unistd.h

问题 I am trying to use pread and pwrite so that I can lseek to the beginning of the file and start reading or writing in one atomic operation. Both of these functions will do that for me however, the issue I am having is that the compiler is giving me warning: implicit declaration of function ‘pread’ even after I added #define _XOPEN_SOURCE 500 and #include<unistd.h> like the man pages said. Am I missing something? The two statements with the function calls are below. Thanks! #include<unistd.h>

问题 I am trying to use pread and pwrite so that I can lseek to the beginning of the file and start reading or writing in one atomic operation. Both of these functions will do that for me however, the issue I am having is that the compiler is giving me warning: implicit declaration of function ‘pread’ even after I added #define _XOPEN_SOURCE 500 and #include<unistd.h> like the man pages said. Am I missing something? The two statements with the function calls are below. Thanks! #include<unistd.h>

问题 I am trying using pipe() to execute ls | wc . The fork part successfully prints as I expected (first parent --> first child --> second parent --> second child), but it does not print out the second child's part ( wc ). I put the exactly same codes for both first child and second child, and I have no idea how I can successfully edit the codes. The entire codes are written below: #include<stdio.h> #include<stdlib.h> #include<unistd.h> #include<sys/types.h> #include<string.h> #include<sys/wait.h

问题 I'm doing a project using Qt creator. I have 3 screen for every screen there are 4 button. when clicked on the first button it will wirte 0 to the file (char) and so on to 3. When i reach the last screen (4. screen) where i will read from the file and display the input from the buttons it doenst show the 3 chars. void fileOperation::openFileWrite(char x, off_t s) { int fd; char c[2] = {x}; fd = open("/home/stud/txtFile", O_CREAT | O_WRONLY, 0666);//open file if(fd == -1) cout << "can't open

问题 When I have searched for the header unistd.h, in The Open Group, I found that it contains the standard symbolic constants & types and for sys/types.h it said for data types. Then I found that both have uid_t, pid_t and several similar types. I am confused why they have divided so and what are the differences between them. I have googled but I didn't get a satisfactory answers. I will be thankful if some one can give me detailed explanation. Thank you. 回答1: The division of the POSIX and C

问题 i have vlc (program to reproduce videos) if i type in a shell: /home/vlc "/home/my movies/the movie i want to see.mkv" it opens up an reproduces the movie. however, when I run the following program: #include <unistd.h> int main(void) { execl("/home/vlc", "/home/my movies/the movie i want to see.mkv",NULL); return 0; } vlc opens up but doesn't reproduce anything. How can I solve this? Things I tried: I guessed execl("/home/vlc", "/home/my movies/the movie i want to see.mkv",NULL); was

问题 i have vlc (program to reproduce videos) if i type in a shell: /home/vlc "/home/my movies/the movie i want to see.mkv" it opens up an reproduces the movie. however, when I run the following program: #include <unistd.h> int main(void) { execl("/home/vlc", "/home/my movies/the movie i want to see.mkv",NULL); return 0; } vlc opens up but doesn't reproduce anything. How can I solve this? Things I tried: I guessed execl("/home/vlc", "/home/my movies/the movie i want to see.mkv",NULL); was

问题 I have written and run the following code: #define _XOPEN_SOURCE #include <iostream> #include <unistd.h> int main() { std::cout << "errno = " << errno << std::endl; std::cout << crypt("sometext", "ab") << std::endl; std::cout << "errno = " << errno <<std:: endl; return 0; } The initial value of errno was 0 , but after a call to the crypt() function it was set to 2 ( ENOENT ). Here is the output: errno = 0 abtAunjzvWWRQ errno = 2 回答1: Here's what the C standard says about errno (§7.5, para 3,

问题 When including the sleep function from unistd.h the program hangs indefinitely: #include <stdio.h> #include <unistd.h> int main() { int i; printf("0 "); for(i = 1; i <20; ++i) { sleep(2); printf("%d ", i); } printf("\n"); return 0; } The rest runs fine when sleep(2) is commented out, any ideas? 回答1: There's nothing wrong with the code, but note that in many cases the output of printf is buffered, meaning that the output appears on the console only if you explicitly call fflush(stdout) , you

问题 When including the sleep function from unistd.h the program hangs indefinitely: #include <stdio.h> #include <unistd.h> int main() { int i; printf("0 "); for(i = 1; i <20; ++i) { sleep(2); printf("%d ", i); } printf("\n"); return 0; } The rest runs fine when sleep(2) is commented out, any ideas? 回答1: There's nothing wrong with the code, but note that in many cases the output of printf is buffered, meaning that the output appears on the console only if you explicitly call fflush(stdout) , you