平台间函数差异与系统移植：snprintf/_snprintf

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　　snprintf函数并不是标准c/c++中规定的函数，但是在许多编译器中，厂商提供了其实现的版本。
在gcc中，该函数名称就snprintf，而在VC中称为_snprintf。
　　由于不是标准函数，没有一个统一的标准来规定该函数的行为，所以导致了各厂商间的实现版本可
能会有差异。今天也的的确确看到了差异，因为这个小小的差异是我的程序无法正常的处理数据。
　　这个小小的差异发生在count参数。在VC中，这个count就是要写入的总字符串字符数，例如：
　　　　//VC
int main(int argc, char * argv[])
...{
    char  buff[100];
    printf("%d ",_snprintf(buff,10,"1234567890ab"));
    printf("%s",buff);
    return 0;
}

//Linxu:gcc/g++
#include <stdio.h>
int main(int argc, char * argv[])
...{
    char  buff[100];
    printf("%d ",snprintf(buff,10,"1234567890ab"));
    printf("%s",buff);
    return 0;
}

 vc程序的输出是：
-1
1234567890@
gcc程序的输出是：
12
123456789
从输出结果可以知道：VC中的_snprintf的count参数表示，会向buff中写入count个字符，不包括'/0'字符，
并且不会在字符串末尾添加'/0'符，并且，如果字符串超过count，函数返回-1以标志可能导致的错误；gcc
中的snprintf函数的count参数表示，向buff中写入10个字符，包括'/0'字符，并且，返回实际的字符串长度，
例子中为12。

　　如果不了解这些函数在各平台间的差异，也许我们的程序在移植过程中会变得很脆弱。我们应该小心各种各样
的陷阱。

下面是MSDN对_snprintf函数的解释：

int _snprintf( char *buffer, size_t count, const char *format [, argument] ... );

Parameters

buffer: Storage location for output
count: Maximum number of characters to store
format: Format-control string
argument: Optional arguments

Libraries

All versions of the C run-time libraries.

Return Value

_snprintf returns the number of bytes stored in buffer, not counting the terminating null character. If the number of bytes required to store the data exceeds count, then count bytes of data are stored in buffer and a negative value is returned.

Remarks

The _snprintf function formats and stores count or fewer characters and values (including a terminating null character that is always appended unless count is zero or the formatted string length is greater than or equal to count characters) in buffer. Each argument (if any) is converted and output according to the corresponding format specification in format. The format consists of ordinary characters and has the same form and function as the format argument for printf. If copying occurs between strings that overlap, the behavior is undefined.

下面是linux下对snprintf函数的解释：

PRINTF(3) Linux Programmer’s Manual PRINTF(3)

NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf - formatted output conversion

SYNOPSIS
#include <stdio.h>

       int printf(const char *format, ...);
       int fprintf(FILE *stream, const char *format, ...);
       int sprintf(char *str, const char *format, ...);
       int snprintf(char *str, size_t size, const char *format, ...);

#include <stdarg.h>

       int vprintf(const char *format, va_list ap);
       int vfprintf(FILE *stream, const char *format, va_list ap);
       int vsprintf(char *str, const char *format, va_list ap);
       int vsnprintf(char *str, size_t size, const char *format, va_list ap);

DESCRIPTION
       The functions in the printf() family produce output according to a format as described below. The functions
       printf() and vprintf() write output to stdout, the standard output stream; fprintf() and vfprintf() write out-
       put to the given output stream; sprintf(), snprintf(), vsprintf() and vsnprintf() write to the character string
       str.

       The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equivalent to the functions printf(),
       fprintf(), sprintf(), snprintf(), respectively, except that they are called with a va_list instead of a vari-
       able number of arguments. These functions do not call the va_end macro. Consequently, the value of ap is unde-
       fined after the call. The application should call va_end(ap) itself afterwards.

       These eight functions write the output under the control of a format string that specifies how subsequent argu-
       ments (or arguments accessed via the variable-length argument facilities of stdarg(3)) are converted for out-
       put.

   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.

