#include <stdio.h>
printf
fwrite fread
ferror perror
gets rename
scanf sprintf
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 **ret, const char *format, ...)
Description
The printf family of functions produces output according to a
format as described below. The printf and vprintf
functions 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
dynamically allocate a new string with malloc.
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) are converted for output.
fwrite, fread
size_t fread (void * restrict ptr,
size_t size, size_t nmemb, FILE * restrict stream)
size_t fwrite (const void *
restrict ptr, size_t size, size_t nmemb, FILE * restrict stream)
The function fread reads nmemb objects, each size bytes long,
from the stream pointed to by stream, storing them at the location given by ptr.
The function fwrite writes nmemb objects, each size bytes long,
to the stream pointed to by stream, obtaining them from the location given by
ptr.
The following code attempts at reading 10
bytes at once from the text file "fred.txt", then it stores them into
buffer and finally it displays this string.
#include <stdio.h>
int main()
{
FILE *f;
char buffer[11];
if (f = fopen("fred.txt",
"rt"))
{
fread(buffer, 1, 10, f);
buffer[10] = 0;
fclose(f);
printf("first 10 characters of the
file:\n%s\n", buffer);
}
return 0;
}
ferror
Description
The function clearerr clears the end-of-file and error indicators for the stream pointed to bystream
.The function feof tests the end-of-file indicator for the stream pointed to by
stream
, returning non-zero if it is set. The
end-of-file indicator can only be cleared by the function clearerr
.The function ferror tests the error indicator for the stream pointed to by
stream
, returning non-zero if it is set. The error
indicator can only be reset by the clearerr function.The function fileno examines the argument
stream
and returns its integer descriptor.The clearerr_unlocked, feof_unlocked, ferror_unlocked and fileno_unlocked functions are equivalent to clearerr, feof, ferror and fileno respectively, except that the caller is responsible for locking the stream with flockfile before calling them. These functions may be used to avoid the overhead of locking the stream and to prevent races when multiple threads are operating on the same stream.
The code below attempts to open the text file "fred.txt" in the current directory and read each character in it until the EOF symbol is encountered. The feof function has been used to test for EOF.
-
#include <stdio.h> int main() { FILE *in; if (in = fopen("fred.txt", "rt")) { for (char c; !feof(in); fscanf(in, "%c", &c)); fclose(in); } return 0; }
perror
void | perror (const char *string) |
Description
The strerror, strerror_r and perror functions look up the error message string corresponding to an error number.The strerror function accepts an error number argument
errnum
and returns a pointer to the corresponding message string.The strerror_r function renders the same result into
strerrbuf
for a maximum of buflen
characters and returns 0 upon success.The perror function finds the error message corresponding to the current value of the global variable errno and writes it, followed by a newline, to the standard error file descriptor. If the argument
string
is non- NULL and does
not point to the null character, this string is prepended to the message
string and separated from it by a colon and space; otherwise, only the error
message string is printed.If the error number is not recognized, these functions return an error message string containing "Unknown error:" followed by the error number in decimal. The strerror and strerror_r functions return EINVAL as a warning. Error numbers recognized by this implementation fall in the range 0 <
errnum
< sys_nerr
.If insufficient storage is provided in
strerrbuf
(as specified in
buflen
) to contain the error string, strerror_r
returns ERANGE and strerrbuf
will contain
an error message that has been truncated and NUL
terminated to fit the length specified by buflen
.The message strings can be accessed directly using the external array sys_errlist. The external value sys_nerr contains a count of the messages in sys_errlist. The use of these variables is deprecated; strerror or strerror_r should be used instead.
-
#include <stdio.h> int main() { char error_message[100] = {0}; perror(error_message); return 0; }
gets
char | fgets (char * restrict str, int size, FILE * restrict stream) | |
char | gets (char *str) |
Description
The fgets function reads at most one less than the number of characters specified bysize
from the given stream
and stores them in the string str
. Reading stops when a newline
character is found, at end-of-file or error. The newline, if any, is retained.
If any characters are read and there is no error, a '%%CUSTOM2%%'
character is appended to end the string.
The gets function is equivalent to fgets with an infinite
size
and a stream
of stdin,
except that the newline character (if any) is not stored in the string. It is
the caller's responsibility to ensure that the input line, if any, is
sufficiently short to fit in the string.
-
#include <stdio.h> int main() { // open the text file "fred.txt" for writing FILE *out = fopen("fred.txt", "wt"); // write some text to the file fprintf(out, "Hello Fred!\n"); // close the stream, so all changes to the file are saved fclose(out); // open the file "fred.txt" for reading FILE *in = fopen("fred.txt", "rt"); // read the first line from the file char buffer[100]; fgets(buffer, 20, in); // display what we've just read printf("first line of \"fred.txt\": %s\n", buffer); // close the stream fclose(in); return 0; }
first line of "fred.txt": Hello Fred!
rename
scanf
|
Description
The scanf family of functions scans input according to aformat
as described below. This format may contain conversion
specifiers; the results from such conversions, if
any, are stored through the pointer arguments. The
scanf function reads input from the standard input stream
stdin, fscanf reads input from
the stream pointer stream
, and sscanf reads
its input from the character string pointed to by str
. The vfscanf
function is analogous to vfprintf
and reads input from the stream pointer stream
using a
variable argument list of pointers (see stdarg). The vscanf
function scans a variable argument list from the standard input and the vsscanf
function scans it from a string; these are analogous to the vprintf and
vsprintf functions respectively. Each successive pointer
argument must correspond properly with each successive conversion
specifier (but see the * and %n
sequence. #include format
string may also
contain other characters. White space (such as blanks, tabs, or newlines)
in the format
string match any amount of white space,
including none, in the input. Everything else matches only itself.
Scanning stops when an input character does not match such a format
character. Scanning also stops when an input conversion cannot be made
(see below).Extended locale versions of these functions are documented in scanf_l. See xlocale for more information.
The code below attempts to open the text file "fred.txt" in the current directory and read each character in it until the EOF symbol is encountered. Reading from the file is achieved using the fscanf function.
-
#include <stdio.h> int main() { FILE *in; if (in = fopen("fred.txt", "rt")) { for (char c; !feof(in); fscanf(in, "%c", &c)); fclose(in); } return 0; }
sprintf
|
Description
The printf family of functions produces output according to aformat
as described below. The printf
and vprintf functions 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 dynamically allocate a new string with malloc.
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) are converted for output.
Also, they return the number of characters printed (not including the trailing '%%CUSTOM2%%' used to end output to strings) or a negative value if an output error occurs, except for snprintf and vsnprintf, which return the number of characters that would have been printed if the
size
were unlimited
(again, not including the final '%%CUSTOM2%%').
The asprintf and vasprintf functions set
*ret
to be a pointer to a buffer sufficiently large to
hold the formatted string. This pointer should be passed to free
to release the allocated storage when it is no longer needed. If
sufficient space cannot be allocated, asprintf and vasprintf
will return -1 and set ret
to be a NULL
pointer.
The snprintf and vsnprintf functions will write at most
size-1
of the characters printed into
the output string (the size'th
character then gets the
terminating '%%CUSTOM2%%'); 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. The output
is always null-terminated.
The sprintf and vsprintf functions effectively assume an infinite
size
.
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. The arguments must correspond properly (after type promotion) with the conversion specifier. After the %, the following appear in sequence:
- An optional field, consisting of a decimal digit string followed by a </span>, specifying the next argument to access. If this field is not provided, the argument following the last argument accessed will be used. Arguments are numbered starting at 1. If unaccessed arguments in the format string are interspersed with ones that are accessed the results will be indeterminate.
- Zero or more of the following flags:
'#' | The value should be converted to an "alternate 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 output 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 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. |
' 0 ' (zero) | Zero padding. For all conversions 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; the converted value is to be left adjusted on the field boundary. Except for n conversions, 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. |
' '(space) | A blank should be left before a positive number produced by a signed conversion ( a, A, d, e, E, f, F, g, G, or i). |
'+' | A sign must always be placed before a number produced by a signed conversion. A + overrides a space if both are used. |
''' | Decimal conversions ( d, u, or i) or the integral portion of a floating point conversion ( f or F) should be grouped and separated by thousands using the non-monetary separator returned by localeconv. |
- An optional separator character( , |
; | :
| _) used for
separating multiple values when printing an AltiVec vector, or other
multi-value unit.
- 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-adjustment flag has been given) to fill out the field width.
- 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 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 conversions.
- An optional length modifier, that specifies the size of the argument. The following length modifiers are valid for the d, i, n, o, u, x, or X conversion:
Modifier | d, i | o, u, x, X | n |
hh | signed char | unsigned char | signed char * |
h | short | unsigned short | short * |
l (ell) | long | unsigned long | long * |
ll (ell ell) | long long | unsigned long long | long long * |
j | intmax_t | uintmax_t | intmax_t * |
t | ptrdiff_t | (see note) | ptrdiff_t * |
z | (see note) | size_t | (see note) |
q (deprecated) | quad_t | u_quad_t | quad_t * |
Note: the t modifier, when applied to a o, u, x, or X conversion, indicates that the argument is of an unsigned type equivalent in size to a ptrdiff_t. The z modifier, when applied to a d or i conversion, indicates that the argument is of a signed type equivalent in size to a size_t. Similarly, when applied to an n conversion, it indicates that the argument is a pointer to a signed type equivalent in size to a size_t.
The following length modifier is valid for the a, A, e, E, f, F, g, or G conversion:
Modifier | a, A, e, E, f, F, g, G |
l (ell) | double (ignored, same behavior as without it) |
L | long double |
The following length modifier is valid for the c or s conversion:
Modifier | c | s |
l (ell) | wint_t | wchar_t * |
The AltiVec Technology Programming Interface Manual also defines five additional length modifiers which can be used (in place of the conventional length modifiers) for the printing of AltiVec vectors:
v | Treat the argument as a vector value, unit length will be determined by the conversion specifier (default = 16 8-bit units for all integer conversions, 4 32-bit units for floating point conversions). |
vh, hv | Treat the argument as a vector of 8 16-bit units. |
vl, lv | Treat the argument as a vector of 4 32-bit units. |
NOTE: The vector length specifiers are AltiVec only extensions onto the printf specification. Behaviour of these values for printf is only defined for operating systems conforming to the AltiVec Technology Programming Interface Manual. (At time of writing this includes only Mac OS X 10.2 and later.)
- A character that specifies the type of conversion to be applied.
A field width or precision, or both, may be indicated by an asterisk '*'
or an asterisk followed by one or more decimal digits and a '</span>'
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. If a single
format directive mixes positional (nn</span>)
and non-positional arguments, the results are undefined.
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 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. |
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. |
eE | 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 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. For a, A, e, E, f, F, g, and G conversions, positive and negative infinity are represented as inf and -inf respectively when using the lowercase conversion character, and INF and -INF respectively when using the uppercase conversion character. Similarly, NaN is represented as nan when using the lowercase conversion, and NAN when using the uppercase conversion. |
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. |
gG | 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. |
aA | The double argument is
rounded and converted to hexadecimal notation in the style[-]0xh.hhhp[
±]d, where the number of digits after the
hexadecimal-point character is equal to the precision
specification. If the precision is missing, it is taken as
enough to represent the floating-point number exactly, and no
rounding occurs. If the precision is zero, no hexadecimal-point
character appears. The p is a literal
character 'p', and the exponent
consists of a positive or negative sign followed by a decimal
number representing an exponent of 2. The A
conversion uses the prefix "0X"
(rather than "0x),"
the letters "ABCDEF" (rather
than "abcdef)"
to represent the hex digits, and the letter 'P'
(rather than 'p') to separate the
mantissa and exponent. Note that there may be multiple valid ways to represent floating-point numbers in this hexadecimal format. For example, 0x3.24p+0, 0x6.48p-1 and 0xc.9p-2 are all equivalent. The format chosen depends on the internal representation of the number, but the implementation guarantees that the length of the mantissa will be minimized. Zeroes are always represented with a mantissa of 0 (preceded by a '-' if appropriate) and an exponent of +0. |
C | Treated as c with the l (ell) modifier. |
c | The int argument is
converted to an unsigned char, and the
resulting character is written. If the l (ell) modifier is used, the wint_t argument shall be converted to a wchar_t, and the (potentially multi-byte) sequence representing the single wide character is written, including any shift sequences. If a shift sequence is used, the shift state is also restored to the original state after the character. |
S | Treated as s with the l (ell) modifier. |
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
character need be present; if the precision is not specified, or
is greater than the size of the array, the array must contain a
terminating NUL character. If the l (ell) modifier is used, the wchar_t * argument is expected to be a pointer to an array of wide characters (pointer to a wide string). For each wide character in the string, the (potentially multi-byte) sequence representing the wide character is written, including any shift sequences. If any shift sequence is used, the shift state is also restored to the original state after the string. Wide characters from the array are written up to (but not including) a terminating wide NUL character; if a precision is specified, no more than the number of bytes specified are written (including shift sequences). Partial characters are never written. If a precision is given, no null character need be present; if the precision is not specified, or is greater than the number of bytes required to render the multibyte representation of the string, the array must contain a terminating wide 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 integer indicated by the int * (or variant) pointer argument. No argument is converted. |
% | A '%' is written. No argument is converted. The complete conversion specification is '%%'. |
The decimal point character is defined in the program's locale (category LC_NUMERIC).
In no case does a non-existent or small field width cause truncation of a numeric field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.
Examples
To print a date and time in the form "Sunday, July 3, 10:02," whereweekday
and month
are pointers to strings:
#include <stdio.h> fprintf(stdout, "%s, %s %d, %.2d:%.2d\n", weekday, month, day, hour, min);
To print 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); }