12.9 Line-Oriented Input

Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream.

Standard C has functions to do this, but they aren’t very safe: null characters and even (for gets) long lines can confuse them. So the GNU C Library provides the nonstandard getline function that makes it easy to read lines reliably.

Another GNU extension, getdelim, generalizes getline. It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character.

All these functions are declared in stdio.h.

Function: ssize_t getline (char **lineptr, size_t *n, FILE *stream)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap | AC-Unsafe lock corrupt mem | See POSIX Safety Concepts.

This function reads an entire line from stream, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in *lineptr.

Before calling getline, you should place in *lineptr the address of a buffer *n bytes long, allocated with malloc. If this buffer is long enough to hold the line, getline stores the line in this buffer. Otherwise, getline makes the buffer bigger using realloc, storing the new buffer address back in *lineptr and the increased size back in *n. See Unconstrained Allocation.

If you set *lineptr to a null pointer, and *n to zero, before the call, then getline allocates the initial buffer for you by calling malloc. This buffer remains allocated even if getline encounters errors and is unable to read any bytes.

In either case, when getline returns, *lineptr is a char * which points to the text of the line.

When getline is successful, it returns the number of characters read (including the newline, but not including the terminating null). This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator.

This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable.

If an error occurs or end of file is reached without any bytes read, getline returns -1.

Function: ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)

Preliminary: | MT-Safe | AS-Unsafe corrupt heap | AC-Unsafe lock corrupt mem | See POSIX Safety Concepts.

This function is like getline except that the character which tells it to stop reading is not necessarily newline. The argument delimiter specifies the delimiter character; getdelim keeps reading until it sees that character (or end of file).

The text is stored in lineptr, including the delimiter character and a terminating null. Like getline, getdelim makes lineptr bigger if it isn’t big enough.

getline is in fact implemented in terms of getdelim, just like this:

ssize_t
getline (char **lineptr, size_t *n, FILE *stream)
{
  return getdelim (lineptr, n, '\n', stream);
}

Function: char * fgets (char *s, int count, FILE *stream)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

The fgets function reads characters from the stream stream up to and including a newline character and stores them in the string s, adding a null character to mark the end of the string. You must supply count characters worth of space in s, but the number of characters read is at most count - 1. The extra character space is used to hold the null character at the end of the string.

If the system is already at end of file when you call fgets, then the contents of the array s are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer s.

Warning: If the input data has a null character, you can’t tell. So don’t use fgets unless you know the data cannot contain a null. Don’t use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. We recommend using getline instead of fgets.

Function: wchar_t * fgetws (wchar_t *ws, int count, FILE *stream)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

The fgetws function reads wide characters from the stream stream up to and including a newline character and stores them in the string ws, adding a null wide character to mark the end of the string. You must supply count wide characters worth of space in ws, but the number of characters read is at most count - 1. The extra character space is used to hold the null wide character at the end of the string.

If the system is already at end of file when you call fgetws, then the contents of the array ws are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer ws.

Warning: If the input data has a null wide character (which are null bytes in the input stream), you can’t tell. So don’t use fgetws unless you know the data cannot contain a null. Don’t use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message.

Function: char * fgets_unlocked (char *s, int count, FILE *stream)

Preliminary: | MT-Safe race:stream | AS-Unsafe corrupt | AC-Unsafe corrupt | See POSIX Safety Concepts.

The fgets_unlocked function is equivalent to the fgets function except that it does not implicitly lock the stream.

This function is a GNU extension.

Function: wchar_t * fgetws_unlocked (wchar_t *ws, int count, FILE *stream)

Preliminary: | MT-Safe race:stream | AS-Unsafe corrupt | AC-Unsafe corrupt | See POSIX Safety Concepts.

The fgetws_unlocked function is equivalent to the fgetws function except that it does not implicitly lock the stream.

This function is a GNU extension.

Deprecated function: char * gets (char *s)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

The function gets reads characters from the stream stdin up to the next newline character, and stores them in the string s. The newline character is discarded (note that this differs from the behavior of fgets, which copies the newline character into the string). If gets encounters a read error or end-of-file, it returns a null pointer; otherwise it returns s.

Warning: The gets function is very dangerous because it provides no protection against overflowing the string s. The GNU C Library includes it for compatibility only. You should always use fgets or getline instead. To remind you of this, the linker (if using GNU ld) will issue a warning whenever you use gets.