标签：c printf
#include <stdio.h>
int main(){
int var1 = -10;
long long var6 = 1000000000;
int var2 = 65;
float var5 = 1234.5678;
char var3[10] = {"hello"};
char var4[10] = "hello";
printf("Hello World! \n");
printf("以有符号输出:%d\n",var1);
printf("以无符号输出:%u\n",var1);
printf("以16进制输出:%x\n",var1);
printf("以 8进制输出:%o\n",var1);
printf("以浮点数输出:%f\n",var1);
printf("以单字符输出:%c\n",var2);
printf("以字符串输出:%s\n",var3);
printf("以字符串输出:%s\n",var4);
printf("以科学计数输出:%e\n",var5);
printf("以长整 数输出:%ld\n",var6);
printf("以长无符号数输出:%lu\n",var6);
printf("以长8进制输出:%lo\n",var6);
printf("以长16进制输出:%lx\n",var6);
printf("以有符号，长度为7输出: %7d \n",var1);
printf("以有符号，长度为7输出左对齐: %-7d \n",var1);
printf("以有符号，长度为7 补0输出: %07d \n",var2);
printf("更多帮助文档,请查看 man 3 printf \n");
}
/*
C:\MinGW\project>gcc main1.c
C:\MinGW\project>a.exe
Hello World!
以有符号输出:-10
以无符号输出:4294967286
以16进制输出:fffffff6
以 8进制输出:37777777766
以浮点数输出:0.000000
以单字符输出:A
以字符串输出:hello
以字符串输出:hello
以科学计数输出:1.234568e+003
以长整 数输出:1000000000
以长无符号数输出:1000000000
以长8进制输出:7346545000
以长16进制输出:3b9aca00
以有符号，长度为7输出: -10
以有符号，长度为7输出左对齐: -10
以有符号，长度为7 补0输出: 0000065
更多帮助文档,请查看 man 3 printf
*/
chunli@pc0003:~$ man 3 printf
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);
Feature Test Macro Requirements for glibc (see fea‐
ture_test_macros(7)):
snprintf(), vsnprintf():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;
or cc -std=c99
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 output to the given output
stream; sprintf(), snprintf(), vsprintf() and vsnprintf() write
to the character string str.
The functions snprintf() and vsnprintf() write at most size
bytes (including the terminating null byte (‘\0‘)) to 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 variable number of arguments. These func‐
tions do not call the va_end macro. Because they invoke the
va_arg macro, the value of ap is undefined after the call. See
stdarg(3).
These eight functions write the output under the control of a
format string that specifies how subsequent arguments (or argu‐
ments accessed via the variable-length argument facilities of
stdarg(3)) are converted for output.
C99 and POSIX.1-2001 specify that the results are undefined if a
call to sprintf(), snprintf(), vsprintf(), or vsnprintf() would
cause copying to take place between objects that overlap (e.g.,
if the target string array and one of the supplied input argu‐
ments refer to the same buffer). See NOTES.
Return value
Upon successful return, these functions return the number of
characters printed (excluding the null byte used to end output
to strings).
The functions snprintf() and vsnprintf() do not write more than
size bytes (including the terminating null byte (‘\0‘)). If the
output was truncated due to this limit then the return value is
the number of characters (excluding the terminating null byte)
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 speci‐
fications, each of which results in fetching zero or more subse‐
quent arguments. 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 precision 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 argument is taken, at each place where an argu‐
ment 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 argu‐
ments, but it may be mixed with "%%" formats which do not con‐
sume 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 character, 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 nonzero 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 (normally, a decimal point appears in
the results of those conversions only if a digit fol‐
lows). 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 converted
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 conver‐
sion (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 conversion.
+ 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 characters 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 nonzero first digit)
specifying a minimum field width. If the converted 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 argu‐
ment, 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 nonexistent or small field width cause trunca‐
tion 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 ‘.‘, the precision is taken to
be zero. A negative precision is taken as if the precision were
omitted. 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 con‐
version.
hh A following integer conversion corresponds to a signed
char or unsigned char argument, or a following n conver‐
sion 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 conver‐
sion 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 following n
conversion corresponds to a pointer to a long int argu‐
ment, or a following c conversion corresponds 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 corre‐
sponds 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 knows about only 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 number 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 pre‐
cision 0, the output is empty.
o, u, x, X
The unsigned int argument is converted to unsigned octal
(o), unsigned decimal (u), or unsigned hexadecimal (x and
X) notation. The letters abcdef are used for x conver‐
sions; the letters ABCDEF are used for X conversions.
