std::fgets

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.

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).

[edit] 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.

[edit] 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