MirBSD manpage: fgets(3), gets(3), gets_s(3)

FGETS(3)                   BSD Programmer's Manual                    FGETS(3)

NAME

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

SYNOPSIS

     #include <stdio.h>

     char *
     fgets(char *str, int size, FILE *stream);

     char *
     gets(char *str);

     char *
     gets_s(char *str, rsize_t size);

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
     on error. The newline, if any, is retained. In any case, 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.

     The gets_s() function is equivalent to fgets() with a stream of stdin,
     however it does not include newline in a result.

RETURN VALUES

     Upon successful completion, fgets(), gets() and gets_s() return a pointer
     to the string. If end-of-file or an error occurs before any characters
     are read, they return NULL. The fgets(), gets() and gets_s() functions do
     not distinguish between end-of-file and error, and callers must use
     feof(3) and ferror(3) to determine which occurred. Whether fgets() can
     possibly fail with a size argument of 1 is implementation-dependent. On
     MirBSD, fgets() will never return NULL when size is 1.

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"). gets_s() conforms to ISO/IEC 9899:2011 ("ISO C11").

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 > 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 NUL ('\0') charac-
                ter. 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 new-
                line.

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

BUGS

     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").

MirBSD #10-current               June 4, 1993                                1