我可以假设 C stdlib 函数不使用 errno 吗？
我正在查看一段 C 代码，
void printerror(char *message)
{
printf ("There was an error: '%s'\n", message);
switch (errno) {
... do stuff depending on errno
}
}
我认为这可能是一个问题，因为 printf 可能会在进入函数和到达 switch 之间更改 errno。但是， printf 的联机帮助页没有说明任何有关设置 errno 的内容，所以我可以假设它永远不会设置它吗？标准中是否有任何内容可以保证哪些函数将使用和不会使用 errno？
I'm looking at a piece of C code which is along the lines of
void printerror(char *message)
{
printf ("There was an error: '%s'\n", message);
switch (errno) {
... do stuff depending on errno
}
}
I think that might be a problem, because printf might change errno between entering the function and reaching the switch. However, printf's manpage doesn't say anything about it setting errno, so can I assume it won't ever set it? Is there anything in the standards that guarantees which functions will and won't use errno?
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任何函数都可以设置 errno，但前提是将其设置为非零值。 ANSI C 规范规定：
因此，如果您使用 errno，最佳实践方法是在可能失败的库调用之前立即将该值设置为 0，并在调用之后立即读取它。在上面的情况下，添加如下内容就足够了：
紧接在 printf 语句之前，并使用 localErrno 作为您的开关。当然，这是假设失败的函数和对 printerror 的调用之间没有库调用。如果有，您需要在调用失败后存储 errno 并将其传递给 printerror 函数。
Any function may set errno, but only if it is setting it to a non-zero value. The ANSI C spec states:
So, if you are using errno the best practices approach is to set the value to 0 immediately before the library call that may fail and read it immediately after. In the case above it would be sufficient to add something like:
immediately before the printf statement and use localErrno for your switch. This, of course, assumes that there are no library calls between the function that failed and your call to printerror. If there are you would need to store errno after the failed call and pass it in to your printerror function.
在极少数情况下，printf 可以设置 errno。如果将 stdout 从程序重定向到文件并且文件系统已满，则 printf() 将返回负数并将 errno 设置为 ENOSPC（设备上没有剩余空间）。您应该在调用 printf() 之前制作 errno 的本地副本。
In rare cases, printf can set errno. If you redirect stdout from your program to a file and the file system is full, printf() will return a negative number and set errno to ENOSPC (No space left on device). You should make a local copy of errno before calling printf().
C99标准规定了以下内容：
因此，简而言之，
errno可以由任何库函数设置。为了确定它是否确实在给定情况下，标准做法是在调用相关函数之前将其设置为零。The C99 standard specifies the following:
So, in short,
errnomay be set by any library function. In order to determine if it actually is in a given case, standard practice is to set it to zero before calling the function in question.