FGETS(3)

FGETS(3) | 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 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 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’) 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 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”).

May 13, 2010 | NetBSD 9.0