HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. |  |
NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |STANDARDS |HISTORY |CAVEATS |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
printf(3) Library Functions Manualprintf(3)printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vdprintf, vsprintf, vsnprintf - formatted output conversion
Standard C library (libc,-lc)
#include <stdio.h>int printf(const char *restrictformat, ...);int fprintf(FILE *restrictstream,const char *restrictformat, ...);int dprintf(intfd,const char *restrictformat, ...);int sprintf(char *restrictstr,const char *restrictformat, ...);int snprintf(size_t size;charstr[restrictsize], size_tsize,const char *restrictformat, ...);int vprintf(const char *restrictformat, va_listap);int vfprintf(FILE *restrictstream,const char *restrictformat, va_listap);int vdprintf(intfd,const char *restrictformat, va_listap);int vsprintf(char *restrictstr,const char *restrictformat, va_listap);int vsnprintf(size_t size;charstr[restrictsize], size_tsize,const char *restrictformat, va_listap); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):snprintf(),vsnprintf(): _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE || /* glibc <= 2.19: */ _BSD_SOURCEdprintf(),vdprintf(): Since glibc 2.10: _POSIX_C_SOURCE >= 200809L Before glibc 2.10: _GNU_SOURCE
The functions in theprintf() family produce output according to aformat as described below. The functionsprintf() andvprintf() write output tostdout, the standard output stream;fprintf() andvfprintf() write output to the given outputstream;sprintf(),snprintf(),vsprintf(), andvsnprintf() write to the character stringstr. The functiondprintf() is the same asfprintf() except that it outputs to a file descriptor,fd, instead of to astdio(3) stream. The functionssnprintf() andvsnprintf() write at mostsize bytes (including the terminating null byte ('\0')) tostr. The functionsvprintf(),vfprintf(),vdprintf(),vsprintf(),vsnprintf() are equivalent to the functionsprintf(),fprintf(),dprintf(),sprintf(),snprintf(), respectively, except that they are called with ava_list instead of a variable number of arguments. These functions do not call theva_end macro. Because they invoke theva_arg macro, the value ofap is undefined after the call. Seestdarg(3). All of these functions write the output under the control of aformat string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities ofstdarg(3)) are converted for output. C99 and POSIX.1-2001 specify that the results are undefined if a call tosprintf(),snprintf(),vsprintf(), orvsnprintf() would cause copying to take place between objects that overlap (e.g., if the target string array and one of the supplied input arguments refer to the same buffer). See CAVEATS.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 arguments. Each conversion specification is introduced by the character%, and ends with aconversion specifier. In between there may be (in this order) zero or moreflags, an optional minimumfield width, an optionalprecision and an optionallength modifier. The overall syntax of a conversion specification is: %[argument$][flags][width][.precision][length modifier]conversion The arguments must correspond properly (after type promotion) with the conversion specifier. By default, the arguments are used in the order given, where each '*' (seeField width andPrecision below) and each conversion specifier asks for the next argument (and it is an error if insufficiently many arguments are given). One can also specify explicitly which argument is taken, at each place where an argument is required, by writing "%m$" instead of '%' and "*m$" instead of '*', where the decimal integerm denotes the position in the argument list of the desired argument, indexed starting from 1. Thus, printf("%*d", width, num); and printf("%2$*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 '$' is used, it must be used throughout for all conversions taking an argument and all width and precision arguments, but it may be mixed with "%%" formats, which do not consume an argument. There may be no gaps in the numbers of arguments specified using '$'; 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 theLC_NUMERICpart of the locale. (Seesetlocale(3).) 