fprintf - print to a file

printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vdprintf, vsprintf, vsnprintf - formatted output conversion

Header File

#include <stdio.h>

Prototype

int fprintf(FILE *stream, const char *format, ...);

Note that ... represents zero or more additional arguments.

#include <stdio.h>
int printf(const char *restrict format, ...);
int fprintf(FILE *restrict stream,
 const char *restrict format, ...);
int dprintf(int fd,
 const char *restrict format, ...);
int sprintf(char *restrict str,
 const char *restrict format, ...);
int snprintf(char str[restrict .size], size_t size,
 const char *restrict format, ...);
int vprintf(const char *restrict format, va_list ap);
int vfprintf(FILE *restrict stream,
 const char *restrict format, va_list ap);
int vdprintf(int fd,
 const char *restrict format, va_list ap);
int vsprintf(char *restrict str,
 const char *restrict format, va_list ap);
int vsnprintf(char str[restrict .size], size_t size,
 const char *restrict format, va_list ap);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE

    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _GNU_SOURCE

This function prints a “formatted string” to a file. It expects as input the pointer to a FILE that was returned by fopen, a “format string” that specifies what to print, and zero or more subsequent arguments. The format string can optionally contain “conversion specifications,” placeholders that begin with % that specify how to format the function’s subsequent arguments, if any. For instance, if file is a pointer to a FILE and c is a char, this function can print the latter to the former as follows using %c:

fprintf(file, "%c\n", c);

Alternatively, this function could format that same value as an int as well using %i, as in an ASCII chart:

fprintf(file, "%c %i\n", c, c);

And this function can print strings without any conversion specifications as well:

fprintf(file, "hello, world\n");

Among this function’s supported conversion specifications are:

To print actual percent sign, use %%.

To specify the “precision” of a float or a double, %f can optionally contain a . after the % followed by a number of decimal places. For instance, this function could format the value of one third to one decimal place using %.1f:

fprintf(file, "%.1f\n", 1.0 / 3.0);

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 function dprintf() is the same as fprintf() except that it outputs to a file descriptor, fd, instead of to a stdio(3) stream.

The functions snprintf() and vsnprintf() write at most size bytes (including the terminating null byte ('\0')) to str.

The functions vprintf(), vfprintf(), vdprintf(), vsprintf(), vsnprintf() are equivalent to the functions printf(), fprintf(), dprintf(), sprintf(), snprintf(), respectively, except that they are called with a va_list instead of a variable number of arguments. These functions do not call the va_end macro. Because they invoke the va_arg macro, the value of ap is undefined after the call. See stdarg(3).

All of 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 output.

C99 and POSIX.1-2001 specify that the results are undefined if a call to sprintf(), snprintf(), vsprintf(), or vsnprintf() 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 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 overall syntax of a conversion specification is:

%[$][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 '*' (see Field width and Precision 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 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 depends on the LC_NUMERIC part of the locale. (See setlocale(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". 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 nonzero 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 m, if errno contains a valid error code, the output of strerrorname_np(errno) is printed; otherwise, the value stored in errno is printed as a decimal number. 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 an integer 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 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 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 -) 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. The Single UNIX Specification 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 information indicates any. (See setlocale(3).) Note that many versions of gcc(1) cannot parse this option and will issue a warning. (SUSv2 did not include %'F, but SUSv3 added it.) Note also that the default locale of a C program is "C" whose locale information indicates no thousands' grouping character. Therefore, without a prior call to setlocale(3), no thousands' grouping characters will be printed.

glibc 2.2 adds one further flag character.

I

For 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 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 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 integer m) to specify that the precision is given in the next argument, or in the m-th argument, respectively, which must be of type int. 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 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.

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 or unsigned short argument, or a following n conversion corresponds to a pointer to a short argument.

l

(ell) A following integer conversion corresponds to a long or unsigned long argument, or a following n conversion corresponds to a pointer to a long 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. On a following a, A, e, E, f, F, g, or G conversion, this length modifier is ignored (C99; not in SUSv2).

ll

(ell-ell). A following integer conversion corresponds to a long long or unsigned long long argument, or a following n conversion corresponds to a pointer to a long long argument.

q

A synonym for ll. This is a nonstandard extension, derived from BSD; avoid its use in new code.

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.)

j

A following integer conversion corresponds to an intmax_t or uintmax_t argument, or a following n conversion corresponds to a pointer to an intmax_t argument.

z

A following integer conversion corresponds to a size_t or ssize_t argument, or a following n conversion corresponds to a pointer to a size_t argument.

Z

A nonstandard synonym for z that predates the appearance of z. Do not use in new code.

t

A following integer conversion corresponds to a ptrdiff_t argument, or a following n conversion corresponds to a pointer to a ptrdiff_t argument.

SUSv3 specifies all of the above, except for those modifiers explicitly noted as being nonstandard extensions. SUSv2 specified only 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).

As a nonstandard extension, the GNU implementations treats ll and L as synonyms, so that one can, for example, write llg (as a synonym for the standards-compliant Lg) and Ld (as a synonym for the standards compliant lld). 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, 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) notation. The letters abcdef are used for x conversions; the letters ABCDEF are used for X conversions. 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, E

The double 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. 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.

(SUSv2 does not know about F and says that character string representations for infinity and NaN may be made available. SUSv3 adds a specification for F. The C99 standard specifies "[-]inf" or "[-]infinity" for infinity, and a string starting with "nan" for NaN, in the case of f conversion, 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 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.

a, A

(C99; not in SUSv2, but added in SUSv3) 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 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 type double. The digit before the decimal point is unspecified for nonnormalized numbers, and nonzero but otherwise unspecified for normalized numbers. The exponent always contains at least one digit; if the value is zero, the exponent is 0.

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(3) function, 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 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 an l modifier is present: the const 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 the wcrtomb(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 of bytes written, not the number of wide characters or screen 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 for lc. Don't use.

S

(Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.) 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 integer pointed to by the corresponding argument. That argument shall be an int *, 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.) Print output of strerror(errno) (or strerrorname_np(errno) in the alternate form). No argument is required.

%

A '%' is written. No argument is converted. The complete conversion specification is '%%'.

This function returns the number of characters printed.

Upon successful return, these functions return the number of bytes printed (excluding the null byte used to end output to strings).

The functions snprintf() and vsnprintf() do not write more than size 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 of size or more means that the output was truncated. (See also below under CAVEATS.)

If an output error is encountered, a negative value is returned.

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 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);

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, ...)
{
    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)