fprintf (3p) - Linux Manuals
fprintf: print formatted output
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fprintf, printf, snprintf, sprintf - print formatted output
SYNOPSIS
#include <stdio.h>
int fprintf(FILE *restrict stream, const char *restrict
format, ...);
int printf(const char *restrict format, ...);
int snprintf(char *restrict s, size_t n,
int sprintf(char *restrict s, const char *restrict
format, ...);
DESCRIPTION
The fprintf() function shall place output on the named output stream. The printf() function shall place output on the standard output stream stdout. The sprintf() function shall place output followed by the null byte, '\0', in consecutive bytes starting at *s; it is the user's responsibility to ensure that enough space is available.
The snprintf() function shall be equivalent to sprintf(), with the addition of the n argument which states the size of the buffer referred to by s. If n is zero, nothing shall be written and s may be a null pointer. Otherwise, output bytes beyond the n-1st shall be discarded instead of being written to the array, and a null byte is written at the end of the bytes actually written into the array.
If copying takes place between objects that overlap as a result of a call to sprintf() or snprintf(), the results are undefined.
Each of these functions converts, formats, and prints its arguments under control of the format. The format is a character string, beginning and ending in its initial shift state, if any. The format is composed of zero or more directives: ordinary characters, which are simply copied to the output stream, and conversion specifications, each of which shall result in the fetching of zero or more arguments. The results are undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments shall be evaluated but are otherwise ignored.
Conversions can be applied to the nth argument after the format in the argument list, rather than to the next unused argument. In this case, the conversion specifier character % (see below) is replaced by the sequence "%n$", where n is a decimal integer in the range [1,{NL_ARGMAX}], giving the position of the argument in the argument list. This feature provides for the definition of format strings that select arguments in an order appropriate to specific languages (see the EXAMPLES section).
The format can contain either numbered argument conversion specifications (that is, "%n$" and "*m$"), or unnumbered argument conversion specifications (that is, % and * ), but not both. The only exception to this is that %% can be mixed with the "%n$" form. The results of mixing numbered and unnumbered argument specifications in a format string are undefined. When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, are specified in the format string.
In format strings containing the "%n$" form of conversion specification, numbered arguments in the argument list can be referenced from the format string as many times as required.
In format strings containing the % form of conversion specification, each conversion specification uses the first unused argument in the argument list.
All forms of the fprintf() functions allow for the insertion of a language-dependent radix character in the output string. The radix character is defined in the program's locale (category LC_NUMERIC ). In the POSIX locale, or in a locale where the radix character is not defined, the radix character shall default to a period ( '.' ).
Each conversion specification is introduced by the '%' character
A field width, or precision, or both, may be indicated by an asterisk
( '*' ). In this case an argument of type
int supplies the field width or precision. Applications shall
ensure that arguments specifying field width, or precision, or
both appear in that order before the argument, if any, to be converted.
A negative field width is taken as a '-' flag
followed by a positive field width. A negative precision is taken
as if the precision were omitted. In format
strings containing the "%n$" form of a conversion
specification, a field width or precision may be
indicated by the sequence "*m$", where m is
a decimal integer in the range [1,{NL_ARGMAX}] giving
the position in the argument list (after the format argument)
of an integer argument containing the field width or
precision, for example:
The flag characters and their meanings are:
The length modifiers and their meanings are:
Specifies that a following d, i, o, u,
x, or X conversion specifier
applies to a long long or unsigned long long argument;
or that a following n conversion specifier applies to
a pointer to a long long argument.
If a length modifier appears with any conversion specifier other than
as specified above, the behavior is undefined.
The conversion specifiers and their meanings are:
A double argument representing an infinity shall be converted
in one of the styles "[-]inf" or
"[-]infinity" ; which style is implementation-defined. A double
argument representing a NaN shall be converted in
one of the styles "[-]nan(n-char-sequence)" or "[-]nan"
; which style, and the meaning of any
n-char-sequence, is implementation-defined. The F conversion
specifier produces "INF",
"INFINITY", or "NAN" instead of "inf", "infinity",
or "nan", respectively.
A double argument representing an infinity or NaN shall be converted
in the style of an f or F
conversion specifier.
A double argument representing an infinity or NaN shall be converted
in the style of an f or F
conversion specifier.
A double argument representing an infinity or NaN shall be converted
in the style of an f or F
conversion specifier.
If an l (ell) qualifier is present, the wint_t argument
shall be converted as if by an ls conversion
specification with no precision and an argument that points to a two-element
array of type wchar_t, the first element of
which contains the wint_t argument to the ls conversion
specification and the second element contains a null wide
character.
If an l (ell) qualifier is present, the argument shall be a
pointer to an array of type wchar_t. Wide characters
from the array shall be converted to characters (each as if by a call
to the wcrtomb() function, with the conversion state described
by an mbstate_t object
initialized to zero before the first wide character is converted)
up to and including a terminating null wide character. The
resulting characters shall be written up to (but not including) the
terminating null character (byte). If no precision is
specified, the application shall ensure that the array contains a
null wide character. If a precision is specified, no more than
that many characters (bytes) shall be written (including shift sequences,
if any), and the array shall contain a null wide
character if, to equal the character sequence length given by the
precision, the function would need to access a wide character one
past the end of the array. In no case shall a partial character be
written.
