fprintf (original) (raw)

The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

A newer edition of this document exists here

NAME

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,
const char *restrict format, ...);
int sprintf(char *restrict s, const char *restrict format, ...);
`

DESCRIPTION

[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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.

[XSI] [Option Start]Conversions can be applied to the _n_th 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 _N_th 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. [Option End]

In format strings containing the % form of conversion specification, each conversion specification uses the first unused argument in the argument list.

[CX] [Option Start] 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 ( '.' ). [Option End]

Each conversion specification is introduced by the '%' character [XSI] [Option Start] or by the character sequence "%n$", [Option End] after which the following appear in sequence:

A field width, or precision, or both, may be indicated by an asterisk ( '*' ). In this case an argument of typeint 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. [XSI] [Option Start] 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:

printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

The flag characters and their meanings are:

'

[XSI] [Option Start]The integer portion of the result of a decimal conversion ( %i, %d, %u, %f, %F,%g, or %G ) shall be formatted with thousands' grouping characters. For other conversions the behavior is undefined. The non-monetary grouping character is used. [Option End]

-

The result of the conversion shall be left-justified within the field. The conversion is right-justified if this flag is not specified.

The result of a signed conversion shall always begin with a sign ( '+' or '-' ). The conversion shall begin with a sign only when a negative value is converted if this flag is not specified.

If the first character of a signed conversion is not a sign or if a signed conversion results in no characters, a shall be prefixed to the result. This means that if the and '+' flags both appear, the flag shall be ignored.

Specifies that the value is to be converted to an alternative form. For o conversion, it increases the precision (if necessary) to force the first digit of the result to be zero. For x or X conversion specifiers, a non-zero result shall have 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G conversion specifiers, the result shall always contain a radix character, even if no digits follow the radix character. Without this flag, a radix character appears in the result of these conversions only if a digit follows it. Forg and G conversion specifiers, trailing zeros shall not be removed from the result as they normally are. For other conversion specifiers, the behavior is undefined.

0

For d, i, o, u, x, X, a, A, e,E, f, F, g, and G conversion specifiers, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the '0' and '-' flags both appear, the '0' flag is ignored. For d, i, o, u, x, and Xconversion specifiers, if a precision is specified, the '0' flag is ignored. [XSI] [Option Start] If the'0' and '" flags both appear, the grouping characters are inserted before zero padding. For other conversions, the behavior is undefined. [Option End]

The length modifiers and their meanings are:

hh

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to signed char or unsigned char before printing); or that a followingn conversion specifier applies to a pointer to a signed char argument.

h

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short or unsigned short argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to short or unsigned short before printing); or that a followingn conversion specifier applies to a pointer to a short argument.

l (ell)

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long or unsigned long argument; that a following n conversion specifier applies to a pointer to a long argument; that a following c conversion specifier applies to a wint_t argument; that a followings conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a,A, e, E, f, F, g, or G conversion specifier.

ll (ell-ell)

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.

j

Specifies that a following d, i, o, u, x, or X conversion specifier applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to an intmax_t argument.

z

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to a size_t argument.

t

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.

L

Specifies that a following a, A, e, E, f, F, g, orG conversion specifier applies to a long double 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:

d, i

The int argument shall be converted to a signed decimal in the style "[-]dddd". The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

o

The unsigned argument shall be converted to unsigned octal format in the style "dddd". The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

u

The unsigned argument shall be converted to unsigned decimal format in the style "dddd". The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

x

The unsigned argument shall be converted to unsigned hexadecimal format in the style "dddd"; the letters "abcdef" are used. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

X

Equivalent to the x conversion specifier, except that letters "ABCDEF" are used instead of "abcdef".

f, F

The double argument shall be converted to decimal notation in the style"[-]ddd.ddd", where the number of digits after the radix character is equal to the precision specification. If the precision is missing, it shall be taken as 6; if the precision is explicitly zero and no'#' flag is present, no radix character shall appear. If a radix character appears, at least one digit appears before it. The low-order digit shall be rounded in an implementation-defined manner.

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.

e, E

The double argument shall be converted in the style"[-]d.ddd_e±_dd", where there is one digit before the radix character (which is non-zero if the argument is non-zero) and the number of digits after it is equal to the precision; if the precision is missing, it shall be taken as 6; if the precision is zero and no '#' flag is present, no radix character shall appear. The low-order digit shall be rounded in an implementation-defined manner. The E conversion specifier shall produce a number with 'E' instead of 'e' introducing the exponent. The exponent shall always contain at least two digits. If the value is zero, the exponent shall be zero.

