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PRINTF(3) Linux Programmer's Manual PRINTF(3) NAME printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vs- nprintf - formatted output conversion SYNOPSIS #include <stdio.h> int printf(const char *format, ...); int fprintf(FILE *stream, const char *format, ...); int sprintf(char *str, const char *format, ...); int snprintf(char *str, size_t size, const char *format, ...); #include <stdarg.h> int vprintf(const char *format, va_list ap); int vfprintf(FILE *stream, const char *format, va_list ap); int vsprintf(char *str, const char *format, va_list ap); int vsnprintf(char *str, size_t size, const char *format, va_list ap); DESCRIPTION The functions in the printf family produce output according to a format as described below. The functions printf and vprintf write output to stdout, the standard output stream; fprintf and vfprintf write output to the given output stream; sprintf, snprintf, vsprintf and vsnprintf write to the character string str. The functions vprintf, vfprintf, vsprintf, vsnprintf are equivalent to the functions printf, fprintf, sprintf, snprintf, respectively, except that they are called with a va_list instead of a variable number of ar- guments. These functions do not call the va_end macro. Consequently, the value of ap is undefined after the call. The application should call va_end(ap) itself afterwards. These eight functions write the output under the control of a format string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of stdarg(3)) are converted for output. Return value Upon successful return, these functions return the number of characters printed (not including the trailing '\0' used to end output to strings). The functions snprintf and vsnprintf do not write more than size bytes (including the trailing '\0'). If the output was truncated due to this limit then the return value is the number of characters (not including the trailing '\0') which would have been written to the final string if enough space had been available. Thus, a return value of size or more means that the output was truncated. (See also below under NOTES.) If an output error is encountered, a negative value is returned. Format of the format string The format string is a character string, beginning and ending in its initial shift state, if any. The format string is composed of zero or more directives: ordinary characters (not %), which are copied un- changed to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments. Each con- version specification is introduced by the character %, and ends with a conversion specifier. In between there may be (in this order) zero or more flags, an optional minimum field width, an optional precision and an optional length modifier. The arguments must correspond properly (after type promotion) with the conversion specifier. By default, the arguments are used in the order given, where each `*' and each conversion specifier asks for the next argument (and it is an error if insufficiently many arguments are given). One can also specify explicitly which argument is taken, at each place where an argument is required, by writing `%m$' instead of `%' and `*m$' instead of `*', where the decimal integer m denotes the position in the argument list of the desired argument, indexed starting from 1. Thus, printf("%*d", width, num); and printf("%2$*1$d", width, num); are equivalent. The second style allows repeated references to the same argument. The C99 standard does not include the style using `$', which comes from the Single Unix Specification. If the style using `$' is used, it must be used throughout for all conversions taking an argument and all width and precision 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 de- pends on the LC_NUMERIC part of the locale. The POSIX locale uses `.' as radix character, and does not have a grouping character. Thus, printf("%'.2f", 1234567.89); results in `1234567.89' in the POSIX locale, in `1234567,89' in the nl_NL locale, and in `1.234.567,89' in the da_DK locale. The flag characters The character % is followed by zero or more of the following flags: # The value should be converted to an ``alternate form''. For o conversions, the first character of the output string is made zero (by prefixing a 0 if it was not zero already). For x and X conversions, a non-zero result has the string `0x' (or `0X' for X conversions) prepended to it. For a, A, e, E, f, F, g, and G conversions, the result will always contain a decimal point, even if no digits follow it (normally, a decimal point appears in the results of those conversions only if a digit follows). For g and G conversions, trailing zeros are not removed from the result as they would otherwise be. For other conversions, the result is undefined. 