Clear the error of a file stream
clearerr (File_Type fp
The clearerr
function clears the error and end-of-file flags
associated with the open file stream fp
.
ferror, feof, fopen
Close a file
Integer_Type fclose (File_Type fp)
The fclose
function may be used to close an open file pointer
fp
. Upon success it returns zero, and upon failure it sets
errno
and returns -1
. Failure usually indicates a that
the file system is full or that fp
does not refer to an open file.
Many C programmers call fclose
without checking the return
value. The S-Lang language requires the programmer to explicitly
handle any value returned by a function. The simplest way to
handle the return value from fclose
is to call it via:
() = fclose (fp);
fopen, fgets, fflush, pclose, errno
Convert a FD_Type file descriptor to a stdio File_Type object
File_Type fdopen (FD_Type, String_Type mode)
The fdopen
function creates and returns a stdio
File_Type
object from the open FD_Type
descriptor fd
. The mode
parameter corresponds to the
mode
parameter of the fopen
function and must be
consistent with the mode of the descriptor fd
. The function
returns NULL
upon failure and sets errno
.
The fclose
function does not close the File_Type
object
returned from this function. The underlying file object must be
closed by the close
function.
fileno, fopen, open, close, fclose
Get the end-of-file status
Integer_Type feof (File_Type fp)
This function may be used to determine the state of the end-of-file
indicator of the open file descriptor fp
. It returns zero
if the indicator is not set, or non-zero if it is. The end-of-file
indicator may be cleared by the clearerr
function.
ferror, clearerr, fopen
Determine the error status of an open file descriptor
Integer_Type ferror (File_Type fp)
This function may be used to determine the state of the error
indicator of the open file descriptor fp
. It returns zero
if the indicator is not set, or non-zero if it is. The error
indicator may be cleared by the clearerr
function.
feof, clearerr, fopen
Flush an output stream
Integer_Type fflush (File_Type fp)
The fflush
function may be used to update the stdio output
stream specified by fp
. It returns 0 upon success, or
-1 upon failure and sets errno
accordingly. In
particular, this function will fail if fp
does not represent
an open output stream, or if fp
is associated with a disk file and
there is insufficient disk space.
This example illustrates how to use the fflush
function
without regard to the return value:
() = fputs ("Enter value> ", stdout);
() = fflush (stdout);
fopen, fclose
Read a line from a file
Integer_Type fgets (SLang_Ref_Type ref, File_Type fp)
fgets
reads a line from the open file specified by fp
and places the characters in the variable whose reference is
specified by ref
.
It returns -1 if fp
is not associated with an open file
or an attempt was made to read at the end the file; otherwise, it
returns the number of characters read.
The following example returns the lines of a file via a linked list:
define read_file (file)
{
variable buf, fp, root, tail;
variable list_type = struct { text, next };
root = NULL;
fp = fopen(file, "r");
if (fp == NULL)
error("fopen %s failed." file);
while (-1 != fgets (&buf, fp))
{
if (root == NULL)
{
root = @list_type;
tail = root;
}
else
{
tail.next = @list_type;
tail = tail.next;
}
tail.text = buf;
tail.next = NULL;
}
() = fclose (fp);
return root;
}
fgetslines, fopen, fclose, fputs, fread, error
Read lines as an array from an open file
String_Type[] fgetslines (File_Type fp [,Int_Type num])
The fgetslines
function reads lines a specified number of
lines as an array of strings from the file associated with the
file pointer fp
. If the number of lines to be read is left
unspecified, the function will return the rest of the lines in the
file. If the file is empty, an empty string array will be returned.
The function returns NULL
upon error.
The following function returns the number of lines in a file:
define count_lines_in_file (file)
{
variable fp, lines;
fp = fopen (file, "r");
if (fp == NULL)
return -1;
lines = fgetslines (fp);
if (lines == NULL)
return -1;
return length (lines);
}
Note that the file was implicitly closed when the variable fp
goes out of scope (in the case, when the function returns).
fgets, fread, fopen, fputslines
Open a file
File_Type fopen (String_Type f, String_Type m)
The fopen
function opens a file f
according to the mode
string m
. Allowed values for m
are:
"r" Read only
"w" Write only
"a" Append
"r+" Reading and writing at the beginning of the file.
"w+" Reading and writing. The file is created if it does not
exist; otherwise, it is truncated.
"a+" Reading and writing at the end of the file. The file is created
if it does not already exist.
In addition, the mode string can also include the letter 'b'
as the last character to indicate that the file is to be opened in
binary mode.
Upon success, fopen
returns a File_Type
object which is
meant to be used by other operations that require an open file
pointer. Upon failure, the function returns NULL
.
The following function opens a file in append mode and writes a string to it:
define append_string_to_file (file, str)
{
variable fp = fopen (file, "a");
if (fp == NULL)
throw OpenError, "$file could not be opened"$;
() = fputs (string, fp);
() = fclose (fp);
}
Note that the return values from fputs
and fclose
were
ignored.
There is no need to explicitly close a file opened with fopen
.
If the returned File_Type
object goes out of scope, the
interpreter will automatically close the file. However, explicitly
closing a file with fclose
and checking its return value is
recommended.
fclose, fgets, fputs, popen
Create and write a formatted string to a file
Int_Type fprintf (File_Type fp, String_Type fmt, ...)
fprintf
formats the objects specified by the variable argument
list according to the format fmt
and write the result to the
open file pointer fp
.
The format string obeys the same syntax and semantics as the
sprintf
format string. See the description of the
sprintf
function for more information.
fprintf
returns the number of bytes written to the file,
or -1 upon error.
fputs, printf, fwrite, message
Write a string to an open stream
Integer_Type fputs (String_Type s, File_Type fp)
The fputs
function writes the string s
to the open file
pointer fp
. It returns -1 upon failure and sets errno
,
otherwise it returns the length of the string.