   Format of the format string
       The format string is a character string, beginning and ending in its initial shift state, if any.   The format
       string is composed of zero or more directives: ordinary characters (not %), which are copied unchanged to the
       output stream; and conversion specifications, each of which results in fetching zero or more subsequent argu-
[root@localhost other]# man snprintf
PRINTF(3)                  Linux Programmer’s Manual                 PRINTF(3)

NAME
printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf - formatted output conversion

SYNOPSIS
#include <stdio.h>

#include <stdarg.h>

   Format of the format string
       The format string is a character string, beginning and ending in its initial shift state, if any.   The format
       string is composed of zero or more directives: ordinary characters (not %), which are copied unchanged to the
       output stream; and conversion specifications, each of which results in fetching zero or more subsequent argu-
       ments.   Each conversion specification is introduced by the character %, and ends with a conversion specifier.
       In between there may be (in this order) zero or more flags, an optional minimum field width, an optional preci-
       sion and an optional length modifier.

       The arguments must correspond properly (after type promotion) with the conversion specifier. By default, the
       arguments are used in the order given, where each ‘*’ and each conversion specifier asks for the next argument
       (and it is an error if insufficiently many arguments are given). One can also specify explicitly which argu-
       ment is taken, at each place where an argument is required, by writing ‘%m$’ instead of ‘%’ and ‘*m$’ instead
       of ‘*’, where the decimal integer m denotes the position in the argument list of the desired argument, indexed
       starting from 1. Thus,
                   printf("%*d", width, num);
       and
                   printf("%2$*1$d", width, num);
       are equivalent. The second style allows repeated references to the same argument. The C99 standard does not
       include the style using ‘$’, which comes from the Single Unix Specification. If the style using ‘$’ is used,
       it must be used throughout for all conversions taking an argument and all width and precision arguments, but it
       may be mixed with ‘%%’ formats which do not consume an argument. There may be no gaps in the numbers of argu-
       ments specified using ‘$’; for example, if arguments 1 and 3 are specified, argument 2 must also be specified
       somewhere in the format string.

       For some numeric conversions a radix character (‘decimal point’) or thousands’ grouping character is used. The
       actual character used depends on the LC_NUMERIC part of the locale. The POSIX locale uses ‘.’ as radix charac-
       ter, and does not have a grouping character. Thus,
                   printf("%’.2f", 1234567.89);
       results in ‘1234567.89’ in the POSIX locale, in ‘1234567,89’ in the nl_NL locale, and in ‘1.234.567,89’ in the
       da_DK locale.

The flag characters
The character % is followed by zero or more of the following flags:

       #      The value should be converted to an ‘‘alternate form’’. For o conversions, the first character of the
              output string is made zero (by prefixing a 0 if it was not zero already). For x and X conversions, a
              non-zero result has the string ‘0x’ (or ‘0X’ for X conversions) prepended to it. For a, A, e, E, f, F,
              g, and G conversions, the result will always contain a decimal point, even if no digits follow it (nor-
              mally, a decimal point appears in the results of those conversions only if a digit follows). For g and
              G conversions, trailing zeros are not removed from the result as they would otherwise be. For other
              conversions, the result is undefined.

       0      The value should be zero padded. For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, the con-
              verted value is padded on the left with zeros rather than blanks. If the 0 and - flags both appear, the
              0 flag is ignored. If a precision is given with a numeric conversion (d, i, o, u, x, and X), the 0 flag
              is ignored. For other conversions, the behavior is undefined.

       -      The converted value is to be left adjusted on the field boundary. (The default is right justification.)
              Except for n conversions, the converted value is padded on the right with blanks, rather than on the
              left with blanks or zeros. A - overrides a 0 if both are given.

　?　? (a space) A blank should be left before a positive number (or empty string) produced by a signed conver-
sion.

+ A sign (+ or -) should always be placed before a number produced by a signed conversion. By default a
sign is used only for negative numbers. A + overrides a space if both are used.

The five flag characters above are defined in the C standard. The SUSv2 specifies one further flag character.

       　?     For decimal conversion (i, d, u, f, F, g, G) the output is to be grouped with thousands’ grouping char-
              acters if the locale information indicates any. Note that many versions of gcc(1) cannot parse this
              option and will issue a warning. SUSv2 does not include %’F.

glibc 2.2 adds one further flag character.

I For decimal integer conversion (i, d, u) the output uses the locale’s alternative output digits, if any.
For example, since glibc 2.2.3 this will give Arabic-Indic digits in the Persian (‘fa_IR’) locale.