0 The value should be zero padded. For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, the converted value is padded on the left with zeros rather than blanks. If the 0 and - flags both appear, the 0 flag is ignored. If a precision is given with a numeric conversion (d, i, o, u, x, and X), the 0 flag is ignored. For other conversions, the behavior is undefined. - The converted value is to be left adjusted on the field bound- ary. (The default is right justification.) Except for n conver- sions, the converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A - overrides a 0 if both are given. ' ' (a space) A blank should be left before a positive number (or empty string) produced by a signed conversion. + A sign (+ or -) always be placed before a number produced by a signed conversion. By default a sign is used only for negative numbers. A + overrides a space if both are used. The five flag characters above are defined in the C standard. The SUSv2 specifies one further flag character. ' For decimal conversion (i, d, u, f, F, g, G) the output is to be grouped with thousands' grouping characters if the locale infor- mation indicates any. Note that many versions of gcc cannot parse this option and will issue a warning. SUSv2 does not in- clude %'F. glibc 2.2 adds one further flag character. I For decimal integer conversion (i, d, u) the output uses the lo- cale's alternative output digits, if any (for example, Arabic digits). However, it does not include any locale definitions with such outdigits defined. The field width An optional decimal digit string (with nonzero first digit) specifying a minimum field width. If the converted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if the left-adjustment flag has been given). Instead of a deci- mal digit string one may write `*' or `*m$' (for some decimal integer m) to specify that the field width is given in the next argument, or in the m-th argument, respectively, which must be of type int. A negative field width is taken as a `-' flag followed by a positive field width. In no case does a non-existent 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. The precision An optional precision, in the form of a period (`.') followed by an optional decimal digit string. Instead of a decimal digit string one may write `*' or `*m$' (for some decimal integer m) to specify that the precision is given in the next argument, or in the m-th argument, re- spectively, which must be of type int. If the precision is given as just `.', or the precision is negative, the precision is taken to be zero. This gives the minimum number of digits to appear for d, i, o, u, x, and X conversions, the number of digits to appear after the radix character for a, A, e, E, f, and F conversions, the maximum number of significant digits for g and G conversions, or the maximum number of characters to be printed from a string for s and S conversions. The length modifier Here, `integer conversion' stands for d, i, o, u, x, or X conversion. hh A following integer conversion corresponds to a signed char or unsigned char argument, or a following n conversion corresponds to a pointer to a signed char argument. h A following integer conversion corresponds to a short int or un- signed short int argument, or a following n conversion corre- sponds to a pointer to a short int argument. l (ell) A following integer conversion corresponds to a long int or unsigned long int argument, or a following n conversion cor- responds to a pointer to a long int argument, or a following c conversion corresponds to a wint_t argument, or a following s conversion corresponds to a pointer to wchar_t argument. ll (ell-ell). A following integer conversion corresponds to a long long int or unsigned long long int argument, or a following n conversion corresponds to a pointer to a long long int argument. L A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argument. (C99 allows %LF, but SUSv2 does not.) q (`quad'. BSD 4.4 and Linux libc5 only. Don't use.) This is a synonym for ll. j A following integer conversion corresponds to an intmax_t or uintmax_t argument. z A following integer conversion corresponds to a size_t or ssize_t argument. (Linux libc5 has Z with this meaning. Don't use it.) t A following integer conversion corresponds to a ptrdiff_t argu- ment. The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx, hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf, Lg, LG). The conversion specifier A character that specifies the type of conversion to be applied. The conversion specifiers and their meanings are: d,i The int argument is converted to signed decimal notation. The precision, if any, gives the