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.Flag characters The character % is followed by zero or more of the following flags:#The value should be converted to an "alternate form". Foroconversions, the first character of the output string is made zero (by prefixing a 0 if it was not zero already). ForxandXconversions, a nonzero result has the string "0x" (or "0X" forXconversions) prepended to it. Fora,A,e,E,f,F,g, andGconversions, 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 follows). ForgandG conversions, trailing zeros are not removed from the result as they would otherwise be. Form, iferrno contains a valid error code, the output ofstrerrorname_np(errno) is printed; otherwise, the value stored inerrno is printed as a decimal number. For other conversions, the result is undefined.0The value should be zero padded. Ford,i,o,u,x,X,a,A,e,E,f,F,g, andGconversions, the converted value is padded on the left with zeros rather than blanks. If the0 and-flags both appear, the0flag is ignored. If a precision is given with an integer conversion (d,i,o,u,x, andX), the0flag 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.) The converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A-overrides a0 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 C99 standard. POSIX 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 as anon-monetary quantity. Misleadingly, this isn't necessarily every thousand: for example Karbi ("mjw_IN"), groups its digits into 3 once, then 2 repeatedly. Comparelocale(7)grouping andthousands_sep, contrast withmon_grouping/mon_thousands_sep andstrfmon(3). This is a no-op in the default "C" locale. glibc 2.2 adds one further flag character.IFor 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.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$" (for some decimal integerm) to specify that the field width is given in the next argument, or in them-th argument, respectively, which must be of typeint. A negative field width is taken as a '-' flag followed by a positive field width. In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.Precision An optional precision, in the form of a period ('.') followed by an optional decimal digit string. Instead of a decimal digit string one may write "*" or "*m$" (for some decimal integerm) to specify that the precision is given in the next argument, or in them-th argument, respectively, which must be of typeint. 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 ford,i,o,u,x, andXconversions, the number of digits to appear after the radix character fora,A,e,E,f, andFconversions, the maximum number of significant digits forgandGconversions, or the maximum number of characters to be printed from a string forsandSconversions.Length modifier Here, "integer conversion" stands ford,i,o,u,x, orX conversion.hhA following integer conversion corresponds to asigned char orunsigned char argument, or a followingnconversion corresponds to a pointer to asigned char argument.hA following integer conversion corresponds to ashort orunsigned short argument, or a followingnconversion corresponds to a pointer to ashort argument.l(ell) A following integer conversion corresponds to along orunsigned long argument, or a followingnconversion corresponds to a pointer to along argument, or a followingcconversion corresponds to awint_t argument, or a followingsconversion corresponds to a pointer towchar_t argument. On a followinga,A,e,E,f,F,g, orG conversion, this length modifier is ignored (C99; not in SUSv2).ll(ell-ell). A following integer conversion corresponds to along long orunsigned long long argument, or a followingn conversion corresponds to a pointer to along long argument.qA synonym forll. This is a nonstandard extension, derived from BSD; avoid its use in new code.LA followinga,A,e,E,f,F,g, orGconversion corresponds to along double argument. (C99 allows %LF, but SUSv2 does not.)jA following integer conversion corresponds to anintmax_t oruintmax_t argument, or a followingnconversion corresponds to a pointer to anintmax_t argument.zA following integer conversion corresponds to asize_t orssize_t argument, or a followingnconversion corresponds to a pointer to asize_t argument.ZA nonstandard synonym forzthat predates the appearance ofz. Do not use in new code.tA following integer conversion corresponds to aptrdiff_t argument, or a followingnconversion corresponds to a pointer to aptrdiff_t argument. SUSv3 specifies all of the above, except for those modifiers explicitly noted as being nonstandard extensions. SUSv2 specified only the length modifiersh(inhd,hi,ho,hx,hX,hn) andl(inld,li,lo,lx,lX,ln,lc,ls) andL(inLe,LE,Lf,Lg,LG). As a nonstandard extension, the GNU implementations treatsllandLas synonyms, so that one can, for example, writellg(as a synonym for the standards-compliantLg) andLd(as a synonym for the standards compliantlld). Such usage is nonportable.Conversion specifiers A character that specifies the type of conversion to be applied. The conversion specifiers and their meanings are:d,iTheint 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 precision 0, the output is empty.o,u,x,X Theunsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (xandX) notation. The lettersabcdefare used forx conversions; the lettersABCDEFare used forXconversions. 