fgets(3) - NetBSD Manual Pages

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FGETS(3)                NetBSD 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 character is found, at end-of-file or
     error.  The newline, if any, is retained, and a `\0' 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.


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 distin-
     guish 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 routine 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') charac-
                ter.  If the first character of a line returned by fgets()
                were null, strchr() would immediately return without consider-
                ing 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 arbitrary length, and because overflowing the input buffer is
     almost invariably a security violation, programs should NEVER use gets().
     The gets() function exists purely to conform to ANSI X3.159-1989
     (``ANSI C89'').

NetBSD 10.1                      May 13, 2010                      NetBSD 10.1