FGETS(3) OpenBSD Programmer's Manual FGETS(3)NAME
fgets, gets - get a line from a stream
SYNOPSIS
#include <stdio.h>
char *
fgets(char *str, int size, FILE *stream);
char *
gets(char *str);
DESCRIPTION
The fgets() function reads at most size-1 characters 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. The string will be NUL-terminated if fgets() succeeds;
otherwise the contents of str are undefined.
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. Whether fgets() can possibly
fail with a size argument of 1 is implementation-dependent. On OpenBSD,
fgets() will never return NULL when size is 1.
ERRORS
[EBADF] The given stream is not a readable stream.
[EINVAL] The given size is less than or equal to 0.
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 ALSOfeof(3), ferror(3), fgetln(3)STANDARDS
The functions fgets() and gets() conform to ANSI X3.159-1989 (``ANSI
C'').
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')
character. If the first character of a line returned by
fgets() were NUL, 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.
It is erroneous to assume that fgets() never returns an empty string when
successful. If a line starts with the NUL character, fgets will store
the NUL and continue reading until it encounters a newline or end-of-
file. This will result in an empty string being returned. The following
bit of code illustrates a case where the programmer assumes the string
cannot be zero length.
char buf[1024];
if (fgets(buf, sizeof(buf), fp) != NULL) {
/* WRONG */
if (buf[strlen(buf) - 1] == '\n')
buf[strlen(buf) - 1] = '\0';
}
If strlen() returns 0, the index into the buffer becomes -1. One way to
concisely and correctly trim a newline is shown below.
char buf[1024];
if (fgets(buf, sizeof(buf), fp) != NULL)
buf[strcspn(buf, "\n")] = '\0';
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
C'').
OpenBSD 4.9 June 2, 2009 OpenBSD 4.9
