| FPRINTF(3P) | POSIX Programmer's Manual | FPRINTF(3P) |
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
dprintf, fprintf, printf, snprintf, sprintf — print formatted output
#include <stdio.h>
int dprintf(int fildes, const char *restrict format, ...); 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, ...);
Excluding dprintf(): 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 POSIX.1‐2017 defers to the ISO C standard.
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 dprintf() function shall be equivalent to the fprintf() function, except that dprintf() shall write output to the file associated with the file descriptor specified by the fildes argument rather than place output on a stream.
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$" form of conversion specification, numbered arguments in the argument list can be referenced from the format string as many times as required.
In format strings containing the % form of conversion specification, each conversion specification uses the first unused argument in the argument list.
All forms of the fprintf() functions allow for the insertion of a language-dependent radix character in the output string. The radix character is defined in the current locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character shall default to a <period> ('.').
Each conversion specification is introduced by the '%' character or by the character sequence "%n$", 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 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:
printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);
The flag characters and their meanings are:
The length modifiers and their meanings are:
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.
Finally, unless the '#' flag is used, any trailing zeros shall be removed from the fractional portion of the result and the decimal-point character shall be removed if there is no fractional portion remaining.
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 last data modification and last file status change timestamps of the file shall be marked for update:
Upon successful completion, the dprintf(), 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 and set errno to indicate the error.
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 dprintf(), fprintf(), and printf() fail and may fail, refer to fputc() or fputwc().
In addition, all forms of fprintf() shall fail if:
The dprintf() function may fail if:
The dprintf(), fprintf(), and printf() 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:
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
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);
...
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);
...
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); } ...
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);
}
...
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%0*ld\n", keystr, elementlen, i);
...
}
The following example creates a pathname using information from a previous getpwnam() function that returned the password database entry of the user.
#include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <unistd.h> ... char *pathname; struct passwd *pw; size_t len; ... // digits required for pid_t is number of bits times // log2(10) = approx 10/33 len = strlen(pw->pw_dir) + 1 + 1+(sizeof(pid_t)*80+32)/33 +
sizeof ".out"; pathname = malloc(len); if (pathname != NULL) {
snprintf(pathname, len, "%s/%jd.out", pw->pw_dir,
(intmax_t)getpid());
... }
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().
#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));
}
...
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);
}
...
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.
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.
If an implementation detects that there are insufficient arguments for the format, it is recommended that the function should fail and report an [EINVAL] error.
None.
Section 2.5, Standard I/O Streams, fputc(), fscanf(), setlocale(), strfmon(), wcrtomb()
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale, <inttypes.h>, <stdio.h>, <wchar.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
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