int main(void) {
close(1); /* stdout file descriptor is avilable now */
/* create the file if doesn't exist, if exist truncate the content to 0 length */
int fd = open("data.txt",O_CREAT|O_TRUNC|O_RDWR,0664); /* fd gets assigned with lowest
available fd i.e 1 i.e nowonwards stdout output
gets rediredcted to file */
if(fd == -1) {
/* @TODO error handling */
return 0;
}
system("echo Hello | base64"); /* system output gets stored in file */
int max_char = lseek(fd,0,2);/* make fd to point to end, get the max no of char */
char *a = malloc(max_char + 1); /* to avoid buffer overflow or
underflow, allocate memory only equal to the max no of char in file */
if(a == NULL) {
/* @TODO error handling if malloc fails */
return 0;
}
lseek(fd,0,0);/* from beginning of file */
int ret = read(fd,a,max_char);/* now read out put of system() from
file as array and print it */
if(ret == -1) {
/* @TODO error handling */
return 0;
}
a[ret] = '\0';/* \0 terminated array */
dup2(0,fd);/*fd 0 duplicated to file descriptor where fd points i.e */
printf("output : %s \n", a);
/* to avoid memory leak, free the dynamic memory */
free(a);
return 0;
}
The popen() function opens a process by creating a pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.
int main(void) {
close(1); /* stdout file descriptor is avilable now */
/* create the file if doesn't exist, if exist truncate the content to 0 length */
int fd = open("data.txt",O_CREAT|O_TRUNC|O_RDWR,0664); /* fd gets assigned with lowest
available fd i.e 1 i.e nowonwards stdout output
gets rediredcted to file */
if(fd == -1) {
/* @TODO error handling */
return 0;
}
system("echo Hello | base64"); /* system output gets stored in file */
int max_char = lseek(fd,0,2);/* make fd to point to end, get the max no of char */
char *a = malloc(max_char + 1); /* to avoid buffer overflow or
underflow, allocate memory only equal to the max no of char in file */
if(a == NULL) {
/* @TODO error handling if malloc fails */
return 0;
}
lseek(fd,0,0);/* from beginning of file */
int ret = read(fd,a,max_char);/* now read out put of system() from
file as array and print it */
if(ret == -1) {
/* @TODO error handling */
return 0;
}
a[ret] = '\0';/* \0 terminated array */
dup2(0,fd);/*fd 0 duplicated to file descriptor where fd points i.e */
printf("output : %s \n", a);
/* to avoid memory leak, free the dynamic memory */
free(a);
return 0;
}
The popen() function opens a process by creating a pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.
