution you suggested with enlarging buffer size.
// -------------------------------------------------------------------------
// FUNCTION: FormatV
// void FormatV(PCSTR szFormat, va_list, argList);
//
// DESCRIPTION:
// This function formats the string with sprintf style format-specs.
// It makes a general guess at required buffer size and then tries
// successively larger buffers until it finds one big enough or a
// threshold (MAX_FMT_TRIES) is exceeded.
//
// PARAMETERS:
// szFormat - a PCSTR holding the format of the output
// argList - a Microsoft specific va_list for variable argument lists
//
// RETURN VALUE:
// -------------------------------------------------------------------------
void FormatV(const CT* szFormat, va_list argList)
{
#ifdef SS_ANSI
int nLen = sslen(szFormat) + STD_BUF_SIZE;
ssvsprintf(GetBuffer(nLen), nLen-1, szFormat, argList);
ReleaseBuffer();
#else
CT* pBuf = NULL;
int nChars = 1;
int nUsed = 0;
size_type nActual = 0;
int nTry = 0;
do
{
// Grow more than linearly (e.g. 512, 1536, 3072, etc)
nChars += ((nTry+1) * FMT_BLOCK_SIZE);
pBuf = reinterpret_cast<CT*>(_alloca(sizeof(CT)*nChars));
nUsed = ssnprintf(pBuf, nChars-1, szFormat, argList);
// Ensure proper NULL termination.
nActual = nUsed == -1 ? nChars-1 : SSMIN(nUsed, nChars-1);
pBuf[nActual+1]= '\0';
} while ( nUsed < 0 && nTry++ < MAX_FMT_TRIES );
// assign whatever we managed to format
this->assign(pBuf, nActual);
#endif
}
I've looked for the same functionality you're talking about, but as far as I know, something as simple as the C99 method is not available in C++, because C++ does not currently incorporate the features added in C99 (such as snprintf).
Your best bet is probably to use a stringstream object. It's a bit more cumbersome than a clearly written sprintf call, but it will work.
Since you're using C++, there's really no need to use any version of sprintf. The simplest thing to do is use a std::ostringstream.
std::ostringstream oss;
oss << a << " " << b << std::endl;
oss.str() returns a std::string with the contents of what you've written to oss. Use oss.str().c_str() to get a const char *. It's going to be a lot easier to deal with in the long run and eliminates memory leaks or buffer overruns. Generally, if you're worrying about memory issues like that in C++, you're not using the language to its full potential, and you should rethink your design.
I am working with an open-source UNIX tool that is implemented in C++, and I need to change some code to get it to do what I want. I would like to make the smallest possible change in hopes of getting my patch accepted upstream. Solutions that are implementable in standard C++ and do not create more external dependencies are preferred.
Here is my problem. I have a C++ class -- let's call it "A" -- that currently uses fprintf() to print its heavily formatted data structures to a file pointer. In its print function, it also recursively calls the identically defined print functions of several member classes ("B" is an example). There is another class C that has a member std::string "foo" that needs to be set to the print() results of an instance of A. Think of it as a to_str() member function for A.
In pseudocode:
class A {
public:
...
void print(FILE* f);
B b;
...
};
...
void A::print(FILE *f)
{
std::string s = "stuff";
fprintf(f, "some %s", s);
b.print(f);
}
class C {
...
std::string foo;
bool set_foo(std::str);
...
}
...
A a = new A();
C c = new C();
...
// wish i knew how to write A's to_str()
c.set_foo(a.to_str());
I should mention that C is fairly stable, but A and B (and the rest of A's dependents) are in a state of flux, so the less code changes necessary the better. The current print(FILE* F) interface also needs to be preserved. I have considered several approaches to implementing A::to_str(), each with advantages and disadvantages:
Change the calls to fprintf() to sprintf()
I wouldn't have to rewrite any format strings
print() could be reimplemented as: fprint(f, this.to_str());
But I would need to manually allocate char[]s, merge a lot of c strings , and finally convert the character array to a std::string
Try to catch the results of a.print() in a string stream
I would have to convert all of the format strings to << output format. There are hundreds of fprintf()s to convert :-{
print() would have to be rewritten because there is no standard way that I know of to create an output stream from a UNIX file handle (though this guy says it may be possible).
Use Boost's string format library
More external dependencies. Yuck.
Format's syntax is different enough from printf() to be annoying:
printf(format_str, args) -> cout << boost::format(format_str) % arg1 % arg2 % etc
Use Qt's QString::asprintf()
A different external dependency.
So, have I exhausted all possible options? If so, which do you think is my best bet? If not, what have I overlooked?
Thanks.
Here's the idiom I like for making functionality identical to 'sprintf', but returning a std::string, and immune to buffer overflow problems. This code is part of an open source project that I'm writing (BSD license), so everybody feel free to use this as you wish.
#include <string>
#include <cstdarg>
#include <vector>
#include <string>
std::string
format (const char *fmt, ...)
