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Use ferror() rather than errno to check whether an error has occurred on a file stream (for example, after a long string chain of stdio calls, for example). The ferror() function tests the error indicator for a specified stream and returns nonzero if and only if the error indicator is set for the stream.

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Many implementations of the stdio package adjust their behavior slightly if stdout is a terminal. To make the determination, these implementations perform some operation that fails (with ENOTTY) if stdout is not a terminal. Although the output operation goes on to complete successfully, errno still contains ENOTTY. This behavior can be mildly confusing, but it is not strictly incorrect , because it is only meaningful for a program to inspect the contents of errno only after an error has been reported. More precisely, errno is meaningful only meaningful after a library function that sets errno on error has returned an error code.

errno = 0;
printf("This\n");
printf("is\n");
printf("a\n");
printf("test.\n");
if (errno != 0) {
  fprintf(stderr, "printf failed: %s\n", strerror(errno));
}

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This compliant solution uses ferror() to detect an error.   In addition, if an early call to printf() fails, later calls may modify errno, whether they fail or not, so the program cannot rely on being able to detect the root cause of the original failure if it waits until after a sequence of library calls to check.

printf("This\n");
printf("is\n");
printf("a\n");
printf("test.\n");
if (ferror(stdout)) {
  fprintf(stderr, "printf failed\n");
}

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Checking errno after multiple calls to library functions can lead to spurious error reporting, possibly resulting in incorrect program operation.

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References

\[[Horton 90|AA. References#Horton 90]\] Section 14, p. 254
\[[ISO/IEC 9899:1999|AA. References#ISO/IEC 9899-1999]\] Section 6.3.1.1, "Boolean, characters, and integers," Section  7.1.4, and Section 7.9.10.3, "The {{ferror}} function"
\[[ISO/IEC PDTR 24772|AA. References#ISO/IEC PDTR 24772]\] "NZN Returning error status"
\[[Koenig 89|AA. References#Koenig 89]\] Section 5.4, p. 73

Bibliography

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