我必须在C中创建一个多级管道,以解释Linux命令,就像Unix控制台一样。我拥有的代码适用于2级管道,但是我必须实现更多(最多16个)。这里的主要问题是我不确定如何获取前两个命令的输出,然后将其重新路由为第三个命令的输入(其他级别等)。我知道我需要使用第一个管道的“ 1”和第二个管道的“ 0”,因为它们分别是stdout和stdin,但是我不确定如何在实践中实现它。
// tokenize is a separate function that uses strtok repeatedly on cmdline, setting the array
// segments to the strings separated by | and numTokens to the number of commands
char* segments[MAX_PIPE_SEGMENTS];
int x = 0;
int* numTokens = &x;
tokenize(segments, cmdline, numTokens, "|");
// the code for one command
if (*numTokens == 1) {
pid_t pid = fork();
if (pid == 0) {
char* strings[MAX_SEGMENT_LENGTH];
int y = 0;
int* numStrings = &y;
tokenize(strings, segments[0], numStrings, " ");
execvp(strings[0], strings);
} else {
wait(0);
return;
}
}
pid_t pid = fork();
if (pid == 0) {
int i = 0;
for (i = 0; i < *numTokens-1; i++) {
pid_t pid2 = fork();
if (pid2 == 0) {
int ps[2];
pipe(ps);
pid_t pid3 = fork();
if (pid3 == 0) {
close(1);
dup2(ps[1], 1);
close(ps[0]);
char* strings[MAX_SEGMENT_LENGTH];
int y = 0;
int* numStrings = &y;
tokenize(strings, segments[i], numStrings, " ");
execvp(strings[0], strings);
} else {
close(0);
dup2(ps[0], 0);
close(ps[1]);
wait(0);
char* strings[MAX_SEGMENT_LENGTH];
int y = 0;
int* numStrings = &y;
tokenize(strings, segments[i+1], numStrings, " ");
execvp(strings[0], strings);
}
} else {
wait(0);
}
}
} else {
wait(0);
return;
}
您会看到,以不同的if
/else
块来处理第一条命令和第二条命令并不能推广到两个以上的命令。可行的是,尽可能相同地对待管道命令和操作,并且只对第一个和最后一个添加特殊条件:
int i, in, out = dup(1); // save standard output descriptor
for (i = 0; i < x; i++)
{
int ps[2];
if (i < x-1) pipe(ps); // if not last in line, make a pipe
pid_t pid = fork();
if (pid == 0)
{
// if not first in line, connect standard input to pipe
if (i) dup2(in, 0), close(in);
// if not last in line, connect standard output to pipe
if (i < x-1) dup2(ps[1], 1), close(ps[1]);
// if last in line, restore standard output to original
else dup2(out, 1), close(out);
char* strings[MAX_SEGMENT_LENGTH];
int y = 0;
int* numStrings = &y;
tokenize(strings, segments[i], numStrings, " ");
execvp(strings[0], strings);
exit(1);
}
if (i) close(in);
close(ps[1]);
in = ps[0]; // the current pipe's read end is the new input
}
close(out);
do ; while (wait(0) > 0);
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