NAME¶

std::fgets - std::fgets

Synopsis¶

Defined in header <cstdio>
char* fgets( char* str, int count, std::FILE* stream );

Reads at most count - 1 characters from the given file stream and stores them in the
character array pointed to by str. Parsing stops if a newline character is found, in
which case str will contain that newline character, or if end-of-file occurs. If
bytes are read and no errors occur, writes a null character at the position
immediately after the last character written to str.

Parameters¶

str - pointer to an element of a char array
count - maximum number of characters to write (typically the length of str)
stream - file stream to read the data from

Return value¶

str on success, null pointer on failure.

If the end-of-file condition is encountered, sets the eof indicator on stream (see
std::feof()). This is only a failure if it causes no bytes to be read, in which case
a null pointer is returned and the contents of the array pointed to by str are not
altered (i.e. the first byte is not overwritten with a null character).

If the failure has been caused by some other error, sets the error indicator (see
std::ferror()) on stream. The contents of the array pointed to by str are
indeterminate (it may not even be null-terminated).

Notes¶

POSIX additionally requires that fgets sets errno if it encounters a failure other
than the end-of-file condition.

Although the standard specification is unclear in the cases where count <= 1, common
implementations do

* if count < 1, do nothing, report error,
* if count == 1,

* some implementations do nothing, report error,
* others read nothing, store zero in str[0], report success.

Example¶

// Run this code

#include <cstdio>
#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <span>

void dump(std::span<const char> buf, std::size_t offset)
{
std::cout << std::dec;
for (char ch : buf)
std::cout << (ch >= ' ' ? ch : '.'), offset--;
std::cout << std::string(offset, ' ') << std::hex
<< std::setfill('0') << std::uppercase;
for (unsigned ch : buf)
std::cout << std::setw(2) << ch << ' ';
std::cout << std::dec << '\n';
}

int main()
{
std::FILE* tmpf = std::tmpfile();
std::fputs("Alan Turing\n", tmpf);
std::fputs("John von Neumann\n", tmpf);
std::fputs("Alonzo Church\n", tmpf);

std::rewind(tmpf);
for (char buf[8]; std::fgets(buf, sizeof buf, tmpf) != nullptr;)
dump(buf, 10);
}

Output:¶

Alan Tu. 41 6C 61 6E 20 54 75 00
ring..u. 72 69 6E 67 0A 00 75 00
John vo. 4A 6F 68 6E 20 76 6F 00
n Neuma. 6E 20 4E 65 75 6D 61 00
nn..uma. 6E 6E 0A 00 75 6D 61 00
Alonzo . 41 6C 6F 6E 7A 6F 20 00
Church.. 43 68 75 72 63 68 0A 00

See also¶

scanf reads formatted input from stdin, a file stream or a buffer
fscanf (function)
sscanf
gets reads a character string from stdin
(deprecated in C++11) (function)
(removed in C++14)
fputs writes a character string to a file stream
(function)
C documentation for
fgets