C语言中通过管道给grep子进程传输入时出现用法错误的问题排查

运行程序时反复收到grep的用法提示：

Usage: grep [OPTION] ... PATTERN [FILE] ...
Try 'grep --help' for more information.

相关代码（省略错误检查） #

int main(int argc, char ** argv) {
    char *file = argv[1];
    char *str = argv[2];
    FILE *fp = fopen(file, "r");
    int grep_stat = 0;
    int pipe_grep[2];
    char word[50];
    pipe(pipe_grep);
    //fork grep process
    if (fork() == 0) {
        //GREP CHILD
        dup2(pipe_grep[0],STDIN_FILENO); //now at std in,0
        close(pipe_grep[1]); //close unused end of pipe
        close(pipe_grep[0]); //shared descriptor now closed
        //parent will pass file contents from pipe to search for string. 
        //should check input from stdin, the pipe
        char *args[] = {"grep", str,NULL};
        //grep_stat =
        execvp("grep", args);
    }
    else {
        //PARENT PROCESS
        close(pipe_grep[0]); //close unused side of pipe
        while (fgets(word,50,fp) != NULL) {
            write(pipe_grep[1], word, strlen(word));
            }

尝试过的调整 #

曾修改子进程中管道的关闭与重定向顺序：

if(fork()==0){
    close(pipe_grep[1]); //close unused end of pipe
    close(0); //close stdin
    dup(pipe_grep[0]); //takes lowest available descriptor, 0
    close(pipe_grep[0]); //close shared descriptor
    
}

问题根源 #

最初怀疑是管道或execvp的使用问题，但最终确认是运行程序时未传入全部所需参数：代码中需要通过命令行传入目标文件（argv[1]）和搜索字符串（argv[2]），如果缺少这两个参数，str会为NULL，导致execvp传递给grep的参数不完整，触发grep的用法提示。

内容的提问来源于stack exchange，提问作者Gillian