std::fsetpos(3)

✨ std::fsetpos(3) ✨

okay so bestie, this page is from the C++ Standard Libary man page, and we're about to make file positioning make sense. no cap, it's giving low-level C++ but make it cute. 🎀

🌸 NAME 🌸

std::fsetpos — literally std::fsetpos, the function that sets the file position indicator and multibyte parsing state for a C file stream. she's iconic for putting the stream exactly where pos says. 💅

🦋 Synopsis 🦋

Defined in header <cstdio>.

int fsetpos( std::FILE* stream, const std::fpos_t* pos );

So here's the vibe: it sets the file position indicator and the multibyte parsing state (if any) for the C file stream stream according to the value pointed to by pos.

Besides establishing new parse state and position, a call to this function undoes the effects of std::ungetc and clears the end-of-file state, if it is set.

If a read or write error occurs, the error indicator (std::ferror) for the stream is set. noted, bestie. 🫶

💖 Parameters 💖

• stream — file stream to modify.
• pos — pointer to a fpos_t object obtained from std::fgetpos called on a stream associated with the same file.

✨ Return value ✨

0 upon success, nonzero value otherwise. Also, sets errno on failure. periodt. 💅

💀 Notes 💀

After seeking to a non-end position in a wide stream, the next call to any output function may render the remainder of the file undefined, e.g. by outputting a multibyte sequence of a different length. this is the part where you go "noted 📝" and stay safe.

🌈 Example 🌈

// Run this code

#include <cstdio>
#include <cstdlib>

int main()
{
    // Prepare an array of floating-point values.
    const int SIZE = 5;
    double A[SIZE] = {1., 2., 3., 4., 5.};
    // Write array to a file.
    std::FILE * fp = std::fopen("test.bin", "wb");
    std::fwrite(A, sizeof(double), SIZE, fp);
    std::fclose(fp);

    // Read the values into array B.
    double B[SIZE];
    fp = std::fopen("test.bin", "rb");
    std::fpos_t pos;
    if (std::fgetpos(fp, &pos) != 0)      // current position: start of file
    {
       std::perror("fgetpos()");
       std::fprintf(stderr, "fgetpos() failed in file %s at line # %d\n",
                    __FILE__, __LINE__-3);
       std::exit(EXIT_FAILURE);
    }

    int ret_code = std::fread(B, sizeof(double), 1, fp);      // read one value
    // current position: after reading one value
    std::printf("%.1f; read count = %d\n", B[0], ret_code);   // print one value and ret_code

    if (std::fsetpos(fp, &pos) != 0)   // reset current position to start of file
    {
       if (std::ferror(fp))
       {
          std::perror("fsetpos()");
          std::fprintf(stderr, "fsetpos() failed in file %s at line # %d\n",
                       __FILE__, __LINE__-5);
          std::exit(EXIT_FAILURE);
       }
    }

    ret_code = std::fread(B, sizeof(double), 1, fp);         // re-read first value
    std::printf("%.1f; read count = %d\n", B[0], ret_code);  // print one value and ret_code
    std::fclose(fp);

    return EXIT_SUCCESS;
}

💫 Output 💫

1.0; read count = 1
1.0; read count = 1

👑 See also 👑

• fgetpos — gets the file position indicator (function).
• ftell — returns the current file position indicator (function).
• fseek — moves the file position indicator to a specific location in a file (function).
• C documentation for fsetpos — the C doc, iykyk.

that's the whole page, bestie 💖 source vibe: http://cppreference.com, version/date: 2024.06.10, and the page is std::fsetpos(3). now go slay those file positions ✨