std:: fread

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.

Parameters

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.

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

See also