{
va_list ap;
va_start (ap, fmt);
std::string buf = vformat (fmt, ap);
va_end (ap);
return buf;
}
std::string
vformat (const char *fmt, va_list ap)
{
// Allocate a buffer on the stack that's big enough for us almost
// all the time.
size_t size = 1024;
char buf[size];
// Try to vsnprintf into our buffer.
va_list apcopy;
va_copy (apcopy, ap);
int needed = vsnprintf (&buf[0], size, fmt, ap);
// NB. On Windows, vsnprintf returns -1 if the string didn't fit the
// buffer. On Linux & OSX, it returns the length it would have needed.
if (needed <= size && needed >= 0) {
// It fit fine the first time, we're done.
return std::string (&buf[0]);
} else {
// vsnprintf reported that it wanted to write more characters
// than we allotted. So do a malloc of the right size and try again.
// This doesn't happen very often if we chose our initial size
// well.
std::vector <char> buf;
size = needed;
buf.resize (size);
needed = vsnprintf (&buf[0], size, fmt, apcopy);
return std::string (&buf[0]);
}
}
EDIT: when I wrote this code, I had no idea that this required C99 conformance and that Windows (as well as older glibc) had different vsnprintf behavior, in which it returns -1 for failure, rather than a definitive measure of how much space is needed. Here is my revised code, could everybody look it over and if you think it's ok, I will edit again to make that the only cost listed:
std::string
Strutil::vformat (const char *fmt, va_list ap)
{
// Allocate a buffer on the stack that's big enough for us almost
// all the time. Be prepared to allocate dynamically if it doesn't fit.
size_t size = 1024;
char stackbuf[1024];
std::vector<char> dynamicbuf;
char *buf = &stackbuf[0];
va_list ap_copy;
while (1) {
// Try to vsnprintf into our buffer.
va_copy(ap_copy, ap);
int needed = vsnprintf (buf, size, fmt, ap);
va_end(ap_copy);
// NB. C99 (which modern Linux and OS X follow) says vsnprintf
// failure returns the length it would have needed. But older
// glibc and current Windows return -1 for failure, i.e., not
// telling us how much was needed.
if (needed <= (int)size && needed >= 0) {
// It fit fine so we're done.
return std::string (buf, (size_t) needed);
}
// vsnprintf reported that it wanted to write more characters
// than we allotted. So try again using a dynamic buffer. This
// doesn't happen very often if we chose our initial size well.
size = (needed > 0) ? (needed+1) : (size*2);
dynamicbuf.resize (size);
buf = &dynamicbuf[0];
}
}
I am using #3: the boost string format library - but I have to admit that I've never had any problem with the differences in format specifications.
Works like a charm for me - and the external dependencies could be worse (a very stable library)
Edited: adding an example how to use boost::format instead of printf:
sprintf(buffer, "This is a string with some %s and %d numbers", "strings", 42);
would be something like this with the boost::format library:
string = boost::str(boost::format("This is a string with some %s and %d numbers") %"strings" %42);
Hope this helps clarify the usage of boost::format
I've used boost::format as a sprintf / printf replacement in 4 or 5 applications (writing formatted strings to files, or custom output to logfiles) and never had problems with format differences. There may be some (more or less obscure) format specifiers which are differently - but I never had a problem.
In contrast I had some format specifications I couldn't really do with streams (as much as I remember)
You can use std::string and iostreams with formatting, such as the setw() call and others in iomanip
The {fmt} library provides fmt::sprintf function that performs printf-compatible formatting (including positional arguments according to POSIX specification) and returns the result as std::string:
std::string s = fmt::sprintf("The answer is %d.", 42);
Disclaimer: I'm the author of this library.
The following might be an alternative solution:
void A::printto(ostream outputstream) {
char buffer[100];
string s = "stuff";
sprintf(buffer, "some %s", s);
outputstream << buffer << endl;
b.printto(outputstream);
}
(B::printto similar), and define
void A::print(FILE *f) {
printto(ofstream(f));
}
string A::to_str() {
ostringstream os;
printto(os);
return os.str();
}
Of course, you should really use snprintf instead of sprintf to avoid buffer overflows. You could also selectively change the more risky sprintfs to << format, to be safer and yet change as little as possible.
You should try the Loki library's SafeFormat header file (http://loki-lib.sourceforge.net/index.php?n=Idioms.Printf). It's similar to boost's string format library, but keeps the syntax of the printf(...) functions.
I hope this helps!
Is this about serialization? Or printing proper?
If the former, consider boost::serialization as well. It's all about "recursive" serialization of objects and sub-object.
Very very late to the party, but here's how I'd attack this problem.
1: Use pipe(2) to open a pipe.
2: Use fdopen(3) to convert the write fd from the pipe to a FILE *.
3: Hand that FILE * to A::print().
4: Use read(2) to pull bufferloads of data, e.g. 1K or more at a time from the read fd.
5: Append each bufferload of data to the target std::string
6: Repeat steps 4 and 5 as needed to complete the task.