   The field width
       An optional decimal digit string (with non-zero first digit) specifying a minimum field width. If the con-
       verted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if
       the left-adjustment flag has been given). Instead of a decimal digit string one may write ‘*’ or ‘*m$’ (for
       some decimal integer m) to specify that the field width is given in the next argument, or in the m-th argument,
       respectively, which must be of type int. A negative field width is taken as a ‘-’ flag followed by a positive
       field width. In no case does a non-existent or small field width cause truncation of a field; if the result of
       a conversion is wider than the field width, the field is expanded to contain the conversion result.

   The precision
       An optional precision, in the form of a period (‘.’) followed by an optional decimal digit string. Instead of
       a decimal digit string one may write ‘*’ or ‘*m$’ (for some decimal integer m) to specify that the precision is
       given in the next argument, or in the m-th argument, respectively, which must be of type int. If the precision
       is given as just ‘.’, or the precision is negative, the precision is taken to be zero. This gives the minimum
       number of digits to appear for d, i, o, u, x, and X conversions, the number of digits to appear after the radix
       character for a, A, e, E, f, and F conversions, the maximum number of significant digits for g and G conver-
       sions, or the maximum number of characters to be printed from a string for s and S conversions.

The length modifier
Here, ‘integer conversion’ stands for d, i, o, u, x, or X conversion.

hh A following integer conversion corresponds to a signed char or unsigned char argument, or a following n
conversion corresponds to a pointer to a signed char argument.

h A following integer conversion corresponds to a short int or unsigned short int argument, or a following
n conversion corresponds to a pointer to a short int argument.

       l      (ell) A following integer conversion corresponds to a long int or unsigned long int argument, or a fol-
              lowing n conversion corresponds to a pointer to a long int argument, or a following c conversion corre-
              sponds to a wint_t argument, or a following s conversion corresponds to a pointer to wchar_t argument.

ll (ell-ell). A following integer conversion corresponds to a long long int or unsigned long long int
argument, or a following n conversion corresponds to a pointer to a long long int argument.

L A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argument. (C99 allows
%LF, but SUSv2 does not.)

q (‘quad’. 4.4BSD and Linux libc5 only. Don’t use.) This is a synonym for ll.

j A following integer conversion corresponds to an intmax_t or uintmax_t argument.

z A following integer conversion corresponds to a size_t or ssize_t argument. (Linux libc5 has Z with this
meaning. Don’t use it.)

t A following integer conversion corresponds to a ptrdiff_t argument.

The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx, hX, hn) and l (in ld, li, lo, lx, lX, ln,
lc, ls) and L (in Le, LE, Lf, Lg, LG).

   The conversion specifier
       A character that specifies the type of conversion to be applied. The conversion specifiers and their meanings
       are:

       d,i    The int argument is converted to signed decimal notation. The precision, if any, gives the minimum num-
              ber of digits that must appear; if the converted value requires fewer digits, it is padded on the left
              with zeros. The default precision is 1. When 0 is printed with an explicit precision 0, the output is
              empty.

       o,u,x,X
              The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadec-
              imal (x and X) notation. The letters abcdef are used for x conversions; the letters ABCDEF are used for
              X conversions. The precision, if any, gives the minimum number of digits that must appear; if the con-
              verted value requires fewer digits, it is padded on the left with zeros. The default precision is 1.
              When 0 is printed with an explicit precision 0, the output is empty.

       e,E    The double argument is rounded and converted in the style [-]d.ddde±dd where there is one digit before
              the decimal-point character and the number of digits after it is equal to the precision; if the preci-
              sion is missing, it is taken as 6; if the precision is zero, no decimal-point character appears. An E
              conversion uses the letter E (rather than e) to introduce the exponent. The exponent always contains at
              least two digits; if the value is zero, the exponent is 00.

       f,F    The double argument is rounded and converted to decimal notation in the style [-]ddd.ddd, where the num-
              ber of digits after the decimal-point character is equal to the precision specification. If the preci-
              sion is missing, it is taken as 6; if the precision is explicitly zero, no decimal-point character
              appears. If a decimal point appears, at least one digit appears before it.