minimum number of digits that must appear; if the converted value requires fewer digits, it is padded on the left with zeros. The default precision is 1. When 0 is printed with an explicit precision 0, the output is empty. o,u,x,X The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (x and X) nota- tion. The letters abcdef are used for x conversions; the let- ters ABCDEF are used for X conversions. The precision, if any, gives the minimum number of digits that must appear; if the con- verted 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,E The double argument is rounded and converted in the style [-]d.ddde+-dd where there is one digit before the decimal-point character and the number of digits after it is equal to the pre- cision; if the precision is missing, it is taken as 6; if the precision is zero, no decimal-point character appears. An E conversion uses the letter E (rather than e) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. f,F The double argument is rounded and converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly zero, no decimal-point character appears. If a decimal point appears, at least one digit appears before it. (The SUSv2 does not know about F and says that character string representations for infinity and NaN may be made available. The C99 standard specifies `[-]inf' or `[-]infinity' for infinity, and a string starting with `nan' for NaN, in the case of f con- version, and `[-]INF' or `[-]INFINITY' or `NAN*' in the case of F conversion.) g,G The double argument is converted in style f or e (or F or E for G conversions). The precision specifies the number of signifi- cant digits. If the precision is missing, 6 digits are given; if the precision is zero, it is treated as 1. Style e is used if the exponent from its 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) For a conversion, the double argument is converted to hexadecimal notation (using the letters abcdef) in the style [-]0xh.hhhhp+-d; for A conversion the prefix 0X, the letters ABCDEF, and the exponent separator P is used. There is one hexadecimal digit before the decimal point, and the number of digits after it is equal to the precision. The default pre- cision suffices for an exact representation of the value if an exact representation in base 2 exists and otherwise is suffi- ciently large to distinguish values of type double. The digit before the decimal point is unspecified for non-normalized num- bers, and nonzero but otherwise unspecified for normalized num- bers. c If no l modifier is present, the int argument is converted to an unsigned char, and the resulting character is written. If an l modifier is present, the wint_t (wide character) argument is converted to a multibyte sequence by a call to the wcrtomb func- tion, with a conversion state starting in the initial state, and the resulting multibyte string is written. s If no l modifier is present: The const char * argument is ex- pected 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 NUL character; if a precision is speci- fied, no more than the number specified are written. If a pre- cision is given, no null character need be present; if the pre- cision is not specified, or is greater than the size of the ar- ray, the array must contain a terminating NUL character. If an l modifier is present: The const wchar_t * argument is ex- pected 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 the wcrtomb 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 re- sulting multibyte characters are written up to (but not includ- ing) 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 deter- mines the number of bytes written, not the number of wide char- acters or screen positions. The array must contain a terminat- ing null wide character, unless a precision is given and it is so small that the number of bytes written exceeds it before the end of the array is reached. C (Not in C99, but in SUSv2.) Synonym for lc. Don't use. S (Not in C99, but in SUSv2.) Synonym for ls. Don't use. p The void * pointer argument is printed in hexadecimal (as if by %#x or %#lx). n The number of characters written so far is stored into the inte- ger indicated by the int * (or variant) pointer argument. No argument is converted. % A `%' is written. No argument is converted. The complete conver- sion specification is `%%'. EXAMPLES To print pi to five decimal places: #include <math.h> #include <stdio.h> fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0)); To print a date and time in the form `Sunday, July 3, 10:02', where weekday and month are pointers to strings: #include <stdio.h> fprintf(stdout, "%s, %s %d, %.2d:%.2d\n", weekday, month, day, hour, min); Many countries use the day-month-year order. Hence, an