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 precision 0, the output is empty.e,EThedouble argument is rounded and converted in the style [-]d.ddde±dd where there is one digit (which is nonzero if the argument is nonzero) 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. AnEconversion uses the letterE(rather thane) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00.f,FThedouble 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. (SUSv2 does not know aboutFand says that character string representations for infinity and NaN may be made available. SUSv3 adds a specification forF. The C99 standard specifies "[-]inf" or "[-]infinity" for infinity, and a string starting with "nan" for NaN, in the case off conversion, and "[-]INF" or "[-]INFINITY" or "NAN" in the case ofFconversion.)g,GThedouble argument is converted in stylefore(orForE forGconversions). 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. Styleeis 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, but added in SUSv3) Foraconversion, thedouble argument is converted to hexadecimal notation (using the letters abcdef) in the style [-]0xh.hhhhp±d; forAconversion the prefix0X, the letters ABCDEF, and the exponent separatorPis used. There is one hexadecimal digit before the radix 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 representation in base 2 exists and otherwise is sufficiently large to distinguish values of typedouble. The digit before the radix point is unspecified for nonnormalized numbers, and nonzero but otherwise unspecified for normalized numbers. The exponent,d, is the appropriate exponent of 2 expressed as a decimal integer; it always contains at least one digit; if the value is zero, the exponent is 0.cIf nolmodifier is present, theint argument is converted to anunsigned char, and the resulting character is written. If anlmodifier is present, thewint_t (wide character) argument is converted to a multibyte sequence by a call to thewcrtomb(3) function, with a conversion state starting in the initial state, and the resulting multibyte string is written.sIf nolmodifier is present: theconst 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 anlmodifier is present: theconst 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 thewcrtomb(3) 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 ofbytes written, not the number ofwidecharacters orscreen 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 or C11, but in SUSv2, SUSv3, and SUSv4.) Synonym forlc. Don't use.S(Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.) Synonym forls. Don't use.pThevoid * pointer argument is printed in hexadecimal (as if by%#xor%#lx).nThe number of characters written so far is stored into the integer pointed to by the corresponding argument. That argument shall be anint *, or variant whose size matches the (optionally) supplied integer length modifier. No argument is converted. (This specifier is not supported by the bionic C library.) The behavior is undefined if the conversion specification includes any flags, a field width, or a precision.m(glibc extension; supported by uClibc and musl, and on Android from API level 29.) Print output ofstrerror(errno) (orstrerrorname_np(errno) in the alternate form). No argument is required.%A '%' is written. No argument is converted. The complete conversion specification is '%%'. Upon successful return, these functions return the number of bytes printed (excluding the null byte used to end output to strings). The functionssnprintf() andvsnprintf() do not write more thansize 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 ofsize or more means that the output was truncated. (See also below under CAVEATS.) On error, a negative value is returned, anderrno is set to indicate the error.Seewrite(2) andputwc(3). In addition, the following error may occur:EOVERFLOW The value to be returned is greater thanINT_MAX. Thedprintf() function may fail additionally if:EBADFThefd argument is not a valid file descriptor.
For an explanation of the terms used in this section, seeattributes(7). ┌───────────────────────────────┬───────────────┬────────────────┐ │Interface│Attribute│Value│ ├───────────────────────────────┼───────────────┼────────────────┤ │printf(),fprintf(), │ Thread safety │ MT-Safe locale │ │sprintf(),snprintf(), │ │ │ │vprintf(),vfprintf(), │ │ │ │vsprintf(),vsnprintf() │ │ │ └───────────────────────────────┴───────────────┴────────────────┘
fprintf()printf()sprintf()vprintf()vfprintf()vsprintf()snprintf()vsnprintf() C11, POSIX.1-2008.dprintf()vdprintf() GNU, POSIX.1-2008.