              (The SUSv2 does not know about F and says that character string representations for infinity and NaN may
              be made available. The C99 standard specifies ‘[-]inf’ or ‘[-]infinity’ for infinity, and a string
              starting with ‘nan’ for NaN, in the case of f conversion, and ‘[-]INF’ or ‘[-]INFINITY’ or ‘NAN*’ in the
              case of F conversion.)

       g,G    The double argument is converted in style f or e (or F or E for G conversions). The precision specifies
              the number of significant digits. If the precision is missing, 6 digits are given; if the precision is
              zero, it is treated as 1.   Style e is used if the exponent from its conversion is less than -4 or
              greater than or equal to the precision. Trailing zeros are removed from the fractional part of the
              result; a decimal point appears only if it is followed by at least one digit.

       a,A    (C99; not in SUSv2) For a conversion, the double argument is converted to hexadecimal notation (using
              the letters abcdef) in the style [-]0xh.hhhhp±d; for A conversion the prefix 0X, the letters ABCDEF, and
              the exponent separator P is used. There is one hexadecimal digit before the decimal point, and the num-
              ber of digits after it is equal to the precision. The default precision suffices for an exact represen-
              tation of the value if an exact representation in base 2 exists and otherwise is sufficiently large to
              distinguish values of type double. The digit before the decimal point is unspecified for non-normalized
              numbers, and non-zero but otherwise unspecified for normalized numbers.

       c      If no l modifier is present, the int argument is converted to an unsigned char, and the resulting char-
              acter is written. If an l modifier is present, the wint_t (wide character) argument is converted to a
              multibyte sequence by a call to the wcrtomb() function, with a conversion state starting in the initial
              state, and the resulting multibyte string is written.

       s      If no l modifier is present: The const char * argument is expected to be a pointer to an array of char-
              acter type (pointer to a string). Characters from the array are written up to (but not including) a
              terminating null byte (’/0’); if a precision is specified, no more than the number specified are writ-
              ten.   If a precision is given, no null byte need be present; if the precision is not specified, or is
              greater than the size of the array, the array must contain a terminating null byte.

              If an l modifier is present: The const wchar_t * argument is expected to be a pointer to an array of
              wide characters.   Wide characters from the array are converted to multibyte characters (each by a call
              to the wcrtomb() function, with a conversion state starting in the initial state before the first wide
              character), up to and including a terminating null wide character. The resulting multibyte characters
              are written up to (but not including) the terminating null byte. If a precision is specified, no more
              bytes than the number specified are written, but no partial multibyte characters are written. Note that
              the precision determines the number of bytes written, not the number of wide characters or screen posi-
              tions.   The array must contain a terminating null wide character, unless a precision is given and it is
              so small that the number of bytes written exceeds it before the end of the array is reached.

C (Not in C99, but in SUSv2.) Synonym for lc. Don’t use.

S (Not in C99, but in SUSv2.) Synonym for ls. Don’t use.

p The void * pointer argument is printed in hexadecimal (as if by %#x or %#lx).

n The number of characters written so far is stored into the integer indicated by the int * (or variant)
pointer argument. No argument is converted.

m (Glibc extension.) Print output of strerror(errno). No argument is required.

% A ‘%’ is written. No argument is converted. The complete conversion specification is ‘%%’.

EXAMPLE
       To print pi to five decimal places:
              #include <math.h>
              #include <stdio.h>
              fprintf(stdout, "pi = %.5f/n", 4 * atan(1.0));

       To print a date and time in the form ‘Sunday, July 3, 10:02’, where weekday and month are pointers to strings:
              #include <stdio.h>
              fprintf(stdout, "%s, %s %d, %.2d:%.2d/n",
                   weekday, month, day, hour, min);

       Many countries use the day-month-year order. Hence, an internationalized version must be able to print the
       arguments in an order specified by the format:
              #include <stdio.h>
              fprintf(stdout, format,
                   weekday, month, day, hour, min);
       where format depends on locale, and may permute the arguments. With the value
              "%1$s, %3$d. %2$s, %4$d:%5$.2d/n"
       one might obtain ‘Sonntag, 3. Juli, 10:02’.

