Difference between revisions of "cpp/io/c/fread"

Latest revision as of 13:30, 2 October 2023

Reads up to count objects into the array buffer from the given input stream stream as if by calling std::fgetc size times for each object, and storing the results, in the order obtained, into the successive positions of buffer, which is reinterpreted as an array of unsigned char. The file position indicator for the stream is advanced by the number of characters read.

If the objects are not TriviallyCopyable, the behavior is undefined.

If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. If a partial element is read, its value is indeterminate.

[edit] Parameters

[edit] Return value

Number of objects read successfully, which may be less than count if an error or end-of-file condition occurs.

If size or count is zero, fread returns zero and performs no other action.

fread does not distinguish between end-of-file and error, and callers must use std::feof and std::ferror to determine which occurred.

[edit] Example

#include <cstddef>
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <vector>
 
int main()
{
    // Prepare file
    std::ofstream("test.txt") << 1 << ' ' << 2 << '\n';
    std::FILE* f = std::fopen("test.txt", "r");
 
    std::vector<char> buf(4); // char is trivially copyable
    const std::size_t n = std::fread(&buf[0], sizeof buf[0], buf.size(), f);
 
    std::cout << "Read " << n << " object" << (n > 1 ? "s" : "") << ": "
              << std::hex << std::uppercase << std::setfill('0');
    for (char n : buf)
        std::cout << "0x" << std::setw(2) << static_cast<short>(n) << ' ';
    std::cout << '\n';
 
    std::vector<std::string> buf2; // string is not trivially copyable
//  This would result in undefined behavior:
//  std::fread(&buf2[0], sizeof buf2[0], buf2.size(), f);
}

Read 4 objects: 0x31 0x20 0x32 0x0A

[edit] See also