Error Handling and errno
When you use system calls in C, it is important to understand how errors are reported and handled. Most system calls indicate an error by returning a special value, often -1 or NULL, depending on the function. When a system call fails, it sets a global variable called errno to a specific integer value that identifies the type of error that occurred. This variable is defined in errno.h and is set by the kernel or the C library wrapper after a failed system call. Common error codes include ENOENT (No such file or directory), EACCES (Permission denied), and EBADF (Bad file descriptor). You should always check the return value of a system call before using its result, as failing to do so can lead to undefined behavior or security issues.
error_handling_example.c
123456789101112131415#include <stdio.h> #include <errno.h> #include <string.h> #include <fcntl.h> #include <unistd.h> int main() { int fd = open("nonexistent_file.txt", O_RDONLY); if (fd == -1) { printf("open() failed with errno %d: %s\n", errno, strerror(errno)); return 1; } close(fd); return 0; }
To make error messages more understandable, C provides functions such as perror and strerror. The perror function prints a descriptive error message to standard error based on the current value of errno, usually after a failed system call. The strerror function returns a pointer to a string that describes the error code passed to it, which you can use for custom error reporting. For robust system-level C code, always check the return value of system calls, use errno to determine the cause of failures, and provide clear error messages. This makes debugging easier and improves the reliability of your programs. Also, avoid overwriting errno before you have handled or reported the error, as it may be changed by subsequent library calls.
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When you use system calls in C, it is important to understand how errors are reported and handled. Most system calls indicate an error by returning a special value, often -1 or NULL, depending on the function. When a system call fails, it sets a global variable called errno to a specific integer value that identifies the type of error that occurred. This variable is defined in errno.h and is set by the kernel or the C library wrapper after a failed system call. Common error codes include ENOENT (No such file or directory), EACCES (Permission denied), and EBADF (Bad file descriptor). You should always check the return value of a system call before using its result, as failing to do so can lead to undefined behavior or security issues.
error_handling_example.c
123456789101112131415#include <stdio.h> #include <errno.h> #include <string.h> #include <fcntl.h> #include <unistd.h> int main() { int fd = open("nonexistent_file.txt", O_RDONLY); if (fd == -1) { printf("open() failed with errno %d: %s\n", errno, strerror(errno)); return 1; } close(fd); return 0; }
To make error messages more understandable, C provides functions such as perror and strerror. The perror function prints a descriptive error message to standard error based on the current value of errno, usually after a failed system call. The strerror function returns a pointer to a string that describes the error code passed to it, which you can use for custom error reporting. For robust system-level C code, always check the return value of system calls, use errno to determine the cause of failures, and provide clear error messages. This makes debugging easier and improves the reliability of your programs. Also, avoid overwriting errno before you have handled or reported the error, as it may be changed by subsequent library calls.
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