fstat vs stat – fstat does not update?

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Summary

The issue at hand is the inconsistent behavior of fstat(2) when compared to stat(2) in updating the st_mtime field after a file modification. This discrepancy arises when using fstat with a file descriptor that was opened prior to the file modification, whereas stat always seems to fetch the latest information.

Root Cause

The root cause of this issue lies in the way file descriptors and file system metadata are handled by the operating system. Specifically:

  • When a file is opened, the operating system caches certain metadata, including the last modification time (st_mtime), associated with the file descriptor.
  • fstat retrieves this cached metadata, which may not reflect recent changes to the file if the file descriptor was opened before these changes occurred.
  • In contrast, stat always fetches the latest metadata from the file system, regardless of when the file was last accessed or modified.

Why This Happens in Real Systems

This behavior occurs in real systems due to several factors:

  • Performance optimization: Caching file metadata improves system performance by reducing the number of disk accesses required to retrieve this information.
  • File system design: The design of file systems, such as the use of inode caching, can lead to this behavior.
  • POSIX standards: The POSIX standard allows for this behavior, as it does not mandate that fstat always return the most up-to-date information.

Real-World Impact

The real-world impact of this issue includes:

  • Inconsistent behavior: Applications may exhibit inconsistent behavior when relying on fstat to determine file modification times.
  • Data inconsistencies: This can lead to data inconsistencies, particularly in applications that rely on accurate file metadata to function correctly.
  • Debugging challenges: Debugging issues related to this behavior can be challenging due to its intermittent nature.

Example or Code

#include 
#include 
#include 
#include 
#include 

int main(int argc, char** argv) {
    FILE* file = NULL;
    int fd = 0;
    const char* filename = NULL;
    struct stat fstat_FILE_sb, fstat_fd_sb, stat_sb;

    if(argc < 2) {
        return 1;
    }

    file = fopen(filename = argv[1], "rb");
    fd = open(filename, O_RDONLY);

    if(!file || !fd) {
        return 1;
    }

    for(;;) {
        if(fstat(fileno(file), &fstat_FILE_sb) != 0) {
            break;
        }
        if(fstat(fd, &fstat_fd_sb) != 0) {
            break;
        }
        if(stat(filename, &stat_sb) != 0) {
            break;
        }
        printf("Comparing [%ld vs %ld vs %ld]\n", fstat_FILE_sb.st_mtime, fstat_fd_sb.st_mtime, stat_sb.st_mtime);
        sleep(1);
    }

    if(file) {
        fclose(file);
    }
    if(fd) {
        close(fd);
    }

    return 0;
}

How Senior Engineers Fix It

Senior engineers can address this issue by:

  • Reopening the file descriptor: Periodically reopening the file descriptor to ensure that the latest metadata is retrieved.
  • Using stat instead of fstat: When possible, using stat instead of fstat to fetch the latest metadata.
  • Implementing custom caching mechanisms: Implementing custom caching mechanisms that account for file modifications and update the cached metadata accordingly.

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of understanding of file system internals: Limited knowledge of how file systems and operating systems handle file metadata.
  • Insufficient experience with POSIX standards: Limited experience with the POSIX standard and its implications on file system behavior.
  • Overreliance on high-level abstractions: Overreliance on high-level programming abstractions that hide the underlying complexities of file system interactions.