printf(3) - NetBSD Manual Pages

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PRINTF(3)               NetBSD Library Functions Manual              PRINTF(3)


NAME
printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf, vsprintf, vsnprintf, vasprintf -- formatted output conversion
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdio.h> int printf(const char * restrict format, ...); int fprintf(FILE * restrict stream, const char * restrict format, ...); int sprintf(char * restrict str, const char * restrict format, ...); int snprintf(char * restrict str, size_t size, const char * restrict format, ...); int asprintf(char ** restrict ret, const char * restrict format, ...); #include <stdarg.h> int vprintf(const char * restrict format, va_list ap); int vfprintf(FILE * restrict stream, const char * restrict format, va_list ap); int vsprintf(char * restrict str, const char * restrict format, va_list ap); int vsnprintf(char * restrict str, size_t size, const char * restrict format, va_list ap); int vasprintf(char ** restrict ret, const char * restrict format, va_list ap);
DESCRIPTION
The printf() family of functions produces output according to a format as described below. 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; asprintf() and vasprintf() write to a dynamically allocated string that is stored in ret. These 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 out- put. These functions return the number of characters printed (not including the trailing `\0' used to end output to strings). If an output error was encountered, these functions shall return a negative value. asprintf() and vasprintf() return a pointer to a buffer sufficiently large to hold the string in the ret argument. This pointer should be passed to free(3) to release the allocated storage when it is no longer needed. If sufficient space cannot be allocated, these functions will return -1 and set ret to be a NULL pointer. Please note that these func- tions are not standardized, and not all implementations can be assumed to set the ret argument to NULL on error. It is more portable to check for a return value of -1 instead. snprintf() and vsnprintf() will write at most size-1 of the characters printed into the output string (the size'th character then gets the ter- minating `\0'); if the return value is greater than or equal to the size argument, the string was too short and some of the printed characters were discarded. If size is zero, nothing is written and str may be a NULL pointer. sprintf() and vsprintf() effectively assume an infinite size. The format string is composed of zero or more directives: ordinary char- acters (not %), which are copied unchanged to the output stream; and con- version specifications, each of which results in fetching zero or more subsequent arguments. Each conversion specification is introduced by the character %. The arguments must correspond properly (after type promo- tion) with the conversion specifier. After the %, the following appear in sequence: ˇ Zero or more of the following flags: - A # character specifying that the value should be converted to an ``alternative form''. For c, d, i, n, p, s, and u conversions, this option has no effect. For o conversions, the precision of the number is increased to force the first character of the out- put string to a zero (except if a zero value is printed with an explicit precision of zero). For x and X conversions, a non-zero result has the string `0x' (or `0X' for X conversions) prepended to it. For e, E, f, F, g, and G conversions, the result will always contain a decimal point, even if no digits follow it (nor- mally, a decimal point appears in the results of those conver- sions only if a digit follows). For g and G conversions, trail- ing zeros are not removed from the result as they would otherwise be. - A zero `0' character specifying zero padding. For all conver- sions except n, the converted value is padded on the left with zeros rather than blanks. If a precision is given with a numeric conversion (d, i, o, u, i, x, and X), the `0' flag is ignored. - A negative field width flag `-' indicates the converted value is to be left adjusted on the field boundary. 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, specifying that a blank should be left before a positive number produced by a signed conversion (d, e, E, f, F, g, G, or i). - A `+' character specifying that a sign always be placed before a number produced by a signed conversion. A `+' overrides a space if both are used. ˇ An optional decimal digit string 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-adjust- ment flag has been given) to fill out the field width. ˇ An optional precision, in the form of a period `.' followed by an optional digit string. If the digit string is omitted, the precision is taken as 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 decimal-point for 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 conversions. ˇ The optional character h, specifying that a following d, i, o, u, x, or X conversion corresponds to a short int or unsigned short int argument, or that a following n conversion corresponds to a pointer to a short int argument. ˇ The optional character j, specifying that a following d, i, o, u, x, or X conversion corresponds to an intmax_t or uintmax_t argument, or that a following n conversion corresponds to a pointer to a intmax_t argument. ˇ The optional character l (ell) specifying that a following d, i, o, u, x, or X conversion corresponds to a long int or unsigned long int argument, or that a following n conversion corresponds to a pointer to a long int argument. ˇ Two optional consecutive l (ell) characters, specifying that a fol- lowing d, i, o, u, x, or X conversion corresponds to a long long or unsigned long long argument, or that a following n conversion corre- sponds to a pointer to a long long argument. ˇ The optional character q, specifying that a following d, i, o, u, x, or X conversion corresponds to a quad_t or u_quad_t argument, or that a following n conversion corresponds to a pointer to a quad_t argu- ment. The q modifier is a BSD extension and should not be used in portable programs. ˇ The optional character t, specifying that a following d, i, o, u, x, or X conversion corresponds to a ptrdiff_t or the corresponding unsigned integer type argument, or that a following n conversion cor- responds to a pointer to a ptrdiff_t argument. ˇ The optional character z, specifying that a following d, i, o, u, x, or X conversion corresponds to a size_t or the corresponding signed integer type argument, or that a following n conversion corresponds to a pointer to a signed integer type corresponding to size_t argu- ment. ˇ The character L specifying that a following e, E, f, F, g, or G con- version corresponds to a long double argument. ˇ A character that specifies the type of conversion to be applied. A field width or precision, or both, may be indicated by an asterisk `*' instead of a digit string. In this case, an int argument supplies the field width or precision. A negative field width is treated as a left adjustment flag followed by a positive field width; a negative precision is treated as though it were missing. The conversion specifiers and their meanings are: diouxX The int (or appropriate variant) argument is converted to signed decimal (d and i), unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (x and X) notation. The letters abcdef are used for x conversions; the letters ABCDEF are used for X conver- sions. 