Unix Prog: Standard I/O -- Open Stream(1)

1. Open stream
fopen, freopen, fdopen system calls are used to create file streams.

Definition:

 ubuntu@ip-172-31-23-227:~$ less /usr/include/stdio.h  
 ......  
 /* Open a file and create a new stream for it.  
   
   This function is a possible cancellation point and therefore not  
   marked with __THROW. */  
 extern FILE *fopen (const char *__restrict __filename,  
           const char *__restrict __modes) __wur;  
 /* Open a file, replacing an existing stream with it.  
   
   This function is a possible cancellation point and therefore not  
   marked with __THROW. */  
 extern FILE *freopen (const char *__restrict __filename,  
            const char *__restrict __modes,  
            FILE *__restrict __stream) __wur;  
 ......  
 /* Create a new stream that refers to an existing system file descriptor. */  
 extern FILE *fdopen (int __fd, const char *__modes) __THROW __wur;  
 ......  

2. File open mode(fopen)

Following mode are allowed at unix:
"r", "rb", "w", "wb", "a", "ab", "r+", "rb+", "w+", "wb+", "a+", "ab+"

fileio.c:

 #include<stdio.h>  
 #include<stdlib.h>  
 #include<unistd.h>  
   
 #define OWN_BUFFSIZE 1024  
   
 int main(int argc, char* argv[])  
 {  
  FILE* fp;  
  char buf[OWN_BUFFSIZE];  
   
  // Open the test1.txt for reading  
  // Note: it won't truncate the file or create the file  
  if((fp = fopen("test1.txt", "r")) == NULL) {  
   printf("fopen r error!\n");  
   exit(1);  
  }  
   
  // Open the test1 for reading, the only difference is  
  // "rb" indicates that test1 is binary file  
  if((fp = fopen("test1", "rb")) == NULL) {  
   printf("fopen rb error!\n");  
   exit(1);  
  }  
   
  // Open the test2.txt for writing only.  
  // If file exists, truncate it to 0  
  // If file doesn't exist, create a new one  
  if((fp = fopen("test2.txt", "w")) == NULL) {  
   printf("fopen w error!\n");  
   exit(1);  
  }  
   
  // Open the test2 for writing only, the only difference is  
  // "wb" indicates that test2 is binary file  
  if((fp = fopen("test2", "wb")) == NULL) {  
   printf("fopen wb error!\n");  
   exit(1);  
  }  
   
  // Open for appending at the end of file.  
  // If file doesn't exist, create new file.  
  if((fp = fopen("test3.txt", "a")) == NULL) {  
   printf("fopen a error!\n");  
   exit(1);  
  }  
   
  // Open for appending at the end of file  
  // The only difference is: "ab" indicates that  
  // test is binary file  
  if((fp = fopen("test3", "ab")) == NULL) {  
   printf("fopen ab error!\n");  
   exit(1);  
  }  
   
  // Open for reading and writing  
  // file has to exist, otherwise, fail  
  if((fp = fopen("test4.txt", "r+")) == NULL) {  
   printf("fopen r+ error!\n");  
   exit(1);  
  }  
   
  // Open for reading and writing, the only difference is:  
  // "rb+"(or "r+b") indicates that test4 is binary file  
  if((fp = fopen("test4", "rb+")) == NULL) {  
   printf("fopen rb+ error!\n");  
   exit(1);  
  }  
   
  // Open for reading and writing  
  // If file doesn't exist, create one  
  // If file already exists, truncate to 0  
  if((fp = fopen("test5.txt", "w+")) == NULL) {  
   printf("fopen w+ error!\n");  
   exit(1);  
  }  
   
  // Open for reading and writing, the only difference is  
  // "wb+" indicates that test5 is one binary file  
  if((fp = fopen("test5", "wb+")) == NULL) {  
   printf("fopen wb+ error!\n");  
   exit(1);  
  }  
   
  // Open for reading and appending.  
  // If file doesn't exist, create one.  
  if((fp = fopen("test6.txt", "a+")) == NULL) {  
   printf("fopen a+ error!\n");  
   exit(1);  
  }  
   
  // Open for reading and appending, the only difference is:  
  // "ab+" indicates that test6 is one binary file  
  if((fp = fopen("test6", "ab+")) == NULL) {  
   printf("fopen ab+ error!\n");  
   exit(1);  
  }  
   
  exit(0);  
 }