international- ized version must be able to print the arguments in an order specified by the format: #include <stdio.h> fprintf(stdout, format, weekday, month, day, hour, min); where format depends on locale, and may permute the arguments. With the value "%1$s, %3$d. %2$s, %4$d:%5$.2d\n" one might obtain `Sonntag, 3. Juli, 10:02'. To allocate a sufficiently large string and print into it (code correct for both glibc 2.0 and glibc 2.1): #include <stdio.h> #include <stdlib.h> #include <stdarg.h> char * make_message(const char *fmt, ...) { /* Guess we need no more than 100 bytes. */ int n, size = 100; char *p; va_list ap; if ((p = malloc (size)) == NULL) return NULL; while (1) { /* Try to print in the allocated space. */ va_start(ap, fmt); n = vsnprintf (p, size, fmt, ap); va_end(ap); /* If that worked, return the string. */ if (n > -1 && n < size) return p; /* Else try again with more space. */ if (n > -1) /* glibc 2.1 */ size = n+1; /* precisely what is needed */ else /* glibc 2.0 */ size *= 2; /* twice the old size */ if ((p = realloc (p, size)) == NULL) return NULL; } } NOTES The glibc implementation of the functions snprintf and vsnprintf con- forms to the C99 standard, i.e., behaves as described above, since glibc version 2.1. Until glibc 2.0.6 they would return -1 when the out- put was truncated. CONFORMING TO The fprintf, printf, sprintf, vprintf, vfprintf, and vsprintf functions conform to ANSI X3.159-1989 (``ANSI C'') and ISO/IEC 9899:1999 (``ISO C99''). The snprintf and vsnprintf functions conform to ISO/IEC 9899:1999. Concerning the return value of snprintf, the SUSv2 and the C99 standard contradict each other: when snprintf is called with size=0 then SUSv2 stipulates an unspecified return value less than 1, while C99 allows str to be NULL in this case, and gives the return value (as always) as the number of characters that would have been written in case the out- put string has been large enough. Linux libc4 knows about the five C standard flags. It knows about the length modifiers h,l,L, and the conversions cdeEfFgGinopsuxX, where F is a synonym for f. Additionally, it accepts D,O,U as synonyms for ld,lo,lu. (This is bad, and caused serious bugs later, when support for %D disappeared.) No locale-dependent radix character, no thousands' separator, no NaN or infinity, no %m$ and *m$. Linux libc5 knows about the five C standard flags and the ' flag, lo- cale, %m$ and *m$. It knows about the length modifiers h,l,L,Z,q, but accepts L and q both for long doubles and for long long integers (this is a bug). It no longer recognizes FDOU, but adds a new conversion character m, which outputs strerror(errno). glibc 2.0 adds conversion characters C and S. glibc 2.1 adds length modifiers hh,j,t,z and conversion characters a,A. glibc 2.2 adds the conversion character F with C99 semantics, and the flag character I. HISTORY Unix V7 defines the three routines printf, fprintf, sprintf, and has the flag -, the width or precision *, the length modifier l, and the conversions doxfegcsu, and also D,O,U,X as synonyms for ld,lo,lu,lx. This is still true for BSD 2.9.1, but BSD 2.10 has the flags #, + and <space> and no longer mentions D,O,U,X. BSD 2.11 has vprintf, vf- printf, vsprintf, and warns not to use D,O,U,X. BSD 4.3 Reno has the flag 0, the length modifiers h and L, and the conversions n, p, E, G, X (with current meaning) and deprecates D,O,U. BSD 4.4 introduces the functions snprintf and vsnprintf, and the length modifier q. FreeBSD also has functions asprintf and vasprintf, that allocate a buffer large enough for sprintf. In glibc there are functions dprintf and vdprintf that print to a file descriptor instead of a stream. BUGS Because sprintf and vsprintf assume an arbitrarily long string, callers must be careful not to overflow the actual space; this is often impos- sible to assure. Note that the length of the strings produced is lo- cale-dependent and difficult to predict. Use snprintf and vsnprintf instead (or asprintf and vasprintf). Linux libc4.[45] does not have a snprintf, but provides a libbsd that contains an snprintf equivalent to sprintf, i.e., one that ignores the size argument. Thus, the use of snprintf with early libc4 leads to se- rious security problems. Code such as printf(foo); often indicates a bug, since foo may contain a % character. If foo comes from untrusted user input, it may contain %n, causing the printf call to write to memory and creating a security hole. SEE ALSO printf(1), asprintf(3), dprintf(3), wcrtomb(3), wprintf(3), scanf(3), locale(5) Linux Manpage 2000-10-16 PRINTF(3)
NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | NOTES | CONFORMING TO | HISTORY | BUGS | SEE ALSO
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