fprintf()printf()sprintf()vprintf()vfprintf()vsprintf() C89, POSIX.1-2001.snprintf()vsnprintf() SUSv2, C99, POSIX.1-2001. Concerning the return value ofsnprintf(), SUSv2 and C99 contradict each other: whensnprintf() is called withsize=0 then SUSv2 stipulates an unspecified return value less than 1, while C99 allowsstr 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. POSIX.1-2001 and later align their specification ofsnprintf() with C99.dprintf()vdprintf() GNU, POSIX.1-2008. Issue 4 of the X/Open Portability Guide (SUSv1, 1994) adds'. glibc 2.1 adds length modifiershh,j,t, andzand conversion charactersaandA. glibc 2.2 adds the conversion characterFwith C99 semantics, and the flag characterI. glibc 2.35 gives a meaning to the alternate form (#) of them conversion specifier, that is%#m.
Some programs imprudently rely on code such as the following sprintf(buf, "%s some further text", buf); to append text tobuf. However, the standards explicitly note that the results are undefined if source and destination buffers overlap when callingsprintf(),snprintf(),vsprintf(), andvsnprintf(). Depending on the version ofgcc(1) used, and the compiler options employed, calls such as the above willnot produce the expected results. The glibc implementation of the functionssnprintf() andvsnprintf() conforms to the C99 standard, that is, behaves as described above, since glibc 2.1. Until glibc 2.0.6, they would return -1 when the output was truncated.
Becausesprintf() andvsprintf() 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. Usesnprintf() andvsnprintf() instead (orasprintf(3) andvasprintf(3)). Code such asprintf(foo);often indicates a bug, sincefoo may contain a % character. Iffoo comes from untrusted user input, it may contain%n, causing theprintf() call to write to memory and creating a security hole.
To printPi 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", whereweekday andmonth 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); whereformat 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, ...) { int n = 0; size_t size = 0; char *p = NULL; va_list ap; /* Determine required size. */ va_start(ap, fmt); n = vsnprintf(p, size, fmt, ap); va_end(ap); if (n < 0) return NULL; size = (size_t) n + 1; /* One extra byte for '\0' */ p = malloc(size); if (p == NULL) return NULL; va_start(ap, fmt); n = vsnprintf(p, size, fmt, ap); va_end(ap); if (n < 0) { free(p); return NULL; } return p; } If truncation occurs in glibc versions prior to glibc 2.0.6, this is treated as an error instead of being handled gracefully.printf(1),asprintf(3),puts(3),scanf(3),setlocale(3),strfromd(3),wcrtomb(3),wprintf(3),locale(5)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-07-19printf(3)Pages that refer to this page:bash(1), find(1), gawk(1), hexdump(1), printf(1), ps(1), time(1), perf_event_open(2), asprintf(3), avc_init(3), curs_printw(3x), curs_termcap(3x), ecvt(3), ecvt_r(3), err(3), errno(3), error(3), FILE(3type), form_field_validation(3x), fwide(3), gcvt(3), intmax_t(3type), intN_t(3type), intptr_t(3type), lber-encode(3), localeconv(3), lttng-ust(3), lttng_ust_tracef(3), lttng_ust_tracelog(3), numa(3), pmnotifyerr(3), pmnumberstr(3), pmprintf(3), pmsprintf(3), printf.h(3head), ptrdiff_t(3type), qecvt(3), rpc(3), scanf(3), sd_bus_error(3), sd_bus_message_new_method_error(3), sd-daemon(3), sd-id128(3), sd_id128_to_string(3), sd_journal_print(3), sd_journal_stream_fd(3), selinux_set_callback(3), setbuf(3), size_t(3type), sscanf(3), stdarg(3), stdin(3), stdio(3), strfmon(3), strfromd(3), strftime(3), syslog(3), uuid_parse(3), void(3type), wprintf(3), slapd.conf(5), slapd-config(5), terminfo(5), feature_test_macros(7), locale(7), signal-safety(7), system_data_types(7), btrfstune(8), rpm(8), slappasswd(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |