(libc.info.gz) Formatted Output Functions
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12.12.7 Formatted Output Functions
----------------------------------
This section describes how to call 'printf' and related functions.
Prototypes for these functions are in the header file 'stdio.h'.
Because these functions take a variable number of arguments, you _must_
declare prototypes for them before using them. Of course, the easiest
way to make sure you have all the right prototypes is to just include
'stdio.h'.
-- Function: int printf (const char *TEMPLATE, ...)
Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe
mem lock corrupt | POSIX Safety Concepts.
The 'printf' function prints the optional arguments under the
control of the template string TEMPLATE to the stream 'stdout'. It
returns the number of characters printed, or a negative value if
there was an output error.
-- Function: int wprintf (const wchar_t *TEMPLATE, ...)
Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe
mem lock corrupt | POSIX Safety Concepts.
The 'wprintf' function prints the optional arguments under the
control of the wide template string TEMPLATE to the stream
'stdout'. It returns the number of wide characters printed, or a
negative value if there was an output error.
-- Function: int fprintf (FILE *STREAM, const char *TEMPLATE, ...)
Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe
mem lock corrupt | POSIX Safety Concepts.
This function is just like 'printf', except that the output is
written to the stream STREAM instead of 'stdout'.
-- Function: int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...)
Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe
mem lock corrupt | POSIX Safety Concepts.
This function is just like 'wprintf', except that the output is
written to the stream STREAM instead of 'stdout'.
-- Function: int sprintf (char *S, const char *TEMPLATE, ...)
Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
POSIX Safety Concepts.
This is like 'printf', except that the output is stored in the
character array S instead of written to a stream. A null character
is written to mark the end of the string.
The 'sprintf' function returns the number of characters stored in
the array S, not including the terminating null character.
The behavior of this function is undefined if copying takes place
between objects that overlap--for example, if S is also given as an
argument to be printed under control of the '%s' conversion.
Copying and Concatenation.
*Warning:* The 'sprintf' function can be *dangerous* because it can
potentially output more characters than can fit in the allocation
size of the string S. Remember that the field width given in a
conversion specification is only a _minimum_ value.
To avoid this problem, you can use 'snprintf' or 'asprintf',
described below.
-- Function: int swprintf (wchar_t *S, size_t SIZE, const wchar_t
*TEMPLATE, ...)
Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
POSIX Safety Concepts.
This is like 'wprintf', except that the output is stored in the
wide character array WS instead of written to a stream. A null
wide character is written to mark the end of the string. The SIZE
argument specifies the maximum number of characters to produce.
The trailing null character is counted towards this limit, so you
should allocate at least SIZE wide characters for the string WS.
The return value is the number of characters generated for the
given input, excluding the trailing null. If not all output fits
into the provided buffer a negative value is returned. You should
try again with a bigger output string. _Note:_ this is different
from how 'snprintf' handles this situation.
Note that the corresponding narrow stream function takes fewer
parameters. 'swprintf' in fact corresponds to the 'snprintf'
function. Since the 'sprintf' function can be dangerous and should
be avoided the ISO C committee refused to make the same mistake
again and decided to not define a function exactly corresponding to
'sprintf'.
-- Function: int snprintf (char *S, size_t SIZE, const char *TEMPLATE,
...)
Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
POSIX Safety Concepts.
The 'snprintf' function is similar to 'sprintf', except that the
SIZE argument specifies the maximum number of characters to
produce. The trailing null character is counted towards this
limit, so you should allocate at least SIZE characters for the
string S. If SIZE is zero, nothing, not even the null byte, shall
be written and S may be a null pointer.
The return value is the number of characters which would be
generated for the given input, excluding the trailing null. If
this value is greater or equal to SIZE, not all characters from the
result have been stored in S. You should try again with a bigger
output string. Here is an example of doing this:
/* Construct a message describing the value of a variable
whose name is NAME and whose value is VALUE. */
char *
make_message (char *name, char *value)
{
/* Guess we need no more than 100 chars of space. */
int size = 100;
char *buffer = (char *) xmalloc (size);
int nchars;
if (buffer == NULL)
return NULL;
/* Try to print in the allocated space. */
nchars = snprintf (buffer, size, "value of %s is %s",
name, value);
if (nchars >= size)
{
/* Reallocate buffer now that we know
how much space is needed. */
size = nchars + 1;
buffer = (char *) xrealloc (buffer, size);
if (buffer != NULL)
/* Try again. */
snprintf (buffer, size, "value of %s is %s",
name, value);
}
/* The last call worked, return the string. */
return buffer;
}
In practice, it is often easier just to use 'asprintf', below.
*Attention:* In versions of the GNU C Library prior to 2.1 the
return value is the number of characters stored, not including the
terminating null; unless there was not enough space in S to store
the result in which case '-1' is returned. This was changed in
order to comply with the ISO C99 standard.
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