--- name: "explain_lseek" section: "3" platform: "linux" package: "libexplain-dev" version: "1.4.D001-16" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/explain_lseek" --- # explain\_lseek(3) explain lseek(2) errors ## NAME explain\_lseek - explain [lseek(2)](/lseek/2.md) errors ## SYNOPSIS \#include \\ const char \*explain\_lseek(int fildes, long long offset, int whence);\ const char \*explain\_errno\_lseek(int errnum, int fildes, long long offset, int whence);\ void explain\_message\_lseek(char \*message, int message\_size, int fildes, long long offset, int whence);\ void explain\_message\_errno\_lseek(char \*message, int message\_size, int errnum, int fildes, long long offset, int whence); ## DESCRIPTION These functions may be used to obtain explanations for [*lseek*(2)](/lseek/2.md) errors. ### explain\_lseek const char \*explain\_lseek(int fildes, long long offset, int whence); The explain\_lseek function may be used to obtain a human readable explanation of what went wrong in an [*lseek*(2)](/lseek/2.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: ``` if (lseek(fd, offset, whence) == (off_t)-1) { fprintf(stderr, '%s0, explain_lseek(fd, offset, whence); exit(EXIT_FAILURE); } ``` - *fildes* The original fildes, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. - *offset* The original offset, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. - *whence* The original whence, exactly as passed to the [*lseek*(2)](/lseek/2.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\_lseek const char \*explain\_errno\_lseek(int errnum, int fildes, long long offset, int whence); The explain\_errno\_lseek function may be used to obtain a human readable explanation of what went wrong in an [*lseek*(2)](/lseek/2.md) system call. The least the message will contain is the value of `strerror(errnum)`, 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: ``` if (lseek(fd, offset, whence) == (off_t)-1) { int errnum = errno; fprintf(stderr, '%s0, explain_errno_lseek(fd, eernum, offset, whence); 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*. - *fildes* The original fildes, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call.\ *offset* The original offset, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call.\ *whence* The original whence, exactly as passed to the [*lseek*(2)](/lseek/2.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\_lseek void explain\_message\_lseek(char \*message, int message\_size, int fildes, long long offset, int whence); The explain\_message\_lseek function may be used to obtain a human readable explanation of what went wrong in an [*lseek*(2)](/lseek/2.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: ``` if (lseek(fd, offset, whence) == (off_t)-1) { char message[3000]; explain_message_lseek(message, sizeof(message), fd, offset, whence); 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. - *fildes* The original fildes, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. - *offset* The original offset, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. - *whence* The original whence, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. ### explain\_message\_errno\_lseek void explain\_message\_errno\_lseek(char \*message, int message\_size, int errnum, int fildes, long long offset, int whence); The explain\_message\_errno\_lseek function may be used to obtain a human readable explanation of what went wrong in an [*lseek*(2)](/lseek/2.md) system call. The least the message will contain is the value of `strerror(errnum)`, 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: ``` if (lseek(fd, offset, whence) == (off_t)-1) { char message[3000]; int errnum = errno; explain_message_errno_lseek(message, sizeof(message), errnum, fd, offset, whence); 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*. - *fildes* The original fildes, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. - *offset* The original offset, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. - *whence* The original whence, exactly as passed to the [*lseek*(2)](/lseek/2.md) system call. ## COPYRIGHT libexplain version 1.4\ Copyright (C) 2008 Peter Miller ## AUTHOR Written by Peter Miller \