If a conversion specification does not match one of the above forms,
the behavior is undefined. If any argument is not the
correct type for the corresponding conversion specification, the behavior
is undefined.
In no case shall a nonexistent or small field width cause truncation
of a field; if the result of a conversion is wider than the
field width, the field shall be expanded to contain the conversion
result. Characters generated by fprintf() and
printf() are printed as if fputc() had been called.
For the a and A conversion specifiers, if FLT_RADIX is
a power of 2, the value shall be correctly rounded to a
hexadecimal floating number with the given precision.
For a and A conversions, if FLT_RADIX is not a power of
2 and the result is not exactly representable in the
given precision, the result should be one of the two adjacent numbers
in hexadecimal floating style with the given precision, with
the extra stipulation that the error should have a correct sign for
the current rounding direction.
For the e, E, f, F, g, and G
conversion specifiers, if the number of
significant decimal digits is at most DECIMAL_DIG, then the result
should be correctly rounded. If the number of significant
decimal digits is more than DECIMAL_DIG but the source value is exactly
representable with DECIMAL_DIG digits, then the result
should be an exact representation with trailing zeros. Otherwise,
the source value is bounded by two adjacent decimal strings
L < U, both having DECIMAL_DIG significant digits; the
value of the resultant decimal string D should
satisfy L <= D <= U, with the extra stipulation
that the error should have a correct sign for the
current rounding direction.
The
st_ctime and st_mtime fields of the file shall be marked
for update between the call to a successful execution of
fprintf() or printf() and the next successful completion
of a call to fflush() or fclose() on the same stream or
a
call to exit() or abort().
Upon successful completion, the fprintf() and printf()
functions shall return the number of bytes transmitted.
Upon successful completion, the sprintf() function shall return
the number of bytes written to s, excluding the
terminating null byte.
Upon successful completion, the snprintf() function shall return
the number of bytes that would be written to s
had n been sufficiently large excluding the terminating null
byte.
If an output error was encountered, these functions shall return a
negative value.
If the value of n is zero on a call to snprintf(), nothing
shall be written, the number of bytes that would have
been written had n been sufficiently large excluding the terminating
null shall be returned, and s may be a null
pointer.
For the conditions under which fprintf() and printf()
fail and may fail, refer to fputc() or fputwc().
In addition, all forms of fprintf() may fail if:
The printf() and fprintf() functions may fail if:
The snprintf() function shall fail if:
The following sections are informative.
The following statement can be used to print date and time using a
language-independent format:
For American usage, format could be a pointer to the following
string:
This example would produce the following message:
For German usage, format could be a pointer to the following
string:
This definition of format would produce the following message:
The following example prints information about the type, permissions,
and number of links of a specific file in a directory.
The first two calls to printf() use data decoded from a previous
stat() call.
The user-defined strperm() function shall return a string similar
to the one at the beginning of the output for the
following command:
The next call to printf() outputs the owner's name if it is
found using getpwuid(); the getpwuid() function shall
return a passwd structure from which the name of the user is
extracted. If the user name is not found, the program instead
prints out the numeric value of the user ID.
The next call prints out the group name if it is found using getgrgid();
getgrgid() is very similar to getpwuid() except that it
shall return group information based on the group number. Once
again, if the group is not found, the program prints the numeric value
of the group for the entry.
The final call to printf() prints the size of the file.
The following example gets a localized date string. The nl_langinfo()
function shall return the localized date string, which specifies the
order and layout of the date. The strftime() function takes
this information and, using the tm structure for values,
places the date and time information into datestring. The printf()
function then outputs datestring and the
name of the entry.
The following example uses fprintf() to write error information
to standard error.
In the first group of calls, the program tries to open the password
lock file named LOCKFILE. If the file already exists,
this is an error, as indicated by the O_EXCL flag on the open()
function. If the call
fails, the program assumes that someone else is updating the password
file, and the program exits.
The next group of calls saves a new password file as the current password
file by creating a link between LOCKFILE and
the new password file PASSWDFILE.
The following example checks to make sure the program has the necessary
arguments, and uses fprintf() to print usage
information if the expected number of arguments is not present.
The following example prints a key and data pair on stdout.
Note use of the '*' (asterisk) in the format string;
this ensures the correct number of decimal places for the element
based on the number of elements requested.
The following example creates a filename using information from a
previous getpwnam() function that returned the HOME directory
of the user.
The following example loops until an event has timed out. The pause()
function
waits forever unless it receives a signal. The fprintf() statement
should never occur due to the possible return values of
pause().
The following example uses strfmon() to convert a number and
store it as a
formatted monetary string named convbuf. If the first number
is printed, the program prints the format and the description;
otherwise, it just prints the number.