The precision, if any, gives the minimum number 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 pre‐
cision 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 precision 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 number of
digits after the decimal-point character is equal to the
precision specification. If the precision 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 conver‐
sion is less than -4 or greater than or equal to the pre‐
cision. 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±; 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 number of digits after it is equal
to the precision. The default precision suffices for an
exact representation of the value if an exact representa‐
tion in base 2 exists and otherwise is sufficiently large
to distinguish values of type double. The digit before
the decimal point is unspecified for nonnormalized num‐
bers, and nonzero but otherwise unspecified for normal‐
ized numbers.
c If no l modifier is present, the int argument is con‐
verted to an unsigned char, and the resulting character
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(3) 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 character 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 written. 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 charac‐
ters. Wide characters from the array are converted to
multibyte characters (each by a call to the wcrtomb(3)
function, with a conversion state starting in the initial
state before the first wide character), up to and includ‐
ing a terminating null wide character. The resulting
multibyte characters are written up to (but not includ‐
ing) the terminating null byte. If a precision is speci‐
fied, no more bytes than the number specified are writ‐
ten, 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
positions. 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 com‐
plete conversion specification is ‘%%‘.
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 contra‐
dict 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 conversions c, d, e,
E, f, F, g, G, i, n, o, p, s, u, x, and X, where F is a synonym
for f. Additionally, it accepts D, O, and U as synonyms for ld,
lo, and lu. (This is bad, and caused serious bugs later, when
support for %D disappeared.) No locale-dependent radix charac‐
ter, 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 modi‐
fiers h, l, L, Z, and q, but accepts L and q both for long dou‐
ble and for long long int (this is a bug). It no longer recog‐
nizes F, D, O, and U, 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, and z and conversion
characters a and A.
glibc 2.2 adds the conversion character F with C99 semantics,
and the flag character I.
NOTES
Some programs imprudently rely on code such as the following
sprintf(buf, "%s some further text", buf);
to append text to buf. However, the standards explicitly note
that the results are undefined if source and destination buffers
overlap when calling sprintf(), snprintf(), vsprintf(), and
vsnprintf(). Depending on the version of gcc(1) used, and the
compiler options employed, calls such as the above will not pro‐
duce the expected results.
The glibc implementation of the functions snprintf() and
vsnprintf() conforms to the C99 standard, that is, behaves as
described above, since glibc version 2.1. Until glibc 2.0.6
they would return -1 when the output was truncated.
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-dependent and difficult to
predict. Use snprintf() and vsnprintf() instead (or asprintf(3)
and vasprintf(3)).
Linux libc4.[45] does not have a snprintf(), but provides a
libbsd that contains an snprintf() equivalent to sprintf(), that
is, 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.
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 interna‐
tionalized 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):
If truncation occurs in glibc versions prior to 2.0.6, this is
treated as an error instead of being handled gracefully.
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
char *
make_message(const char *fmt, ...)
{
int n;
int size = 100; /* Guess we need no more than 100 bytes */
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);
/* Check error code */
if (n < 0)
return NULL;
/* If that worked, return the string */
if (n < size)
return p;
/* Else try again with more space */
size = n + 1; /* Precisely what is needed */
if ((np = realloc (p, size)) == NULL) {
free(p);
return NULL;
} else {
p = np;
}
}
}
SEE ALSO
printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcr‐
tomb(3), wprintf(3), locale(5)
COLOPHON
This page is part of release 3.54 of the Linux man-pages
project. A description of the project, and information about
reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.
GNU 2013-09-04 PRINTF(3)本文出自 “魂斗罗” 博客，请务必保留此出处http://990487026.blog.51cto.com/10133282/1769204
标签：c printf
原文地址：http://990487026.blog.51cto.com/10133282/1769204