The following function opens a file in append mode and uses the
fputs
function to write to it.
define append_string_to_file (str, file)
{
variable fp;
fp = fopen (file, "a");
if (fp == NULL)
throw OpenError, "Unable to open $file"$;
if ((-1 == fputs (s, fp))
or (-1 == fclose (fp)))
throw WriteError, "Error writing to $file";
}
One must not disregard the return value from the fputs
function. Doing so may lead to a stack overflow error.
To write an object that contains embedded null characters, use the
fwrite
function.
fclose, fopen, fgets, fwrite
Write an array of strings to an open file
Int_Type fputslines (String_Type[]a, File_Type fp)
The fputslines
function writes an array of strings to the
specified file pointer. It returns the number of elements
successfully written. Any NULL
elements in the array will be
skipped.
if (length (lines) != fputslines (fp, lines))
throw WriteError;
fputs, fgetslines, fopen
Read binary data from a file
UInt_Type fread (Ref_Type b, DataType_Type t, UInt_Type n, File_Type fp)
The fread
function may be used to read n
objects of type
t
from an open file pointer fp
. Upon success, it
returns the number of objects read from the file and places the
objects in variable specified by b
. Upon error or
end-of-file, it returns -1 and sets errno
accordingly.
If more than one object is read from the file, those objects will be placed in an array of the appropriate size.
The following example illustrates how to read 50 integers from a file:
define read_50_ints_from_a_file (file)
{
variable fp, n, buf;
fp = fopen (file, "rb");
if (fp == NULL)
throw OpenError;
n = fread (&buf, Int_Type, 50, fp);
if (n == -1)
throw ReadError, "fread failed";
() = fclose (fp);
return buf;
}
Use the pack
and unpack
functions to read data with a
specific byte-ordering.
The fread_bytes
function may be used to read a specified number of
bytes in the form of a binary string (BString_Type
).
fread_bytes, fwrite, fgets, fopen, pack, unpack
Read bytes from a file as a binary-string
UInt_Type fread_bytes (Ref_Type b, UInt_Type n, File_Type fp)
The fread_bytes
function may be used to read n
bytes
from from an open file pointer fp
. Upon success, it returns
the number of bytes read from the file and assigns to the variable
attached to the reference b
a binary string formed from the
bytes read. Upon error or end of file, the function returns
-1 and sets errno
accordingly.
Use the pack
and unpack
functions to read data with a
specific byte-ordering.
fread, fwrite, fgets, fopen, pack, unpack
Reposition a stdio stream
Integer_Type fseek (File_Type fp, LLong_Type ofs, Integer_Type whence
The fseek
function may be used to reposition the file position
pointer associated with the open file stream fp
. Specifically,
it moves the pointer ofs
bytes relative to the position
indicated by whence
. If whence
is set to one of the symbolic
constants SEEK_SET
, SEEK_CUR
, or SEEK_END
, the
offset is relative to the start of the file, the current position
indicator, or end-of-file, respectively.
The function returns 0 upon success, or -1 upon failure and sets
errno
accordingly.
define rewind (fp) { if (0 == fseek (fp, 0, SEEK_SET)) return; vmessage ("rewind failed, reason: %s", errno_string (errno)); }
ftell, fopen
Obtain the current position in an open stream
LLong_Type ftell (File_Type fp)
The ftell function may be used to obtain the current position in the
stream associated with the open file pointer fp
. It returns
the position of the pointer measured in bytes from the beginning of
the file. Upon error, it returns -1
and sets errno
accordingly.
fseek, fopen
Write binary data to a file
UInt_Type fwrite (b, File_Type fp)
The fwrite
function may be used to write the object represented by
b
to an open file. If b
is a string or an array, the
function will attempt to write all elements of the object to the
file. It returns the number of elements successfully written,
otherwise it returns -1
upon error and sets errno
accordingly.
The following example illustrates how to write an integer array to a
file. In this example, fp
is an open file descriptor:
variable a = [1:50]; % 50 element integer array
if (50 != fwrite (a, fp))
throw WriteError;
Here is how to write the array one element at a time:
variable ai, a = [1:50];
foreach ai (a)
{
if (1 != fwrite(ai, fp))
throw WriteError;
}
Not all data types may be supported the fwrite
function. It
is supported by all vector, scalar, and string objects.
fread, fputs, fopen, pack, unpack
Close a process pipe
Integer_Type pclose (File_Type fp)
The pclose
function waits for the process associated with
fp
to exit and the returns the exit status of the command.
pclose, fclose
Open a pipe to a process
File_Type popen (String_Type cmd, String_Type mode)
The popen
function executes a process specified by cmd
and opens a unidirectional pipe to the newly created process. The
mode
indicates whether or not the pipe is open for reading
or writing. Specifically, if mode
is "r"
, then the
pipe is opened for reading, or if mode
is "w"
, then the
pipe will be open for writing.
Upon success, a File_Type
pointer will be returned, otherwise
the function failed and NULL
will be returned.
This function is not available on all systems.
pclose, fopen
Create and write a formatted string to stdout
Int_Type printf (String_Type fmt, ...)
printf
formats the objects specified by the variable argument
list according to the format fmt
and write the result to
stdout
. This function is equivalent to fprintf
used
with the stdout
file pointer. See fprintf
for more
information.
printf
returns the number of bytes written or -1 upon error.
Many C programmers do not check the return status of the
printf
C library function. Make sure that if you do not care
about whether or not the function succeeds, then code it as in the
following example:
() = printf ("%s laid %d eggs\n", chicken_name, num_egg);
fputs, printf, fwrite, message