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NetBSD 6.1.5 - man page for fgets (netbsd section 3)

FGETS(3)			   BSD Library Functions Manual 			 FGETS(3)

NAME
     fgets, gets -- get a line from a stream

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     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 by size
     from the given stream and stores them in the string str.  Reading stops when a newline char-
     acter is found, at end-of-file or error.  The newline, if any, is retained, and a '\0' char-
     acter 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.

RETURN VALUES
     Upon successful completion, fgets() and gets() return a pointer to the string.  If end-of-
     file or an error occurs before any characters are read, they return NULL.	The fgets() and
     gets() functions do not distinguish between end-of-file and error, and callers must use
     feof(3) and ferror(3) to determine which occurred.

ERRORS
     [EBADF]		The given stream is not a readable stream.

     The function fgets() may also fail and set errno for any of the errors specified for the
     routines fflush(3), fstat(2), read(2), or malloc(3).

     The function gets() may also fail and set errno for any of the errors specified for the rou-
     tine getchar(3).

SEE ALSO
     feof(3), ferror(3), fgetln(3)

STANDARDS
     The functions fgets() and gets() conform to ANSI X3.159-1989 (``ANSI C89'') and IEEE Std
     1003.1-2001 (``POSIX.1'').  The IEEE Std 1003.1-2008 (``POSIX.1'') revision marked gets() as
     obsolescent.

CAVEATS
     The following bit of code illustrates a case where the programmer assumes a string is too
     long if it does not contain a newline:

	     char buf[1024], *p;

	     while (fgets(buf, sizeof(buf), fp) != NULL) {
		     if ((p = strchr(buf, '\n')) == NULL) {
			     fprintf(stderr, "input line too long.\n");
			     exit(1);
		     }
		     *p = '\0';
		     printf("%s\n", buf);
	     }

     While the error would be true if a line longer than 1023 characters were read, it would be
     false in two other cases:

	   1.	If the last line in a file does not contain a newline, the string returned by
		fgets() will not contain a newline either.  Thus strchr() will return NULL and
		the program will terminate, even if the line was valid.

	   2.	All C string functions, including strchr(), correctly assume the end of the
		string is represented by a null ('\0') character.  If the first character of a
		line returned by fgets() were null, strchr() would immediately return without
		considering the rest of the returned text which may indeed include a newline.

     Consider using fgetln(3) instead when dealing with untrusted input.

SECURITY CONSIDERATIONS
     Since it is usually impossible to ensure that the next input line is less than some arbi-
     trary length, and because overflowing the input buffer is almost invariably a security vio-
     lation, programs should NEVER use gets().	The gets() function exists purely to conform to
     ANSI X3.159-1989 (``ANSI C89'').

BSD					   May 13, 2010 				      BSD


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