--- name: "explain_fopen" section: "3" platform: "linux" package: "libexplain-dev" version: "1.4.D001-16" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/explain_fopen" --- # explain\_fopen(3) explain fopen(3) errors ## NAME explain\_fopen - explain [fopen(3)](/fopen/3.md) errors ## SYNOPSIS \#include \\ const char \*explain\_fopen(const char \*path, const char \*mode);\ const char \*explain\_errno\_fopen(int errnum, const char \*path, const char \*mode);\ void explain\_message\_fopen(char \*message, int message\_size, const char \*path, const char \*mode);\ void explain\_message\_errno\_fopen(char \*message, int message\_size, int errnum, const char \*path, const char \*mode); ## DESCRIPTION These functions may be used to obtain explanations for [*fopen*(3)](/fopen/3.md) errors. ### explain\_fopen const char \*explain\_fopen(const char \*path, const char \*mode); The explain\_fopen function is used to obtain an explanation of an error returned by the [*fopen*(3)](/fopen/3.md) system call. The least the message will contain is the value of `strerror(errno)`, but usually it will do much better, and indicate the underlying cause in more detail. The *errno* global variable will be used to obtain the error value to be decoded. This function is intended to be used in a fashion similar to the following example: ``` FILE *fp = fopen(path, mode); if (!fp) { const char *message = explain_fopen(path, mode); fprintf(stderr, '%s0, message); exit(EXIT_FAILURE); } ``` - *path* The original path, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. - *mode* The original mode, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. - Returns: The message explaining the error. This message buffer is shared by all libexplain functions which do not supply a buffer in their argument list. This will be overwritten by the next call to any libexplain function which shares this buffer, including other threads. **Note:** This function is **not** thread safe, because it shares a return buffer across all threads, and many other functions in this library. ### explain\_errno\_fopen const char \*explain\_errno\_fopen(int errnum, const char \*path, const char \*mode); The explain\_errno\_fopen function is used to obtain an explanation of an error returned by the [*fopen*(3)](/fopen/3.md) system call. The least the message will contain is the value of `strerror(errno)`, but usually it will do much better, and indicate the underlying cause in more detail. This function is intended to be used in a fashion similar to the following example: ``` FILE *fp = fopen(path, mode); if (!fp) { const char *message = explain_errno_fopen(err, path, mode); fprintf(stderr, '%s0, message); exit(EXIT_FAILURE); } ``` - *errnum* The error value to be decoded, usually obtained from the *errno* global variable just before this function is called. This is necessary if you need to call **any** code between the system call to be explained and this function, because many libc functions will alter the value of *errno*. - *path* The original path, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. - *mode* The original mode, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. - Returns: The message explaining the error. This message buffer is shared by all libexplain functions which do not supply a buffer in their argument list. This will be overwritten by the next call to any libexplain function which shares this buffer, including other threads. **Note:** This function is **not** thread safe, because it shares a return buffer across all threads, and many other functions in this library. ### explain\_message\_fopen void explain\_message\_fopen(char \*message, int message\_size, const char \*path, const char \*mode); The explain\_message\_fopen function is used to obtain an explanation of an error returned by the [*fopen*(3)](/fopen/3.md) system call. The least the message will contain is the value of strerror(errno), but usually it will do much better, and indicate the underlying cause in more detail. The *errno* global variable will be used to obtain the error value to be decoded. This function is intended to be used in a fashion similar to the following example: ``` FILE *fp = fopen(path, mode); if (!fp) { char message[3000]; explain_message_fopen(message, sizeof(message), path, mode); fprintf(stderr, '%s0, message); exit(EXIT_FAILURE); } ``` - *message* The location in which to store the returned message. Because a message return buffer has been supplied, this function is thread safe. - *message\_size* The size in bytes of the location in which to store the returned message. - *path* The original path, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. - *mode* The original mode, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call ### explain\_message\_errno\_fopen void explain\_message\_errno\_fopen(char \*message, int message\_size, int errnum, const char \*path, const char \*mode); The explain\_message\_errno\_fopen function is used to obtain an explanation of an error returned by the [*fopen*(3)](/fopen/3.md) system call. The least the message will contain is the value of `strerror(errno)`, but usually it will do much better, and indicate the underlying cause in more detail. This function is intended to be used in a fashion similar to the following example: ``` FILE *fp = fopen(path, mode); if (!fp) { int err = errno; char message[3000]; explain_message_errno_fopen(message, sizeof(message), err, path, mode); fprintf(stderr, '%s0, message); exit(EXIT_FAILURE); } ``` - *message* The location in which to store the returned message. Because a message return buffer has been supplied, this function is thread safe. - *message\_size* The size in bytes of the location in which to store the returned message. - *errnum* The error value to be decoded, usually obtained from the *errno* global variable just before this function is called. This is necessary if you need to call **any** code between the system call to be explained and this function, because many libc functions will alter the value of *errno*. - *path* The original path, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. - *mode* The original mode, exactly as passed to the [*fopen*(3)](/fopen/3.md) system call. ## COPYRIGHT libexplain version 1.4\ Copyright (C) 2008 Peter Miller ## AUTHOR Written by Peter Miller \