The following example prints a series of wide characters. Suppose
that "L`@`" expands to three bytes:
In the last line of the example, after processing three characters,
nine bytes have been output. The fourth character must then
be examined to determine whether it converts to one byte or more.
If it converts to more than one byte, the output is only nine
bytes. Since there is no fourth character in the array, the behavior
is undefined.
If the application calling fprintf() has any objects of type
wint_t or wchar_t, it must also include the <wchar.h>
header to have these objects defined.
fputc(), fscanf(), setlocale(), strfmon(),
wcrtomb(),
the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 7,
Locale, <stdio.h>, <wchar.h>
printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);
RETURN VALUE
ERRORS
EXAMPLES
Printing Language-Independent Date and Time
printf(format, weekday, month, day, hour, min);
"%s, %s %d, %d:%.2d\n"
Sunday, July 3, 10:02
"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"
Sonntag, 3. Juli, 10:02
Printing File Information
ls -l
#include <stdio.h>
#include <sys/types.h>
#include <pwd.h>
#include <grp.h>
char *strperm (mode_t);
...
struct stat statbuf;
struct passwd *pwd;
struct group *grp;
...
printf("%10.10s", strperm (statbuf.st_mode));
printf("%4d", statbuf.st_nlink);
if ((pwd = getpwuid(statbuf.st_uid)) != NULL)
printf(" %-8.8s", pwd->pw_name);
else
printf(" %-8ld", (long) statbuf.st_uid);
if ((grp = getgrgid(statbuf.st_gid)) != NULL)
printf(" %-8.8s", grp->gr_name);
else
printf(" %-8ld", (long) statbuf.st_gid);
printf("%9jd", (intmax_t) statbuf.st_size);
...
Printing a Localized Date String
#include <stdio.h>
#include <time.h>
#include <langinfo.h>
...
struct dirent *dp;
struct tm *tm;
char datestring[256];
...
strftime(datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm);
printf(" %s %s\n", datestring, dp->d_name);
...
Printing Error Information
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#define LOCKFILE "/etc/ptmp"
#define PASSWDFILE "/etc/passwd"
...
int pfd;
...
if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
exit(1);
}
...
if (link(LOCKFILE,PASSWDFILE) == -1) {
fprintf(stderr, "Link error: %s\n", strerror(errno));
exit(1);
}
...
Printing Usage Information
#include <stdio.h>
#include <stdlib.h>
...
char *Options = "hdbtl";
...
if (argc < 2) {
fprintf(stderr, "Usage: %s -%s <file\n", argv[0], Options); exit(1);
}
...
Formatting a Decimal String
#include <stdio.h>
...
long i;
char *keystr;
int elementlen, len;
...
while (len < elementlen) {
...
printf("%s Element%0*ld\n", keystr, elementlen, i);
...
}
Creating a Filename
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
...
char filename[PATH_MAX+1];
struct passwd *pw;
...
sprintf(filename, "%s/%d.out", pw->pw_dir, getpid());
...
Reporting an Event
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
...
while (!event_complete) {
...
if (pause() != -1 || errno != EINTR)
fprintf(stderr, "pause: unknown error: %s\n", strerror(errno));
}
...
Printing Monetary Information
#include <monetary.h>
#include <stdio.h>
...
struct tblfmt {
char *format;
char *description;
};
struct tblfmt table[] = {
{ "%n", "default formatting" },
{ "%11n", "right align within an 11 character field" },
{ "%#5n", "aligned columns for values up to 99999" },
{ "%=*#5n", "specify a fill character" },
{ "%=0#5n", "fill characters do not use grouping" },
{ "%^#5n", "disable the grouping separator" },
{ "%^#5.0n", "round off to whole units" },
{ "%^#5.4n", "increase the precision" },
{ "%(#5n", "use an alternative pos/neg style" },
{ "%!(#5n", "disable the currency symbol" },
};
...
float input[3];
int i, j;
char convbuf[100];
...
strfmon(convbuf, sizeof(convbuf), table[i].format, input[j]);
if (j == 0) {
printf("%s %s %s\n", table[i].format,
convbuf, table[i].description);
}
else {
printf(" %s\n", convbuf);
}
...
Printing Wide Characters
wchar_t wz [3] = L"@@"; // Zero-terminated
wchar_t wn [3] = L"@@@"; // Unterminated
fprintf (stdout,"%ls", wz); // Outputs 6 bytes
fprintf (stdout,"%ls", wn); // Undefined because wn has no terminator
fprintf (stdout,"%4ls", wz); // Outputs 3 bytes
fprintf (stdout,"%4ls", wn); // Outputs 3 bytes; no terminator needed
fprintf (stdout,"%9ls", wz); // Outputs 6 bytes
fprintf (stdout,"%9ls", wn); // Outputs 9 bytes; no terminator needed
fprintf (stdout,"%10ls", wz); // Outputs 6 bytes
fprintf (stdout,"%10ls", wn); // Undefined because wn has no terminator
APPLICATION USAGE
RATIONALE
FUTURE DIRECTIONS
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
SEE ALSO