A double argument representing an infinity or NaN shall be converted in the style of an f or Fconversion specifier.

g, G

The double argument shall be converted in the style f or e (or in the style F or Ein the case of a G conversion specifier), with the precision specifying the number of significant digits. If an explicit precision is zero, it shall be taken as 1. The style used depends on the value converted; style e (or E ) shall be used only if the exponent resulting from such a conversion is less than -4 or greater than or equal to the precision. Trailing zeros shall be removed from the fractional portion of the result; a radix character shall appear only if it is followed by a digit or a '#' flag is present.

A double argument representing an infinity or NaN shall be converted in the style of an f or Fconversion specifier.

a, A

A double argument representing a floating-point number shall be converted in the style"[-]0x_h_.hhhh_p±_d", where there is one hexadecimal digit (which shall be non-zero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point character and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision shall be sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision shall be sufficient to distinguish values of type double, except that trailing zeros may be omitted; if the precision is zero and the '#' flag is not specified, no decimal-point character shall appear. The letters"abcdef" shall be used for a conversion and the letters "ABCDEF" for A conversion. TheA conversion specifier produces a number with 'X' and 'P' instead of 'x' and 'p'. The exponent shall always contain at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent shall be zero.

A double argument representing an infinity or NaN shall be converted in the style of an f or Fconversion specifier.

c

The int argument shall be converted to an unsigned char, and the resulting byte shall be written.

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.

s

The argument shall be a pointer to an array of char. Bytes from the array shall be written up to (but not including) any terminating null byte. If the precision is specified, no more than that many bytes shall be written. If the precision is not specified or is greater than the size of the array, the application shall ensure that the array contains a null byte.

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.

p

The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printable characters, in an implementation-defined manner.

n

The argument shall be a pointer to an integer into which is written the number of bytes written to the output so far by this call to one of the fprintf() functions. No argument is converted.

C

[XSI] [Option Start]Equivalent to lc. [Option End]

S

[XSI] [Option Start]Equivalent to ls. [Option End]

%

Print a '%' character; no argument is converted. The complete conversion specification shall be %%.

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.

[CX] [Option Start] 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(). [Option End]

RETURN VALUE

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.

ERRORS

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:

[EILSEQ]

[XSI] [Option Start]A wide-character code that does not correspond to a valid character has been detected. [Option End]

[EINVAL]

[XSI] [Option Start]There are insufficient arguments. [Option End]

The printf() and fprintf() functions may fail if:

[ENOMEM]

[XSI] [Option Start]Insufficient storage space is available. [Option End]

The snprintf() function shall fail if:

[EOVERFLOW]

[XSI] [Option Start]The value of n is greater than {INT_MAX} or the number of bytes needed to hold the output excluding the terminating null is greater than {INT_MAX}. [Option End]

The following sections are informative.

EXAMPLES

Printing Language-Independent Date and Time

The following statement can be used to print date and time using a language-independent format:

printf(format, weekday, month, day, hour, min);

For American usage, format could be a pointer to the following string:

"%s, %s %d, %d:%.2d\n"

This example would produce the following message:

Sunday, July 3, 10:02

For German usage, format could be a pointer to the following string:

"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"

This definition of format would produce the following message:

Sonntag, 3. Juli, 10:02

Printing File Information

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:

ls -l

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.

#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

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.

#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

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.

#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

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.

#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

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.

#include <stdio.h> ... long i; char keystr; int elementlen, len; ... while (len < elementlen) { ... printf("%s Element%0ld\n", keystr, elementlen, i); ... }

Creating a Filename

The following example creates a filename using information from a previous getpwnam() function that returned the HOME directory of the user.

#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

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 ofpause().

#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

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.

#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

The following example prints a series of wide characters. Suppose that "L`@`" expands to three bytes:

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

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.

APPLICATION USAGE

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

fputc(), fscanf(), setlocale(), strfmon(), wcrtomb(), the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 7, Locale, <stdio.h>, <wchar.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Aligned with ISO/IEC 9899:1990/Amendment 1:1995 (E). Specifically, the l (ell) qualifier can now be used withc and s conversion specifiers.

The snprintf() function is new in Issue 5.

Issue 6

Extensions beyond the ISO C standard are marked.

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

The DESCRIPTION is updated to use the terms "conversion specifier" and "conversion specification" consistently.

ISO/IEC 9899:1999 standard, Technical Corrigendum 1 is incorporated.

An example of printing wide characters is added.

End of informative text.

UNIX ® is a registered Trademark of The Open Group.
POSIX ® is a registered Trademark of The IEEE.
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