       To allocate a sufficiently large string and print into it (code correct for both glibc 2.0 and glibc 2.1):
              #include <stdio.h>
              #include <stdlib.h>
              #include <stdarg.h>

              char *
              make_message(const char *fmt, ...) {
                 /* Guess we need no more than 100 bytes. */
                 int n, size = 100;
                 char *p, *np;
                 va_list ap;

if ((p = malloc (size)) == NULL)
return NULL;

                 while (1) {
                    /* Try to print in the allocated space. */
                    va_start(ap, fmt);
                    n = vsnprintf (p, size, fmt, ap);
                    va_end(ap);
                    /* If that worked, return the string. */
                    if (n > -1 && n < size)
                       return p;
                    /* Else try again with more space. */
                    if (n > -1)    /* glibc 2.1 */
                       size = n+1; /* precisely what is needed */
                    else           /* glibc 2.0 */
                       size *= 2; /* twice the old size */
                    if ((np = realloc (p, size)) == NULL) {
                       free(p);
                       return NULL;
                    } else {
                       p = np;
                    }
                 }
              }

NOTES
       The glibc implementation of the functions snprintf() and vsnprintf() conforms to the C99 standard, i.e.,
       behaves as described above, since glibc version 2.1. Until glibc 2.0.6 they would return -1 when the output was
       truncated.

CONFORMING TO
The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions conform to C89 and C99.
The snprintf() and vsnprintf() functions conform to C99.

       Concerning the return value of snprintf(), SUSv2 and C99 contradict each other: when snprintf() is called with
       size=0 then SUSv2 stipulates an unspecified return value less than 1, while C99 allows str to be NULL in this
       case, and gives the return value (as always) as the number of characters that would have been written in case
       the output string has been large enough.

       Linux libc4 knows about the five C standard flags. It knows about the length modifiers h,l,L, and the conver-
       sions cdeEfFgGinopsuxX, where F is a synonym for f. Additionally, it accepts D,O,U as synonyms for ld,lo,lu.
       (This is bad, and caused serious bugs later, when support for %D disappeared.) No locale-dependent radix char-
       acter, no thousands’ separator, no NaN or infinity, no %m$ and *m$.

       Linux libc5 knows about the five C standard flags and the ’ flag, locale, %m$ and *m$. It knows about the
       length modifiers h,l,L,Z,q, but accepts L and q both for long doubles and for long long integers (this is a
       bug). It no longer recognizes FDOU, but adds the conversion character m, which outputs strerror(errno).

glibc 2.0 adds conversion characters C and S.

glibc 2.1 adds length modifiers hh,j,t,z and conversion characters a,A.

glibc 2.2 adds the conversion character F with C99 semantics, and the flag character I.

HISTORY
       Unix V7 defines the three routines printf(), fprintf(), sprintf(), and has the flag -, the width or precision
       *, the length modifier l, and the conversions doxfegcsu, and also D,O,U,X as synonyms for ld,lo,lu,lx. This is
       still true for 2.9.1BSD, but 2.10BSD has the flags #, + and <space> and no longer mentions D,O,U,X. 2.11BSD
       has vprintf(), vfprintf(), vsprintf(), and warns not to use D,O,U,X. 4.3BSD Reno has the flag 0, the length
       modifiers h and L, and the conversions n, p, E, G, X (with current meaning) and deprecates D,O,U. 4.4BSD
       introduces the functions snprintf() and vsnprintf(), and the length modifier q.   FreeBSD also has functions
       asprintf() and vasprintf(), that allocate a buffer large enough for sprintf(). In glibc there are functions
       dprintf() and vdprintf() that print to a file descriptor instead of a stream.

BUGS
       Because sprintf() and vsprintf() assume an arbitrarily long string, callers must be careful not to overflow the
       actual space; this is often impossible to assure. Note that the length of the strings produced is locale-depen-
       dent and difficult to predict. Use snprintf() and vsnprintf() instead (or asprintf() and vasprintf).

       Linux libc4.[45] does not have a snprintf(), but provides a libbsd that contains an snprintf() equivalent to
       sprintf(), i.e., one that ignores the size argument. Thus, the use of snprintf() with early libc4 leads to
       serious security problems.

       Code such as printf(foo); often indicates a bug, since foo may contain a % character.   If foo comes from
       untrusted user input, it may contain %n, causing the printf() call to write to memory and creating a security
       hole.

SEE ALSO
printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3), wprintf(3), locale(5)

Linux Manpage 2000-10-16 PRINTF(3)