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. DOU The long int argument is converted to signed decimal, unsigned octal, or unsigned decimal, as if the format had been ld, lo, or lu respectively. These conversion characters are deprecated, and will eventually disappear. fF 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. If the double argument represents an infinity it is converted in the style [-]inf. If the double argument represents a NaN it is converted in the style [-]nan. An F conversion produces [-]INF and [-]NAN, respectively. eE The double argument is rounded and converted in the style [-]d.dddeądd 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 con- version uses the letter E (rather than e) to introduce the expo- nent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. Double arguments representing infinities or NaNs are converted in the same styles as in the f and F conversions. gG The double argument is converted in style f or e (or in style F or E for G conversions). 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. 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. Double arguments representing infinities or NaNs are converted in the same styles as in the f and F conversions. c The int argument is converted to an unsigned char, and the resulting character is written. s The ``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 NUL character; if a precision is specified, no more than the number specified are written. If a precision is given, no null charac- ter need be present; if the precision is not specified, or is greater than the size of the array, the array must contain a ter- minating NUL character. 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 con- version specification is `%%'. 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.
RETURN VALUES
Upon successful completion printf(), fprintf(), vprintf(), and vfprintf() return the number of characters printed. Otherwise a negative value is returned and errno is set to indicate the error. Upon successful completion sprintf() and vsprintf() return the number of characters written to str, excluding the terminating NUL character. Oth- erwise a negative value is returned and errno is set to indicate the error. Upon successful completion snprintf() and vsnprintf() return the number of characters that would have been written to a sufficiently sized str, excluding the terminating NUL character. Otherwise a negative value is returned and errno is set to indicate the error. Upon successful completion asprintf() and vasprintf() return the number of characters written to ret, excluding the terminating NUL character. Otherwise -1 is returned, ret is set to NULL, and errno is set to indi- cate the error.
EXAMPLES
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); To print pi to five decimal places: #include <math.h> #include <stdio.h> fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0)); To allocate a 128 byte string and print into it: #include <stdio.h> #include <stdlib.h> #include <stdarg.h> char *newfmt(const char *fmt, ...) { char *p; va_list ap; if ((p = malloc(128)) == NULL) return (NULL); va_start(ap, fmt); (void) vsnprintf(p, 128, fmt, ap); va_end(ap); return (p); }
SEE ALSO
printf(1), scanf(3), printf(9)
STANDARDS
The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions conform to ISO/IEC 9899:1990 (``ISO C90''). The conversion format modifiers %j, %t and %z conform to ISO/IEC 9899:1999 (``ISO C99''). The snprintf() and vsnprintf() functions conform to ISO/IEC 9899:1999 (``ISO C99'').
HISTORY
The functions snprintf() and vsnprintf() first appeared in 4.4BSD. The functions asprintf() and vasprintf() are modeled on the ones that first appeared in the GNU C library.
CAVEATS
Because sprintf() and vsprintf() assume an infinitely long string, callers must be careful not to overflow the actual space; this is often impossible to assure. For safety, programmers should use the snprintf() and asprintf() family of interfaces instead. Unfortunately, the snprintf() interfaces are not available on older systems and the asprintf() interfaces are not yet portable. It is important never to pass a string with user-supplied data as a for- mat without using `%s'. An attacker can put format specifiers in the string to mangle your stack, leading to a possible security hole. This holds true even if you have built the string ``by hand'' using a function like snprintf(), as the resulting string may still contain user-supplied conversion specifiers for later interpolation by printf(). Be sure to use the proper secure idiom: snprintf(buffer, sizeof(buffer), "%s", string); There is no way for printf to know the size of each argument passed. If you use positional arguments you must ensure that all parameters, up to the last positionally specified parameter, are used in the format string. This allows for the format string to be parsed for this information. Failure to do this will mean your code is non-portable and liable to fail. In this implementation, passing a NULL char * argument to the %s format specifier will output (null) instead of crashing. Programs that depend on this behavior are non-portable and may crash on other systems or in the future.
BUGS
The conversion formats %D, %O, and %U are not standard and are provided only for backward compatibility. The effect of padding the %p format with zeros (either by the `0' flag or by specifying a precision), and the benign effect (i.e. none) of the `#' flag on %n and %p conversions, as well as other nonsensical combinations such as %Ld, are not standard; such combinations should be avoided. NetBSD 4.0 September 25, 2006 NetBSD 4.0
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